CONSTRUCTABILITY of Interior Design

INTRODUCTION

Constructability is the sum of the requirements produced by a detail itself, independent of the basic function or aesthetic needs of the detail. E.g., every detail must be structurally sound, use appropriate connections, accommodate tolerances, and be durable enough to suit its intended life cycle. This section discusses these and other aspects of constructability and offers some ways to address these universal concerns found in all detailing problems.

Although constructability issues are a part of all detailing problems, the methods by which they are satis fied may con flict with design intent, constraints, or function. It’s the designer's task to resolve these con flicts in the best way possible. E.g., the design intent of a service counter in a restaurant may suggest a highly articulated assembly of countertop, opening framing, and overhead lighting, while the basic constructability issues of cleanability, durability, and ease of construction may require a simpler assemblage of materials and connections.

STRUCTURAL REQUIREMENTS

Strength and structure are terms referring to the inherent ability of a material, product, or assembly to withstand any loads that may be placed on it. This may be as simple as the ability of one part of a detail to support the weight of another part of a detail, or as complex as the ability of an assembly to withstand complex gravity and wind loads that would require a structural engineer to calculate.

For many interior details structural requirements are not critical; the ability of one component to support another and common loads placed on the detail are satis fied by standard methods of construction. E.g., the typical methods by which a hollow metal door frame is anchored to metal studs is su fficient to hold the frame in place, support the door, withstand the forces of opening and closing the door, and resist the occasional impact on the frame caused by people or objects moving through the opening. However, if the door is detailed as wider, higher, or heavier, or if a custom frame con figuration is developed the required structural connections will need to be examined and designed to support the detail's unique loading.

In other cases, the particular function and constraints of the detail require the designer to develop a unique solution to the problem. E.g., a countertop spanning two supporting cabinets will require structural support su fficient to hold any equipment or materials placed on it, while accounting for the possibility that someone might lean or sit on the countertop.

These loads must be resisted by materials and con figurations based on the material and span of the countertop, how much deflection is allowable, and required knee space below the countertop.

ill. 1 Basis structural loads compression tension shear moment torque

Basic Concepts of Structure

Although complex details with unique structural requirements must be designed by a structural engineer, the interior designer can develop many light-loading details with an understanding of some basic structural concepts. With this knowledge, the designer can make decisions regarding the basic con figuration of a detail, the types of connections required, and the size of the components. If required, a structural engineer can then verify the adequacy of the structural portions of the detail.

There are several basis types of loads that building elements must resist. These are shown diagrammatically in ill. 1.

Compression loads push the elements of a material together. These loads are resisted by making the structural element su fficiently large and/or by using a material that is su fficiently strong. Every material has a particular ability to resist loading per unit area. E.g., steel can resist more compressive force per square inch than wood. Compressive loads are typically the easiest to resist with a variety of materials. Wood, steel, aluminum, brass, stone, concrete, and even plastic are all good choices for interior details with compressive loads.

In addition to simple compression there is another aspect of materials resisting compressive loads that must be considered. This is called the buckling load and is the point at which compressive forces in a column or other vertical member cause the member to bend outward (and break, if the load is su fficiently large), even though it’s capable of resisting the basic compressive load. Buckling commonly happens when the vertical element is long relative to its thickness. E.g., very thin legs on a tall table or countertop supporting a heavy weight could be subject to buckling loads. In most cases, however, the commonly used materials and sizes of most interior details eliminate problems with buckling loads.

Tension loads tend to pull the elements of a material apart. A wire supporting a suspended weight is in tension. Some materials, such as concrete or masonry, are good at resisting compressive loads, while very poor at resisting tensile forces. Steel, on the other hand, has great strength in both compression and tension and is typically used for wire supports.

When a material is subject to tensile loads, it becomes longer, or stretches. The amount of stretch depends on the weight being supported or force being applied, as well as the strength and size of the supporting member. For most interior design and detailing applications, the change in length is not significant, but if heavy loads are anticipated, this elongation must be taken into account and calculated by a structural engineer.

Shear loads tend to cause the elements of a material to slide relative to each other. While shear stress is always present in large structural members, such as a beam supporting a load, for interior details shear stresses are also commonly present in smaller elements, such as a screw supporting a wall panel or cabinet.

Moment is a property by which a force applied to an element tends to cause the element to rotate about a point or line. The amount of moment is proportional to the amount of load applied and the distance from the line of action of the load and the point of rotation.

In building construction, moment is most often found in cantilevered beams where the beam is supported at only one end or a shelf is supported along only one edge. When one material is connected to another material and the connection must resist moment forces, the connection must be carefully considered and designed. Even if the connection is su fficient to support the load, the cantilevered element may deflect by an unacceptable amount. The con figuration of interior details can be simplified by avoiding situations where moment loads are present.

Torque is the result of a force tending to produce a rotation. A very simple example of torque is using a wrench to tighten a bolt. For most interior details, torque forces are not present or are of magnitudes small enough not to be problematic.

The following suggestions include approaches to dealing with structural issues in interior details.

ill. 2 Beam action compression tension (a) beam in bending (b) incorporate beam action

Use Simple Direct Bearing Connections Whenever Possible

In most cases, resting one material directly on another can easily accommodate weight or applied load in a detail. This sets up simple compressive forces that nearly any material can resist and simplifies any connection required. E.g., setting a privacy partition directly on the floor is an easier structural connection to make than hanging it from the wall. In the first case, the weight of the partition rests directly on the floor, while in the second case the weight must be transferred with moment connections or with fasteners loaded in shear and tension, in which case both the fasteners and the substrate to which they are applied must be adequate to support the weight.

Incorporate Beam Action

Beams are the most basic type of structure in which a horizontal element rests on two or more vertical elements. Uniform or concentrated loads on the beam are transferred through bending action to the supports. As diagrammed in ill. 2(a), when a load is applied to a simple rectangular beam, the beam bends with portions of the beam above the centerline, called the neutral axis, tending to shorten, while the portions below the centerline tend to lengthen. Thus, the top of the beam is in compression and the bottom of the beam is in tension, with the maximum stresses being at the extreme distances from the neutral axis.

Because of the way beams resist loads, it’s most e fficient to locate as much of the area of the beam as far away from the neutral axis as possible. This is why steel beams are formed in an I- or H-shape and some manufactured wood joists use a thin plywood web with thicker, solid members at the top and bottom of the web to form an I-shaped assembly.

For simple rectangular shapes, beam action can be most e fficient and result in a stiffer and stronger beam if the rectangle is placed such that the orientation is vertical rather than horizontal. See ill. 2(b). In practical terms, E.g., if a 1 × 2 piece of wood is being used to support a countertop, it’s better to orient it vertically rather than horizontally. This principle can be used with wood, aluminum, steel, or any other material. When space is limited for a detail incorporating beam action, steel or aluminum strips or angles can be used instead of wood.

Use the Simplest Connections Possible

Because the forces acting on most interior details are minimal, connections can be kept as simple as possible to minimize cost and shorten construction time. As long as the individual components are su fficiently supported and attached to each other, the most direct method of making connections should be used. E.g., adhesives may be used in place of screws or clips can be used instead of ledgers.

Use Redundant Connections or Bearing When Required

If the structural integrity of the detail is critical consider using redundant connections in case one weakens or fails. E.g., three bolts may be used instead of two or one detail component may be placed directly on top of another for a simple gravity bearing connection, while using fasteners to secure the two elements in place.

The decision to use redundant connections, however, must be balanced with the goal of using simple structural connections and as few as possible, as suggested below. The designer must weigh the need for ensuring that the detail is structurally sound against the requirements of cost, construction time, aesthetics, and ease of assembly.

Use Structural Connections Approved by the Manufacturer

For details that use premanufactured components, the recommendations provided by the manufacturer should be used. Manufacturers often provide instructions for installing their products on a variety of substrates and give recommendations on the types of fasteners to use. However, if the manufactured item is being used in a manner that is different than intended, the connections should be reviewed by a structural engineer.

Use Removable Connections for Reuse Potential

Although not typically considered with current construction methods, a building or detail can be designed for disassembly. This allows the building component to be taken apart for reuse, recycling, or proper disposal as part of a larger program of sustainable design. Designing for disassembly must be considered during detailing, at the beginning of the building life cycle rather than at the end. Connections are one of the key components in designing for disassembly, because taking apart a building must be reasonably easy or the decision will be made to simply dispose of materials rather than reusing or recycling them.

Connections can be made for disassembly in a variety of ways. E.g., screws or bolts can be used instead of nails or adhesive. Clips or pressure joints can also be used instead of rigid connections, where appropriate. As part of the disassembly mindset, the fewest number of components should also be used to minimize the time required for taking the detail apart.

As with other aspects of detailing, the designer must balance the practicality of designing for disassembly with the sustainability bene fits that may be gained and factors of cost, function, and the many other aspects of detailing.

CONNECTIONS

Connections are the ways the various parts of a detail are attached to each another and to the substrate to which the detail is anchored. Connections can be made with adhesives, nails, screws, bolts, tape, or Velcro or by power fastening, crimping, clipping, welding, or soldering. Which method is selected depends, of course, on the materials being fastened, but other factors may include appearance, cost, strength, safety, simplicity, clearance available, adjustability, and the ability to disassemble the detail, as mentioned in the previous section.

E.g., steel can be welded, bolted, or screwed to other steel members, while it can only be bolted or screwed to wood members. Even if two pieces of steel need to be connected, considerations of safety and the need for a skilled welder may suggest using another method.

ill. 3 Types of bolt and screw heads hex cap bolt acorn cap bolt hex flange locknut square head bolt wing nut thumb screw carriage bolt step bolt lag screw flat head wood screw round head wood screw oval head wood screw

Use the Appropriate Method for Rigid Connections

Rigid connections are those that are not intended to move or accommodate incidental movement. Rigid connections include fasteners, such as nails, screws, bolts, and clamps, as well as welding and soldering. Adhesives, tape, and crimping provide a slightly less rigid connection, allowing for very minor movement if the force acting on the two joined materials is great enough. Screws and bolts tend to hold better than nails, adhesives, and clamping and are useful if some amount of adjustability is required. If the designer wants to emphasize the method of connection as part of the aesthetics of the detail, screws or bolts can be used and even oversized. Many types of screws and bolts are available if the designer wants something other than a standard connection. There are available in steel, stainless steel, and some other materials depending on the connector type. Some of these are shown in ill. 3. Most of the screws are available with various types of heads, including slotted, Phillips, square, slotted hex, and one-way tamperproof.

Determine Movable Connection Type Based on Use

Many times a detail must have moving parts, such as an access panel on a cabinet or a sliding platform in a display case. There are a variety of methods and hardware types of movable connections. Which type is selected depends on whether the movable portion must be completely removable (loose), sliding, swinging, rolling, or some combination. Loose components are a low-cost and simple method of providing a removable panel when only occasional access is required. The panel can be af fixed by screws or bolts. For panels, cabinet doors, and other construction elements that must be opened repeatedly, some type of hinge should be used. These may include butts, wraparounds, pivots, continuous hinges, and concealed hinges.

Some of the many types of hinges are shown.

Other specialized hardware is available for attaching doors or drawers to cabinets and other millwork. These include flap stays, lid stays, flipper door slides, swing-up fittings, slides, running track, and sliding door hardware.

ill. 4 Hinge types door shown from inside open 90° cabinet shown from the inside (a) pivot hinge shown from outside (b) wrap around hinge shown from outside (d) surface hinge (c) concealed hinge side panel door side panel door side panel door

Minimize Number and Types of Connections

Unless redundant connections are required for safety and in critical circumstances, as mentioned in the previous section, try to minimize the variety and number of connections. E.g., use only one size and type of bolt for every connection in a detail, and if possible, in all details. This makes it less likely that workers will confuse one type of connector for another in different construction assemblies. Minimizing both number and types also generally reduces construction time and minimizes cost.

Make Connections Accessible

When any type of connection is shown in a detail there must be su fficient clearance to install the connection during initial construction, to replace the connection if necessary, and to disassemble the construction. For instance, there must be room to install and tighten a bolt or screw, to apply adhesive, to solder a connection, or to use a power tool. Space must be provided to both accommodate the worker's hand as well as any tools being used.

MOVEMENT

Movement is that aspect of a detail that makes provision for any anticipated displacement of the detail itself or its components as well as overall building motion that the detail must accommodate. Buildings move for a variety of reasons, and interior construction must accommodate this movement. Movement can be caused by temperature changes, de flection of the structure, wind-induced sway, seismic motion, building settlement, or warping of wood.

The amount of movement expected will determine the detailing response.

ill. 5 Recommended average moisture content for interior wood products

ill. 6 Relieved backs kerfed plowed out

ill. 7 Interior control and expansion joints eased edge width four times expected movement; depth = 1/2 width control, isolation, or construction joint sealant backer rod controlled crack varies joint compound (b) wallboard or plaster (a) flush wood paneling (c) ceramic tile

Use Acclimated Materials

Materials that respond to changes in moisture and temperature, such as wood, should be seasoned and installed at approximately the same humidity and temperature levels that will be present during their use. If they come from a climate substantially different than the one in which they will be used they should be stored on site for a period of time as recommended by the manufacturer before installation. ill. 5 shows the recommended moisture content levels for woodwork used for interior applications.

Use Relieved Backs on Wood

Flat, solid pieces of wood are subject to cupping distortions caused by differences in shrinkage on opposite sides of the board. This tendency can be minimized by relieving the back, or nonvisible side, of the board, as shown in ill. 6. This can be done by cutting small kerfs in the back of the board or by cutting out a wide portion, which also makes it easier to attach the board to two slightly uneven surfaces. Normally, shop-fabricated wood trim comes with relieved backs. Custom-fabricated wood pieces may need to be relieved on site or the requirement clearly stated in the architectural woodwork specifications.

Use Control Joints

Control joints are small joints purposely built into a construction component to cause any incidental cracking that occurs to happen in a predetermined location. E.g., control joints are commonly seen in concrete slabs as small grooves. These grooves weaken the concrete slightly so that minor cracking will be concealed within the joint rather than show up as a random crack elsewhere in the slab. In interior construction, control joints, sometimes incorrectly called expansion joints, are used in large expanses of wood paneling or flooring.

For flush wood panels, eased joints should be used between two panels. An eased joint is one with the visible corners slightly beveled about 1/16 in. (1.6 mm). See ill. 7(a). Although the joint is visible it looks like a deliberate design decision and is not objectionable and can conceal any slight shrinkage of the wood panels. Without it, any shrinkage at joints would be noticeable. Control joints are also used in terrazzo floors by placing a zinc strip in the floor, which is flush with the floor and may be part of a decorative pattern to separate different colors of terrazzo.

Provide Expansion Joints

Expansion joints provide for greater movement than control joints. They are used in large expanses of gypsum wallboard, plaster, ceramic tile, wood paneling, and other materials that expand or contract or may move slightly from other causes. E.g., expansion joints in ceramic tile floors accommodate slight de flection of the supporting slab and expansion and contraction of the flooring itself. Ill.s 4-7(b) and (c) show some common control and expansion joints used in interior detailing.

If large movement is expected, then slip joints or building expansion joints should be used, as described below.

Use Sliding/Overlapping Joints

A simple method to accommodate movement, either as a control joint or as an expansion joint is to detail materials that overlap or otherwise allow for sliding movement. See ill. 8. This generic type of detail can be used with wood, metal, plastic, or proprietary products. Sliding joints can be combined with reveals to allow a material to move without the movement being noticeable. E.g., as diagrammed in ill. 8(e), wall panels can be mounted flush with each other on wall clips with a reveal space between them. The clips hold the panels in place and take the gravity loads, while allowing horizontal movement. The reveals between the panels effectively conceal any slight movement. As an added bene fit, the clips allow the panels to be easily removed for repair or replacement.

ill. 8 Sliding joints (a) sliding/overlapping joints (b) metal sliding joint (c) sliding joint with trim (d) reveal (e) panels on clips panel

ill. 9 Site constructed slip joint 1/2" (13) min. 1/2" (13) max. metal stud 5/8" (16) type X gypsum wallboard acoustical insulation if required suspended ceiling system fire-rated sealant attach wallboard to studs and not to top track long-leg runner

ill. 10 Proprietary fire-rated slip joint deflection amount metal stud 5/8" (16) type X gypsum wallboard suspended ceiling system fire-rated sealant attach wallboard to studs and not to top track; verify fastener requirements with manufacturer proprietary top track extend wallboard into fluted metal deck; mineral fiber insulation between layers

ill. 11 Relief joints at perimeter walls 1/2" (13) max. 1/2" (13) min. acoustical insulation if required acoustical insulation if required single or double layer gypsum wallboard attach wallboard to outside stud acoustical sealant or gasket continuous aluminum channel attached to mullion wallboard screwed to stud and finished with joint compound curtain wall mullion stud attached to top and bottom runners metal runner attached to structure gypsum wallboard trim (b) relief joint at wall or column (a) relief joint at mullion

Use Slip Joints

When a significant amount of movement is expected in a detail, slip joints should be used to allow the detail to move without damaging the detail or surrounding materials. E.g., in covering the joint between a fire-rated partition and the underside of a floor slab the potential de flection of the floor slab must be considered in addition to the requirement for properly sealing the joint against fire and smoke penetration. A typical site-constructed detail is shown in ill. 9. This detail provides a slip joint that creates a fire-rated seal, while allowing the slab to de flect up to approximately 1/2 inch (13 mm) without buckling the partition. Another approach is to use a proprietary product as shown in ill. 10, which does the same job, but allows greater de flection. Several proprietary products are available that make constructing partition slip joints easier, while allowing for various amounts of slab de flection.

Significant horizontal movement in high-rise buildings can be caused by wind loading. It an interior partition is detailed against a window mullion with no provision for accommodating the movement, partition cracking or buckling can occur, just as with vertical deflection. A slip joint, such as that shown in ill. 11, should be provided. This can be constructed with generic materials or a proprietary product can be used. The amount of deflection expected should be verified by a structural engineer.

Use Building Expansion Joints

When very large movement is expected a different type of joint must be used. Such movement is caused by entire sections of a building moving at different rates or by earthquakes. Joints for these kinds of movement are called building separation joints and seismic separation joints.

They are typically located and designed by the architect and structural engineer as part of the design of the building. Proprietary products are available to accommodate different magnitudes of movement in floors, walls, and ceilings and the interior designer may encounter these types of joints in large buildings. Two such joints are shown in ill. 12. Generally, manufacturers provide a method to incorporate the adjacent finish material on the joint.

Provide Clear Space

The simplest way to accommodate small or large building movement is to separate interior construction assemblies and components from the building structure. This is diagrammed in ill. 13. This allows the building to move without transferring any damaging forces to interior materials or components, and it allows interior construction to move separately from the building structure. Of course, this is only possible if the connection can be open from a functional standpoint, E.g., a partition held away from the ceiling and exterior walls could not provide complete visual or audio separation.

ill. 12 Building separation joints W base and wall finish as required cover for fire-rated joint assemblies are available joint width determined by movement expected (a) floor joint (b) wall joint W slip plate expansion joint cover assembly insulation if required cover for fire-rated joint assemblies are available floor covering

ill. 13 Provide clear space fixed construction

Tbl. 1 Common Industry-Standard Construction Tolerances

Building Element Tolerance Source

Concrete slab on grade, level from stated elevation ±3/4_ ++ (±19) ACI Concrete slab on grade, flatness under 10 ++ (3 m) straightedge ±3/8_ ++ (±10) ACI Concrete suspended slab, level from stated elevation ±3/4_ ++ (±19) ACI Concrete suspended slab, flatness under 10 ++ (3 m) straightedge ±1/2_ ++ (±13) ACI Position of concrete beams and walls ±1_ ++ (±25) ACI Concrete opening size, vertical openings, like windows and doors +1_ ++ , -1/2_ ++ (+25, -13) ACI Size of concrete columns over 12_ ++ (305 mm) +1/2 _ ++ , -3/8_ ++ (+13, -10) ACI Concrete block, position in plan ±1/2_ ++ in 20 ++ (±12.7 in 6.1 m) ACI Concrete block, plumb ±1/4_ ++ in 10 ++ (±6.4 in 3.05 m) ACI Interior stone wall cladding size ±1/8_ ++ (3) MIA Stone tile size ±1/32_ ++ (0.8) MIA Rough lumber framing, position ±1/4_ ++ (6) RCPS Rough lumber framing, plumb ±1/4_ ++ in 10 ++ (6 in 3050) RCPS Rough floor framing with sub flooring in 10_ ±1/4_ ++ in 10 ++ (6 in 3050) RCPS Rough floor framing with sub flooring overall ±1/2_ ++ in 20 ++ (13 in 6100) RCPS Woodwork field joint installation of wood to wood items:

Flushness and gap width, premium grade ±0.012_ ++ (±0.3) AWI Flushness and gap width, custom grade ±0.025_ ++ (±0.65) AWI Woodwork field joint installation of wood to nonwood items:

Flushness and gap width, premium grade ±0.025_ ++ (±0.65) AWI Flushness and gap width, custom grade ±0.050_ ++ (±1.3) AWI Curtain wall and storefront installation, plumb ±1/8_ ++ in 12 ++ (±3 in 3600) GANA Gypsum wallboard partitions in horizontal position ±1/4_ ++ (6) Various Gypsum wallboard partitions, plumb and ceiling level ±1/4 _ ++ in 10 ++ (±6 in 3050) Various Maximum bow of 1/4_ ++ tempered glass from 71_ ++ to 83_ ++ long 0.47_ ++ (12 mm) ASTM Original source of industry standard:

ACI American Concrete Institute MIA Marble Institute of America RCPS Residential Construction Performance Guidelines 3rd ed., Association of Home Builders Remodelors, AWI Architectural Woodwork Quality Standards.

GANA Glass Association of North America, Glazing Manual ASTM American Society for Testing and Materials, C1048 For a complete listing of construction tolerances see guide of Construction Tolerances 2nd ed., David Kent Ballast. John Wiley and Sons.

TOLERANCES

Tolerance is the acceptable deviation in size, position, shape, or location from a theoretically exact value. Tolerances in building construction recognize that nothing can be built perfectly.

The amount of tolerance of a building material or the installation of a product depends on the material itself, manufacturing tolerances, fabrication tolerances, and installation tolerances.

Some tolerances in construction are very small while some are quite large. E.g., architectural woodwork built in the shop may have tolerances in the range of hundredths of an inch, while site-cast concrete may have tolerances of several inches.

The interior designer must understand both the manufacturing and installation tolerances for construction components and detail accordingly. Tbl. 1 gives some common industry standard tolerances for various construction components, including both basic architectural tolerances on which interior construction is based, as well as interior components themselves.

Of course, for architectural tolerances, such as a concrete floor slab, the interior designer must accommodate whatever tolerances and existing conditions are present with the interior construction. For interior components, the designer can choose to design for industry-standard tolerances or specify more restrictive tolerances, recognizing that requiring smaller tolerance values may increase the cost and installation time of a construction element.

The following guidelines are some of the methods that can be used to accommodate tolerances when developing a detail. These are diagrammed in ill. 14.

ill. 14 Methods of accommodating tolerances (a) provide shim space (b) provide scribe (c) use reveals (d) use offsets (e) use filler strips (f) use sliding fit/overlap (g) use adjustable connections (h) use clearance/intermediate attachment out of parallel out of plane trim offset

Provide Shim Space

A shim space allows one construction component that must be installed with close tolerance to be placed within or adjacent to another construction component that may be built with greater tolerances. E.g., a door frame must be placed perfectly plumb and level in order for the door to operate properly but is placed within, and attached to, a rough opening whose edges are usually not plumb or level. See ill. 14(a). Shims are used to provide for the variable space between the two components. Shims are thin pieces of wood (usually tapered), metal, or plastic, which are used to fill the space. Shimming is typically used for the installation of doors, glazed openings, cabinets, wall panels, and any construction element that must be installed plumb and level. In most cases, the resulting irregular shim space and shims must be covered with trim or otherwise concealed by the detail.

Provide Scribe

A scribe, or scribe piece, is a slightly oversized piece of material that is custom trimmed on site to follow the irregularities of another edge. As diagrammed in ill. 14(b) a scribe may be used to set a perfectly straight and plumb cabinet edge against a wall that is bowed or slightly out of plumb. When the gaps are filled by the scribe the variation is generally not noticeable to the eye.

Use Reveals

Reveals visually separate one construction element with another by using a small space, generally in the range of 3/8 in. to 1 in. (10 mm to 25 mm). See ill. 14(c). Reveals may be as deep as allowed by the materials or at the designer's discretion. If the edges and surfaces of the two elements are not parallel or flush, the reveal separates them enough to be unnoticeable. Reveals are an effective device for use in a wide range of construction details. In addition to accommodating tolerances, reveals provide a way to separate and finish different materials and to establish interesting shadow lines. A scribe piece may be recessed to create a reveal if the gap must be physically closed off. Reveals may be finished with the same color and material as one or both of the adjacent materials to help minimize its presence, painted black, or finished with a contrasting color.

Use Offsets

An offset, diagrammed in ill. 14(d) is similar to a reveal in that it visually conceals slight differences in plane between two elements. Offsets are often referred to as reveals. Offsets are most often used to conceal the situation of two adjacent surfaces being slight out of plane with each other. They are commonly used when two pieces of wood trim are joined. Instead of setting the two wood pieces perfectly aligned along their edges, one is set back from the other by a fraction of an inch; E.g., the casing trim of a door opening is usually set back from the face of the door frame itself by about 1/4 in. (6 mm).

Use Filler Strips

Filler strips are separate construction elements that extend beyond two edges or planes to visually separate and disguise any discrepancies in flushness or alignment. See ill. 14(e).

They conceal minor irregularities in the same way a reveal does, but a filler strip becomes a prominent design element in its own right by extending beyond the primary materials. A

filler strip can also be recessed from the two adjacent surfaces to create a reveal but with a separate finish to highlight the strip instead of trying to conceal it. Filler strips can also act as scribe pieces.

Use Sliding or Overlapping Fit

A sliding or overlapping fit is a common way of detailing to conceal imperfect fits between construction elements. As diagrammed in ill. 14(f), a finished piece of wood molding applied over the joint between two adjacent pieces of paneling easily conceals any imperfection in plumb or flush alignment, as well as poorly finished edges of the paneling. Most molding or trim pieces in construction are used for this purpose. A rubber base, E.g., conceals the imperfect gap between the floor and the wall surface. Door casing trim covers not only the shim space but also the out-of-plumb tolerance of the rough opening.

Use Adjustable Connections

When two components of a detail must be joined, provide for a connection that can be adjusted slightly. Slotted bolt holes are one way to provide for easy adjustment when detailing attachment of one rigid piece to another, as diagrammed in ill. 14(g). Floor levelers on desks and other cabinets are another way to build in adjustment in a detail and are an example of using a screw fitting to provide a fine level of adjustment that can be used in many types of details. Depending on the detail, adjustment capability may need to be provided for in one-, two-, or three-dimensions.

Give Adequate Clearance and Incorporate Intermediate Attachments

In many, cases the best way to deal with construction tolerances is to provide space for an intermediate material that be used to connect the finish object with an imperfect construction element. E.g., as diagrammed in ill. 14(h), if a panel must be installed perfectly plumb, level, and flat over a rough concrete wall one or more hanging strips can be installed on the concrete using any required shimming and structural fasteners independent of the

finished panel itself. The panel is then suspended from the hanging strips as required by its construction. This approach requires that sufficient space be detailed between the concrete surface and the back of the panel for the hanging strip, shims, and any mechanical fasteners.

CLEARANCES

Clearance is the space between construction elements required for installation, making connections, allowing for tolerances, and providing space for other construction items. The requirement for clearance is often forgotten by detailers because the detail is viewed and conceived on paper as a completed assembly and often in isolation, without regard for the construction and required maneuvering space around it. The best way to avoid clearance problems when detailing is to imagine how the fabricators and installation workers would actually perform the work to build the detail.

Allow Space for Working and Assembly

A detail must provide sufficient space for a worker to use tools and manipulate hands and arms for assembly. Most hand tools don’t require a large space, but some power tools may require 12 in. (300 mm) or more of space in addition to space for the worker's hand and arm.

E.g., a small gap shown on a detail to be enclosed with gypsum wallboard finish must provide space for attaching the wallboard, taping the joints, and applying joint compound.

Sometimes, just a 90-degree change in orientation of a screw or bolt connection may be enough to solve a problem with a fastener installation.

Provide Space for Installation of Pieces

Individual pieces of a detail must be placed into position. This often requires a component to be lifted over something else, tilted into place, rotated, or otherwise manipulated in a space larger than required for its final position. E.g., sufficient plenum space must be provided below ducts and piping to tilt and install acoustical ceiling tile, even though the tile and suspension grid themselves only require a narrow envelope of space when they are in their final position.

Allow for Tolerances

As mentioned in the previous section, clearance should be provided to allow the various construction elements to be installed slightly off their theoretical position and for materials to vary in size according to their manufacturing tolerances. When allowing clearances for tolerances the detailer must also consider the accumulation of tolerances, which may be more than the individual tolerances of the various components.

DURABILITY

Durability is the requirement of a material or detail to withstand the rigors of use. E.g., the corner of a partition in a heavily traveled corridor should be durable enough to resist denting, scratching, and chipping as people or goods impact it.

Durability is also an important aspect of sustainability. If a material or detail component must be replaced or repaired frequently, more energy and resources must be used to maintain the detail. Refer to Section 3 for more information on detailing for sustainability.

The following suggestions list some of the ways to build durability into a detail.

Self-Durable

Details can be made durable by themselves without any applied protection.

USE HARD, HEAVY,DURABLE MATERIALS

The materials used in a detail, especially the portions exposed to abuse, can be specified as hard, heavy, or otherwise durable. Materials such as concrete, brick, stone, some metals are generally very durable for interior use. Other materials, such as wood, vary in hardness depending on the specific type used. Oak, E.g., is harder than pine or poplar. Plaster is a harder finish than gypsum wallboard, although abuse-resistant wallboard is available. Many manufacturers supply variations of their products that are durable. E.g., acoustical ceiling tiles are available that are intended for use in schools and other occupancies where intentional or accidental damage are concerns. Very durable materials may cost more initially but have a lower life-cycle cost when replacement and maintenance costs are considered.

The designer can also specify and detail materials that may show wear over time but which age well and can be re finished easily. E.g., hardwoods such as oak hold up to impact, rubbing, and similar wear but take on a unique patina with age that many people find attractive. When the finish wears excessively, the wood can be sanded and re finished.

LOCATE FRAGILE SURFACES AWAY FROM ABUSE

If a detail does have a finish, surface, or edge that is susceptible to damage, the designer may be able to position that portion away from damage. E.g., instead of using a 90-degree corner on gypsum wallboard, a rounded trim piece can be used. A decorative soft metal strip can be recessed below a wall surface instead of being flush or extending beyond the surface.

Decorative wood molding can be located above shoulder level instead of being in a lower position.

Applied Protection

If materials that are susceptible to damage must be used for reasons of cost, availability, design intent, function, or other constructability issues, there are various strategies than can be built into the detail.

COVER FINISH WITH SHIELD

The most obvious method to protect something is to cover it. A kick plate or armor plate on a wood door are simple examples of this strategy. Clear plastic panels can be installed over more fragile wall surfaces or glass can be placed on horizontal wood surfaces. However, the designer must decide whether covering one part of a detail or surface with a protective material compromises the design intent or creates more problems than it may solve. In many cases, if a material must be covered with a shield, it may be best to replace it with a more durable material or one that can be easily repaired or replaced, as discussed in the next section on maintainability. Alternate, similar materials can also be considered. E.g., instead of using a real wood surface for a countertop, a plastic laminate finish with a wood-grain pattern can be substituted.

INSTALL GUARDS AT CRITICAL EDGES and SURFACES

Instead of covering a finish or portion of a surface, just critical parts of it may need protection.

A corner guard on the exterior corner of a partition is an examples of this. If protective guards are needed, the designer may consider making them a design feature instead of trying to hide them. E.g., instead of using clear plastic corner guards on a partition, a large, stainless steel pole could be placed at the outside edge of the corner.

SEPARATE FINISH WITH RAILINGS/POSTS

A surface, edge, or other portion of a detail can be physically separated to protect it from damage by maintaining a distance from the material and whatever the damage may come from. E.g., a railing or series of simple metal bars can be used in front of a wall surface to prevent carts, people, and goods from damaging the lower portion of a partition.

MAINTAINABILITY

Maintainability is the quality of a detail that allows it to be kept in its original state or level of use during the life cycle of the interior space. This may involve the ability to clean easily and to make adjustments and repairs to, and replacements of, the detail or its components.

Cleanability

MAKE CLEANING and OTHER MAINTENANCE EASY

The easier it’s to maintain something, the more likely it’s to be maintained. This includes details and finishes that are maintained by the users or occupants, such as residences, or by separate maintenance personnel. Making a detail easy to clean is especially important for occupancies that must be maintained in a sanitary condition on a daily basis such as restaurants, hospitals, restrooms, kitchens, hotel rooms, and the like. Interior components that must be cleaned frequently should have the fewest number of materials and connections possible. Different materials may require different cleaning products, but in practice, cleaning personnel often use only a limited number. Many connections, edges, corners, and joints also make a detail di fficult and time-consuming to clean.

USE SMOOTH,NONABSORBENT SURFACES

In most cases, the smoother and more nonabsorbent a material is, the easier it’s to clean.

However, some materials, such as brass or flat-painted gypsum wallboard, may scratch or streak easily if not cleaned properly. As with all aspects of cleanability, the designer must understand who will be maintaining the space and what procedures will be used. Professional cleaning personnel may be more likely to correctly clean materials and surfaces than user/occupants.

AVOID SHARP INSIDE CORNERS and SMALL AREAS

Sharp, inside corners and small gaps and recesses are especially di fficult to clean. Cove bases and other types of rounded corners allow mops and cleaning rags to be wiped over surfaces quickly and easily. In addition, any type of connector, edge, or joint that has a small dimension should also be avoided when sanitation or ease of cleaning is an issue. Something as simple as a screw head can collect dirt and grease and be nearly impossible to clean. If possible, individual components that create small joints or corners can be made removable to make cleaning easier.

MINIMIZE NUMBER OF CONNECTIONS TO CLEANABLE SURFACE

The simplest surface to clean is one that is smooth, flat, and without interruptions. Floors are relatively easy to clean, except for all the cabinets, table and chair legs, and other furnishings and construction elements that intersect the floor. The detailer must consider how frequently and by what method something will be cleaned. E.g., a clear glass panel used for a sign suspended from a partition with standoffs may be an interesting design feature, but any dust or dirt accumulation behind the glass may be di fficult to remove because of the various connectors and the small gap between the glass and partition. An alternative detail could mount the sign on a four-sided frame set directly against the partition providing just one large element to clean around. Benches or cabinets can be cantilevered or suspended from partitions instead of resting on the floor to make mopping easier.

SELECT PRODUCTS THAT DO NOT NEED TOXIC CLEANERS

Although maintenance of facilities is not the responsibility of the interior designer, materials and finishes can be selected and specified that don’t require complex cleaning procedures or toxic cleaners that may compromise indoor air quality. As part of the selection process, the interior designer should review the manufacturer's cleaning instructions to determine if any toxic substances are required for routine maintenance. The material safety data sheets for recommended cleaners can also be reviewed to help determine toxicity.

Adjustability

Many times a construction detail must have some degree of adjustability, to accommodate tolerances when the detail is constructed, to provide for realignment after a period of use, or to allow the detail to function as intended. Some building products, such as door closers, have adjustability built in, while other times the interior designer must design the detail to be adjustable.

PROVIDE LOOSE CONNECTIONS

Loose connections are those that provide some amount of movement while still holding materials in place. Loose connections are generally employed in details that must provide for movement, from either material expansion and contraction or movement of the adjacent construction, as well as to allow for minor adjustability or easy disassembly. Examples of loose connections include the following.

++ Wire-suspended elements. Suspended ceilings, E.g., allow for fine adjustment to level the ceiling during construction, while allowing for de flection of the floor above without compromising the integrity of the ceiling.

++ Clips that provide for one-way sliding. E.g., wall-mounted panels set on Z-clips can be shimmed to accommodate wall tolerances, provide for adjustment during construction, and allow the panels to accommodate slight movement during use.

++ Clips or other components that provide for friction fit. E.g., freestanding wall panels can be connected with clips for easy installation and removal. Glass is held in place with tightly fitting glazing tape or elastomeric material.

++ Loose components held in place by gravity. Adjustable shelving can rest directly on supports or slide into dados to make repositioning easy. Connections using metal tabs that fit into slots also utilize gravity to hold materials in place, as with adjustable shelving standards.

++ Joints in wood without adhesive or mechanical fasteners. Spline and tongue-and-groove joints, E.g., allow the wood to expand and contract and can accommodate minor building movement.

++ Sliding joints such as those illustrated.

ill. 15 Repositionable connections (a) leveling bolt (b) slotted connection (c) setscrew

USE REPOSITIONABLE FASTENERS and CONNECTIONS

Repositionable fasteners create a tight connection after installation but allow for adjustment during construction as well as repositioning of a building component after use. E.g., an adjustable hinge allows a cabinet door to be reset if it sags slightly during use. Because this type of connection must be capable of multiple tightening and loosening, it most often must employ some type of bolt and nut or set screw in a threaded fitting. ill. 15 shows three types of repositionable connections.

A leveling bolt, shown in ill. 15(a) allows for adjustment parallel to the length of the bolt. Once the piece is in position both bolts are tightened against it to hold it tight. Used in conjunction with a slotted bolt hole, this type of connection can provide for adjustment in two directions. Although ill. 15(a) shows vertical adjustment, the detail can be oriented in any direction to provide for fine adjustment. Floor levelers used on desks and cabinets are one example of a leveling bolt.

Slotted connections, shown in ill. 15(b) allow for adjustment perpendicular to the length of the bolt. Used in conjunction with a slotted hole in the adjacent piece this connection can provide for two-way adjustment.

Setscrews are small screw-type fasteners that are threaded into mating holes in one of the adjustable pieces. See ill. 15(c). When the two elements are in position the setscrew is tightened and holds the two pieces together by pressing one against the other. This type of connection requires that one of the pieces be con figured so that one piece is held in place by the other. Setscrew fasteners generally provide for adjustment in only one direction.

USE FLEXIBLE JOINTS

A flexible joint is a semirigid joint that uses the stiffness of the connector to hold two pieces together while still allowing movement. Springs, metal spring clips, and hard elastomeric sheets are examples of flexible joints. Flexible plumbing connections are often used to connect drain lines to prevent buckling or breaking caused by building movement. Although flexible joints are not used much in interior detailing, they can provide a useful way to join two construction elements.

Repair and Replacement

Nearly all interior finishes wear out eventually or become damaged through normal use or accidents. Interior details should provide methods to easily repair all or a portion of a detail or replace it, especially those vulnerable to damage.

USE PATCHABLE MATERIAL

The simplest way to provide for repair is to use materials and finishes that can be patched and re finished. Gypsum wallboard and plaster are two materials that can be patched and repainted by qualified trade workers to look like the original finish.

If a material cannot be successfully patched in a small area, the designer can detail the

finishes in small sections so that only one section needs to be replaced edge to edge instead of an entire surface. E.g., a large wall can be built in sections separated by reveals. If a small area is damaged just one section between the reveals needs to be replaced or repaired.

Any minor differences between the existing wall and the repaired section is not noticeable.

USE INTERCHANGEABLE PIECES

When a material or finish cannot be successfully patched the detail can be designed to use replaceable parts. Carpet tile and ceiling tile are two common examples of this approach.

For other types of interior details, individual pieces can be attached with removable fasteners or otherwise designed so a damaged part can be replaced with a new one. If the pieces are unusual, unique, or are from a particular dye lot, the designer should specify that replacement pieces be provided as attic stock for maintenance by the client.

E.g., walls in areas susceptible to abuse can be finished with individual panels mounted on clips so that, if one is damaged, it can easily be removed and replaced with an identical unit. Likewise, a woodwork detail may use a removable strip of molding along an edge that receives the most wear. When the wood is dented, scratched, or chipped, it can be replaced with a newly finished member.

ALLOW FOR EASY MAINTENANCE ACCESS

Details should be con figure d to make whatever maintenance may be required easy to accomplish. This includes providing su fficient clearance for workers and tools, using connections that can be easily disassembled, and orienting fasteners in accessible locations.

CONSTRUCTION PROCESS

Construction process is the sequence of steps taken by various trade workers to complete the building of an interior space. Regardless of the design intent of a detail or how it meets functional needs, all details reflect the fact that buildings are composed of both manufactured and field-fabricated elements, require a certain sequence of activities, and are built by trade workers who may belong to different unions and who have various levels of skills.

In addition to simple physical buildability, the construction process is most often closely related to cost and time. A complicated detail using a variety of materials will most likely cost more and take more time to build than a simpler detail. The following suggestions provide some ways to make construction more e fficient when designing interior details.

Number of Parts

MINIMIZE TOTAL NUMBER OF PIECES

The detailer should try to minimize the total number of pieces and parts of a detail. This simplifies construction, reduces the chances for errors, speeds construction time, and minimizes cost. As described below, using the maximum amount of prefabricated components also simplifies construction and generally improves quality.

MINIMIZE VARIATION IN SIZE and TYPE OF COMPONENTS

If possible, the individual components of a detail should be the same size, type, and con figuration. Not only is it more di fficult to order and stock a variety of materials, but the chances of a worker using the wrong one also increases as the number of different materials increases, especially if they are similar in size, shape, and material. However, minimizing variation in individual components does not necessarily mean a uniform, simplistic appearance in the

final design. By coordinating all the detailing on a project, the designer can develop a few well-crafted details and use them in a variety of ways to satisfy the design intent of the problem while resolving the other functional issues. The following are some ideas to consider.

++ Limit the number of different partition types. E.g., a fire-rated partition may also be used as an acoustical partition in some situations.

++ Develop a limited vocabulary of wallboard trim and finishing.

++ Minimize the number of different door, frame, and glazing types.

++ Develop a few ceiling details and use them in different ways throughout the job.

++ Use a limited number of wood trim pro files and sizes. This not only provides more design consistency but also may reduce costs.

++ Minimize the number of different types and sizes of ornamental metals.

++ When fasteners are needed, they should all be the same size and material.

Sequence

SIMPLIFY SEQUENCE OF CONSTRUCTION

Most details require more than one worker to assemble and a certain number of steps for the complete process. Whenever possible, design details to require the least number of individual steps consistent with the design intent, constraints, and functional issues. Also consider what tools and equipment will be needed. E.g., using an aluminum frame will require fewer steps to complete the installation of an interior glass panel than one using wood that may require several steps for the installation of the frame, glazing stop, and casing trim.

SEQUENCE CONSTRUCTION FROM ROUGH TO FINISH

Think through how a contractor will need to construct a detail. Generally, design a detail as though all rough construction will be completed first and progressively move to more re fined

finishing. Don’t develop a detail that requires rough construction, such as metal or wood framing, to be installed after some of the finished part of the detail. All dirty and wet work, such as wall framing, mechanical installation, wallboard finishing, plastering, and tile work, should be complete before finish work and installation of architectural woodwork.

EMPLOY REPETITIOUS ASSEMBLY WHEN POSSIBLE

As on an assembly line, there is e fficiency and lower cost in repetitive work. Details, and all interior construction in general, should be designed to use as much repetitive assembly as possible. E.g., all wall panels should be installed using the same method, doors should

fit into frames in a consistent manner, glazing details should be consistent, and tile installation should be uniform. Modular units should be used whenever possible.

Trade Division of Labor

MINIMIZE NUMBER OF TRADES NEEDED FOR ASSEMBLY

Because labor is a significant portion of total construction cost anything that reduces labor saves cost. Generally, a detail that requires the fewest number of different trades will cost less than one that uses more. This approach has the added advantage of reducing the possibility that trade disputes will develop on the job site.

SEQUENCE SO THAT EACH TRADE IS INVOLVED ONLY ONCE

Details should be designed so each required trade is involved only once. E.g., framers and wallboard finishers should not build a portion of a detail or wall and then have to return to the job site to complete additional work after carpenters or millwork installers have done their portion of the work. Imagining construction of a detail from a rough-to- finish progression as described in the previous section is one way to think about the sequence of trade workers.

CONSIDER UNION JURISDICTIONS

Depending on the geographical region where a project is being constructed, union jurisdiction may in fluence how a designer develops a detail. In some areas, unions may have very strict rules about the limits of each unions work and how workers may (or may not) cooperate.

Limits on what type of work is done may result in a single trade having to be involved more than once as discussed in the paragraph above. If the designer is working in a new geographical area for the first time, they should talk to other designers or contractors in the area to get an idea of unique problems or concerns related to union requirements.

CONSIDER AVAILABILITY OF LOCAL LABOR

If the interior construction of a project is complex and incorporates nonstandard construction techniques the job may require highly skilled and specialized workers or a large number of workers. It may be di fficult to find such workers in some geographical areas. If this is the case, either the quality will suffer, if less qualified workers are used, or workers will have to be brought in from another area, raising the cost of the project. Therefore, the availability of local labor, in terms of both number and quality, may in fluence how a project is designed and detailed. If certain types of skilled labor are unavailable, the designer may consider simplifying details or using prefabricated components rather than site-built construction.

Off-Shelf versus Custom Parts

Whenever possible, the designer should incorporate standard, off-the-shelf components in stead of designing custom parts. This costs less and makes repair and replacement easier. E.g., instead of developing a custom pro file for a piece of ornamental metal, the designer should investigate the standard shapes and sizes of whatever metal type is being considered.

Likewise, instead of detailing a gypsum wallboard edge trim or sof fit for a ceiling detail, there may be standard manufactured components that will function as well or better at a lower cost and with a faster construction time.

Shop versus Field Fabricated

Although much of construction includes standard practices regarding what is site built and what is manufactured or shop fabricated, there are still times when the contractor or designer can decide on how a particular part of a job can be completed. E.g., many woodwork items can be built on site by a qualified finish carpenter or fabricated in a mill shop and installed by the mill shop workers. On-site construction has the advantage of providing for tolerances and making an exact fit, while shop-fabricated items are generally higher quality and shorten construction time because they can be built while other construction is taking place. Shop-fabricated items may also be necessary if qualified labor is not available for on-site work, as described in the section above. When a decision must be made, it’s a matter of balancing quality, time, cost, and availability of labor.

Previous: (FUNCTION of Interior Design)
Next: DIVIDE / CREATE SPACE USING PERMANENT BARRIERS

top of page | Home