The Structural Floor: Its Design or Renovation--Home/Apartment Renovations--TECHNICAL DECISIONS

We firmly believe that you should know something about structures before tampering with the bones of your building. We have heard too many stories of buildings that have collapsed around the ears of novice renovators who at tempted surgery on the body of their buildings without having studied structural anatomy. The old adage “a little bit of knowledge is a dangerous thing” does not necessarily apply here. We hope that a little bit of structural knowledge, at the very least, will engender a respect for the complexity of structure and that you will be wise enough to hire an experienced carpenter if you aren't sure you can handle a particular procedure. If you are designing the floor system for the extension or if you are planning to cut an opening into your existing floor, be sure to have an architect or structural engineer review your structural plans and calculations.

We have included the following four sections primarily for those of you who are planning to construct an addition to your house. The sections are critical, however, for all renovators who are planning to make even minor modifications to the floor structure, walls, roof, or foundations of the existing house. Many novices are unaware that any penetration of these areas may affect the integrity of the structure. For instance, even the addition of a skylight may require a modification of the roof structure and , while still in the design stages, you should have an understanding of how roofs work. Apartment renovators will probably be able to skip the sections on roofs and foundations but will need the one on walls since it explains partition construction. The section on floors is critical if you are combining two apartments to make a duplex and have to cut into the floor to install a stair, add a partition, or put in a bathtub.


Although buildings are constructed from the foundations up, it's easiest to design the structure by considering the floor system first, the walls next, the roof third, and the foundations last. It is difficult to make decisions about the foundations without knowing how you are going to frame the floor, where you are going to locate the load- bearing partitions, and how the roof is going to work. On the other hand, you can’t finalize any aspect of the structural design until you have considered all aspects. This makes the design process complicated since it does not fall into a neat step- by-step order. If you are constructing an extension, we suggest that you read about all of the parts—floors, walls, roofs, and foundations— before making any decisions or calculations. (Read the construction sections in Part Four of this guide as well.) When you have an overall understanding of the process, return to this topic and design the floor system.

Most houses built since the 1950’s use the plat form floor system of construction since it's the easiest to build. We advise you to use this method when constructing your extension for much the same reason.

It is easier to understand the design of the floor system if you have a clear understanding of how the floor system works. In its simplest terms, each component of the floor acts to transfer the weight placed on it to the foundations. The floorboards receive the live loads and transfer that weight plus their own weight to the joists. The joists add their own structural weight and transfer the loads to the girders or beams, which in turn transfer the loads to the vertical supports or directly to the foundations. Eventually all of the live and dead loads are transferred by the foundations’ footings to the ground. The trick is to get the loads transferred evenly to the ground without putting undue stress on any of the components. A poorly designed floor will sag, squeak, or, at worst, fail.

Laying Out the Platform Floor

The platform floor is exactly that, a platform. It consists of a subfloor (the finished floor is laid much later, after the danger of its being ruined by rain, cement droppings, paint, etc., has passed), which is nailed to the floor joists. The subfloor is usually made of sheets of plywood. The floor joists are long, slender horizontal members spaced either 12”, 16”, or 24” o.c.

These joists span between supports. Both supports could be continuous foundation walls, or if the span is too long, the joists will span between a foundation wall and an intermediate girder. In the case of an addition to an existing house, the joists will, be supported by two new foundation walls or one new foundation wall and the side of the existing house (with or without inter mediate supports).

If foundation piers are used instead of a continuous foundation wall, the joists will span from a perimeter girder to an intermediate girder (, or in the case of an extension, from the existing house to an intermediate or perimeter girder.

Floor joists are the lightweight horizontal members that carry the subfloor. Joists are easily differentiated from beams and girders in that they are always spaced closely together.

In a house with continuous foundation walls the joists frame into a member called the header. If the house sits on piers, the joists frame either into a header or directly into the girder. The header is usually the same size (the same cross- sectional dimensions) as the joists. It is used as a nailing surface for the joists (giving them added rigidity) and also serves to enclose the space and to keep out unwanted animals and drafts. The end joist that sits on the foundation wall parallel to the run of the joists is called the trimmer joist. It closes off those ends of the house and carries the end load of the floor and the wall.

*Why the 12”, 16”, or 24” spacing? This spacing is used for studs, floor joists, and roof rafters. It is a modular unit derived from the standard sheet size of most sheathing, subflooring, paneling, and plywood, which is 4’ X 8’. Four feet is equal to 48”, which is evenly divided by 24” (into two segments), 12” (into four segments), or 16” (into three segments). Any of these spacings ensures that the 4’ X 8’ sheets have a nailing surface at each end.

Historically, this spacing derives from the fact that in Eng land firewood was cut to 16” lengths (to fit into a fireplace constructed of 8” -long bricks). This firewood was often split into lath strips to support plaster. The wood uprights that supported the lath were then spaced 16” apart.

The “o.c.” refers to “on center,” meaning that the distances between structural components are to be measured from the center line of one joist to the center line of another type. Beams and girders are heavier structural members and are harder to tell apart. Generally, a girder is heavier than a beam, meaning that it either spans a wider gap or carries a heavier load or supports beams. In buildings that have joists, beams, and girders, the joists carry the direct floor loads, transferring these loads to the beams, which transfer the loads to the girders, which transfer the loads to the vertical supports or to the foundations. Few house designs require both beams and girders. We will call the intermediate support for the joists the girder.

Both the headers and the trimmers sit on a mud sill (pressure-treated to resist rot and insects), a wood board generally a 2 X 6 or 2 X 8, which lies directly on top of the foundation wall and is secured to it with anchor bolts. The remaining structural element of the floor system is the bridging. The bridging (cross or solid) helps make the floor more rigid. The bridging holds the joists in straight, reducing the potential movement in the floor. A well-designed and well-constructed floor shouldn't creak. Although a tiny bit of resilience is tolerable, if not actually desirable, a creaky floor isn't .

Designing the Framing Plan of the Extension

It is likely that your extension will abut the house on one side. If the extension is longer than it's narrow, and its long side is perpendicular to the house, it makes sense to run the joists between your new foundation walls (or new girders sup ported by piers). If the length of the extension is parallel to the adjacent wall of the house, it makes sense to run the joists from the existing house to the new foundation wall. If you are going to frame the joists into the existing wall so that the new and existing floors will be level, plan to frame into the existing header or trimmer.

Determining Loads and Spans

The design of the floor system is determined by the loads on the floor, the distance the beams and joists must span, and the strength of the lumber you are going to use. Knowing all of these factors, you can specify the dimensions, spacing, and length of the joists and beams.

Building codes in most regions require that for single-family residences the first-floor framing be designed to support a live load of 40 pounds per square foot (psf) and the second-floor framing be designed to support a live load of 30 psf. This means that you must design the floor so that it can carry 40 pounds on every square foot of space. (Picture a 10’ X 10’ room with 100 little kids jammed into it.) Our design chart (Table A)* is based on the 40 psf design requirement. The 40 psf requirement covers normal household furniture, including pianos. If you are considering in stalling anything heavier, consult the building department. Most garages must be designed for a 50 psf load, but you would use a reinforced- concrete slab and not wood for the floor. (It is unlikely that your code allows a wood floor in the garage.)

The design span is determined by the floor plan of the extension. If the extension is to be less than 16’ wide, you may be able to span from foundation wall (or girder) to foundation wall (or girder) without needing any intermediate supports. If the extension is over 18’ wide, you will need an intermediate support, which will reduce the span by half. There are borderline cases. For in stance, if the extension is 18’ wide, you could specify floor joists that are 18’ long, but your floor might have too much bounce in it. For the border line cases we recommend breaking the span in two. In addition, lumber over 14’ long might be difficult to come by. The length of your joists and girders might be determined by what is available in the local lumberyards.

*The tables provided in this guide are for preliminary design purposes only and shouldn't be used for final selection of structural sections. Since each municipality follows its own codes and standards, what is valid in one community may not be valid in another. In addition, the calculations in these tables (and all tables in this guide) are based on the strengths of lumber that's “stress-graded.” Since the lumber available in the lumberyards isn't stress-graded, when using the tables in this guide assume that the lumber is no. 3 grade for making your preliminary selections.

Selecting the Joists

The design of the floor joists is made relatively simple by the availability of joist charts, which are a compilation on one page of all the necessary mathematical calculations. All that's left for you to do is to feed the charts the required span of the joists and the type of lumber you will be using. The chart identifies the cross-sectional dimensions of the appropriate structural member.

As an example, assume the extension is 24’ wide. (The width, remember, is the shorter dimension. If your extension is 14’ X 24’, the width is 14’. The joists are designed to span the shorter distance.) We decide that the 24’ span will be broken into two spans of 12’ each. Therefore, the joists will span 12’. (If your extension is less than 16’ wide, you can span from foundation wall to foundation wall and eliminate the intermediate girder support.) Our first choice for the lumber is Douglas fir, but we discover that it's out of stock. We choose hem-fir (not as strong, but good enough for our purposes) and decide to use no. 1. As a general rule, architects and contractors select stronger materials for horizontal structural members, such as joists and girders, which are subjected to bending stresses. Vertical structural members, such as columns and studs, are subjected to compression stresses, which are less critical. It isn't uncommon for a contractor to use no. 1 grade for the joists and no. 2 grade for the studs.

Going to the simplified joist table (Table A), designed specifically for a live load of 40 psf, we read down the left-hand column and find hem-fir (north) no. 1. The numbers in the boxes are the span designations in feet and inches. Reading across the line from left to right we pass 10’-ó”, etc., and stop at a number that's longer than our 12’ span, which is 12’-7”. We look up to the top of the column to learn that the structural member made out of no. 1 hem-fir that can span 12’-7” with a live load of 40 psf is a 2 X 8 joist spaced 16” on center. If we examine the line further, we see that the same 2 x 5 joist would span as much as 13’-10” if it were spaced 12” on center. Obviously, we would require more joists for this spacing and our lumber bill (and labor) would be greater. Besides, the 16” spacing will do the job.

If our design span was 13’-6”, we could use 2 x 10’s at 16” o.c. (good for a 16’ span) or 2 x 8’s at 12” o.c. To determine which is more economical, add up the number of joists you will need and multiply it by 14’. (Remember that lumber is often sold in lengths that increase in 2’ intervals.) You will have to pay for the 14’ joists, but you may need the extra length to overlap the girder. Multiply this figure by the cost of the joists per square foot. Compare the price of the 2 X 8’s with that of the 2 X 10’s. There are other considerations: the 16” spacing will require less labor and the 2 X 10 floor will be firmer than the floor with joists that aren't as deep.


(to be used for preliminary selections only)

Joist size (inches) | Joist spacing (inches)

*10 be used for activity floors.

Maximum allowable span (feet-inches)

Species | Grade| 2 x 6| 2 x 8 | 2 X 10 | 2 X 12

Coast sitka spruce

select structural


Designing the Girder

The girder is used as the intermediate support between foundation walls or, if you are using foundation piers instead of a continuous wall, to span between the piers. The girder must be strong enough to carry the weight of the joists, which carry the weight of the floor system, as well as the partitions resting on it and the live loads.

We design the girders by trial and error. We make some tentative design decisions based on our 24’-wide extension. We have already divided the span into two parts of 12’ each which the joists, will span. As an example, let us say the extension is 32’ long, which is much too long a span for the girder. A rule of thumb for girder design suggests 10’ as the maximum practical span for a wood girder. We could make the span longer if we are willing to use a very deep built- up section. If we divide our 32’ length into three parts, we get spans of 10’-8” each. If we divide the length into four parts, we get short spans of 8’. How we divide the girder will determine how many foundation piers will be needed. We will evaluate both of these choices to determine which division is best. If we need a very long girder we may turn to steel.

In order to determine the cross section (size) of the girder, we must evaluate how much weight it will be supporting. First we must determine which portion of the floor will be supported at the perimeter of the house and which must be sup ported by the girder. We have sketched our potential floor-framing plan in Illustration 8. Under the drawing is a section through that plan at the point indicated by the lines with the arrows at the ends. (A-A’ identifies the section and shows exactly where on the plan the cut was taken.)



(for preliminary design purposes only)

It isn't necessary to frame the house completely in lumber. Steel beams and girders can span longer distances with less chance of bending than can wooden beams, and are therefore often used in place of heavy wooden girders in the framing of floor systems. A 6” -deep steel section can substitute for a wooden member 14” deep, thus saving headroom. The table gives the allowable loads for steel beams and it indicates two types of steel sections. The well-known I-beam is designated on the table as an S section. The ‘W’ designation refers to a steel section with wider flanges than the I section. Illustration 1-2 shows framing details.


We will analyze the requirements for four spans of 8’ each first. Since the joists are spanning the north-south (assuming north to be at the top of the page), it's obvious that the east and west perimeters of the building need to support only the weight of the walls above them. The center girder supports half the weight of the floor joists, as indicated on the framing plan (by the shaded portion. If the total floor is 24’ wide and the girder is placed midway between the north and south perimeters of the building, the girder will be supporting half of each of the 12’ spans— that's , a total of 12’ of width. If we isolate the part of the floor system directly supported by the girders, we can add up the loads that will be placed on it.

Live loads are the first consideration. Since we are designing a one-story extension, we are required to design for a minimum of 40 psf. The dead loads (the weight of the materials in the joists, subfloor, finished floor, etc.) are about 10 psf (Inset II). The total live loads and dead loads are 50 psf. The load on the floor segment can now be calculated: 50 psf X 12’ X 8’ = 4,800 lbs.

If there are load-bearing partitions that rest either on the girder or on the section of floor that the girder supports, these loads must be considered too, as they will be taken on by the girder. The next calculation requires us to guess the weight of the girder itself. Since we don’t know what the girder will be, we will assume a girder that's 4 X 10. We consult Table II-A for the weights of the various structural members. The 4 X 10 weighs 9 lbs. per linear foot: 9 lbs X 8’ = 72 lbs.

To complete the calculations:

Add up the live and dead loads:

Add the dead load of girder segment:

Add the dead load of a non-bearing partition: 12’ long X 9’ high X 8.2 psf

(The roof is supported by the peripheral walls and not by the girder.)

Total load: 5,757

Table B will give us the proper girder for the given loads. For the 8’ span we may use a 4 x 10, which is good for up to 6,820 lbs. Since this is such a light member, we should try the calculations for the 12’ spacings and compare the two. We don’t know an easier way to design girders. We leave you with this advice: When in doubt, overdesign.


When calculating loads, both live loads and dead loads must be accounted for. Table II-A gives the weights of various building materials and composite structural items. Most of the listings are per square foot, which means that if you are trying to determine the weight of a 2 x 4 stud wall with 1/2” gypsum-board walls on both sides of the studs, you would multiply the weight of the wall per square foot (8.2 lbs.) x the dimensions of the wall (let us say, 8 high by 12 long) = 787.2 lbs.


Floors plyscore 1 .5

per square foot (8.2 lbs.) x the dimensions of the hardwood flooring 4.0

wall (let us say, 8 high by 12 long) = 787.2 lbs.

joists with hardwood floor and plyscore subfloor:

2X6 10.5

2X8 11.5

2X10 12.0

2X12 12.5

Roofing shingles:

Wood 2.5

fiberglass-asphalt strip 2.5

plywood sheathing 1.5

Windows For windows and sliding glass doors constructed of wood frame 3.5 and double glazing (insulating glass) use 3.5 psf for approximating purposes.

Ceilings 1/2” gypsum board 2.1

Roof Plank 2” thick 5

3” thick 8

(The following aren't in pounds per square foot):

Pounds per linear foot

Reams and Girders 4 x 8 7.2

3X10 6.6

4X10 9.1

4X12 11.0

6X10 13.8

4X14 13.0

6X12 16.7

Stairs Weight for a complete simple stair about 3’ wide: 300 lbs.


Framing the Joists to the Girder

Joists can be framed into a girder in one of three ways depending on the framing situation. The most common method is to lap the joists over the girder, nailing the joists to each other and to the girder. The advantage of the lap joint is that the joists don't require absolutely precise measuring and cutting. The disadvantage is that it steals a lot of potential headroom from the basement or crawl space. The second method is to butt the joists end to end over the girder and to use metal ties to connect them to each other and nails to join them to the girder. The advantage of this method is ease in joist layout. (The joists are now laid in straight and not staggered lines.) The disadvantage is that the joists must be cut precisely to size. The third method resolves the head room problem by hanging the joists to the girder so that the top of the girder and the top of the joists are on the same plane. This can be accomplished with metal joist hangers designed for this purpose.

Framing the Joists and Girder to the Foundation Wall

Joists are framed into a continuous foundation wall by nailing the ends of the joists to both the mud sill and the header. The intermediate girder is supported on the extreme ends by the foundation wall. If you are using the joist lap or butt methods, the girder will join the foundation wall at some point a few inches below the top of the wall. In this case the girder is framed into a “pocket” that was made when the foundation wall was constructed (). Even if you use the joist hanger method, it's likely that the girder will need a small pocket in the foundation wall. (Its depth is usually a few inches greater than the joists.) Whatever method used, be careful to coordinate the heights of foundation wall, foundation piers, girders, and sills to ensure that the joists are perfectly level when joined to both sill and inter mediate girder.

Stabilizing the Floor with Bridging and Subflooring

The joists are held in position, to some extent, by being nailed to the headers or joined to the girders. The joists are further stabilized by diagonal or solid pieces of lumber nailed between them, called bridging. In addition to its value in keeping the joists straight, the bridging helps in the distribution of a concentrated load to the other joists. Cross bridging joins the joists by means of two diagonal pieces of wood, usually 1 x 2’s that cross each other in an X pattern. Metal cross bridging is available that eliminates the cutting and simplifies the nailing. Solid bridging uses pieces of lumber the same dimensions as the joists. The pieces are measured, cut, and nailed solidly into place. A row of bridging joins the joists every 6’ to 8’ .


This section covers framing conditions that are likely in the design of the extension’s floor sys tem. In addition, it covers framing modifications that must be made to the existing house as a result of the renovations. Before modifying the floor of an existing house, you will have to determine the layout of the existing joists. For modifications of the first floor, go down into the basement or crawl space and draw the framing plan to scale. If you are making changes to the second floor, you may have to remove a small portion of the first-floor ceiling to determine the way the joists are running as well as their depth and spacing. The thoroughness of this analysis depends on what you are planning to build above the floor. You will need an exact framing plan if you are framing an opening for a stair. If you are adding lightweight partitions for a closet, you may not need to be all that cautious. On the other hand, if you are planning to put an oversized Jacuzzi midspan on the second floor, you will probably have to reconstruct whole segments of the second-floor framing system.

The final floor of the house will have two layers: the subfloor, or the rough floor, and the finished floor, which may be of wood planks, parquet squares, tile, or carpet. The subfloor is constructed immediately after the joists are secure and the bridging in place. It generally consists of 4’ X 8’ sheets of plywood, 3/4” thick, nailed to the joists in a basket-weave pattern. Some old houses may have boards that were nailed on the diagonal to the joists as subflooring. The subfloor, bridging, and joists are nailed tightly together to form a structural unit that acts to resist stresses and transfer loads. In addition, the subfloor serves as a platform on which the wall units may be constructed. The finished floor isn't installed until much later.

Framing under a Partition or Bathtub

The floor under a partition or a bathtub must be reinforced in order to carry the added load. When the partition or tub runs parallel to the run of the joists, the joists are simply doubled beneath it . If the partition running parallel to the joists is to carry plumbing, it's necessary to create a space through which these lines will pass. The floor joists are doubled and separated a few inches (they are placed to straddle the plate of the partition above). Solid bridging is nailed between them every 16” so that the two joists will act in unison.

If the partition runs perpendicular to the joists, some say there is no need for added reinforcement. Conservative carpenters recommend solid blocking between the joists directly beneath the partitions. In the case of a load-bearing partition or an oversized bathtub running perpendicular to the joists, it makes sense to be cautious and double every other joist.

Framing an Opening

You will need to frame an opening for a stair, a fireplace flue, or a fireplace foundation. No matter which way the joists run, parallel or perpendicular to the opening, some of the joists must be cut and , thereby, “crippled.” Generally, the crippled joists are supported by a doubled or tripled header which is framed into doubled or tripled trimmer joists. The opening is thus framed by a box made of doubled headers and trimmers. If the opening does not line up with the spacing of the joists, it might even be necessary to throw in an extra joist or two.

Framing a Cantilever

A cantilever is an overhang or projection which is supported on one side. Large bays, porches, and balconies can be designed as cantilevers. It is relatively easy to incorporate a cantilever in the design of an extension. On the other hand, because of the nature of its framing, it's very difficult to add a cantilevered element to a house that's al ready built.

A cantilever is actually an extension of the floor system, rather than an element added on at the end. The joists that support the cantilevered portion must be extensions of the joists of the adjacent floor system. Because the cantilever must be fully counterweighted by the structure behind it, you should limit its width to 6’ to 8’. The part that overhangs the support shouldn't be longer than one-fourth the length of the adjacent (fully supported) part. The non-cantilevered end must be either counterweighted or firmly anchored to its support to counter the upward tendency that's produced when a load is placed on the overhang.

If the cantilever occurs parallel to the run of the joists, this rule is easily applied. The regular joists are merely extended so that they span the length between the supports and also the, cantilevered length. A header is placed at the end of the projection to supply rigidity to the joists. The trimmer joists on the sides of the projected portion are doubled. Solid blocking (short pieces of lumber) is constructed between the joists (over the sup port) to maintain structural integrity. It is important to connect the non-projecting ends of the joists to the girder using metal framing anchors to combat both the gravitational force of the ordinary floor and the upward tendency of the cantilever.

In a case where the extension’s joists are running perpendicular to the cantilever, you must revise part of the framing plan to ensure that the projecting portions of the joists are counter balanced by three times their lengths. Essentially, we have a condition similar to framing an opening, in that we are left with crippled joists.


If you are renovating a house that's more than forty years old, or if the house is stuccoed and multistoried, there is a good chance that it's a balloon frame. Illustration 30 in Section 16 covers framing details for the balloon as seen from the inside of the building (see also Section 16, ). Adding a joist for a partition or a bathtub may be a little difficult. If you are trying to double a joist under a midspan weight and you can’t get the new joist to rest on the sill, bolt the new joist to the existing one to reinforce it. (Remember that the main stresses on the joists will be bending stresses at midspan rather than the shear stresses at the end supports.) If you are thinking of ex tending your existing balloon-frame house, use the platform frame for the extension. Balloon-frame construction requires fire stops between floors. Be sure not to remove the existing ones.


Many apartment buildings are constructed using a slab called a flat plate, which is about 5” to 10” thick and reinforced with steel rods and wire mesh. There are no beams in this system and the bottom of the slab is the ceiling of the floor below.

Generally, there is a good deal of special reinforcement at the points where the slab is joined to the column. We are reluctant to give you advice on how to cut into the slab for an opening. We suggest that you consult an architect or a structural engineer. Depending on the location of the hole and its size, the opening may or may not need special reinforcing. In addition to the structural problems, there may be plumbing and electrical lines buried in the slab that may be disturbed.

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