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Designing or Modifying the Roof--Design and Modification--Home/Apartment Renovations--TECHNICAL DECISIONS



There are four probable renovation approaches that would result in the modification of your roof’s profile: (1) the penetration of the roof for skylights, (2) the construction of an extension to the original house, (3) the inclusion of porches, verandas, or other overhangs, and (4) the addition of another story onto the house.

With the exception of the skylights, all of these modifications must be structured as if they were new roofs. The most important consideration in the structural design of the roof, as in the design of floor and wall framing, is the transfer of loads from one structural member to another. In addition to the dead loads (the weight of roofing materials), the roof is expected to carry live loads, which in this case are the weight of accumulated snow and the lateral pressures of wind and wind- driven rain. Because the weight of accumulated snow is so great in northern climates, roofs often are designed with slopes that shed the piles of snow. Flat roofs in snowy climates must be designed to carry greater live loads.

Keep in mind that in addition to structural and aesthetic considerations, there are a number of legal imperatives. Check with the zoning authorities to see if there are height (or style) restrictions for the roof. Check with the building department to see if they require you to use fireproof structural or finishing materials and to determine the roof’s design loads. If you are thinking of building on top of an apartment house in an urban neighborhood, you will have to clear it with at least a half dozen authorities.

Roof design, even for a small extension, re quires some education in structures. Also, the roof, being exposed to the elements, presents a more complex design problem than the structural design of the house’s interior floor systems. The criteria for roof design vary considerably from one geographical area to another. Some areas of the country are subject to occasional earthquakes, tornadoes, and gale winds, not to mention the more ordinary problems of heavy rain and snow fall. Therefore, we urge you to consult an architect or structural engineer for any new roof or for the modification of an existing roof. The instructions and tables included in this section are to serve for preliminary design purposes and aren't to be used so that you can design your own roof.

THE FLAT ROOF

No roof, not even a “flat” roof, is ever designed to be perfectly level. The roof is always pitched to a drain that carries water down through the house or through drainpipes along the exterior walls to a dry well or storm sewer. It is never a good idea to allow water or ice to accumulate on a roof. In the first place, water is heavy and its weight may stress the roof beyond its structural capacity. Second, a large, standing pool of water may cause seepage to the interior below if the roof member is any less than perfect.

Framing the Flat Roof for an Extension

For the most part the flat roof is structured like a floor with some degree of variety. A flat roof may or may not have an overhang. The beams and joists can be concealed by a ceiling, or exposed to reveal the structure. The roof can be constructed of joists and beams or can be of the post-and- beam variety (see the section on plank-and-beam construction).

The flat roof is framed as if it were a floor system. Review the procedure for the framing and sizing of structural members. Openings for the chimney and skylights are framed as you would frame an opening for a stair.

Building codes must be consulted to determine the live loads to be used for the design of the roofing in your particular weather zone. These loads will vary considerably from one section of the country to the other. Because the roof is flat, and must support snow loads as well as the ceiling, the built-up roofing, and whatever mechanical pipes and ducts are hung from it, the loads are likely to be greater than those used for an interior floor system or for a pitched roof. The absolute minimum design live load for a roof in a climate not exposed to snow, earthquake, or major winds is 20 pounds per square foot (psf). To this basic figure you must add the snow and wind loads for your area and the dead loads for the roof’s ply- wood sheathing and the membrane materials. If the live and dead loads for a flat roof in your area are under 40 psf, you may use Table A for the preliminary selection of the joists. Framing details for flat-roof construction are similar to those used in floor construction. Refer to the illustrations in Section 21 for framing details. When designing the roof, keep in mind that the roof must slope about 1/4” per 1’ to drain water. Roofing details and end conditions can be found here.

Roofing Insulation

It is likely that the roof will be insulated. Flat roofs can be insulated both under and over the deck surface. Traditionally, insulated batts, at least 8” thick and with a vapor barrier, are stapled

between the roof joists. A bit of an air space should be left between the batts and the deck to allow any seeped moisture to dissipate. Rigid insulation in the form of boards (usually composed of a sandwich of foam plastic, fiberglass, and other materials) may be installed above the roof deck with the roofing membrane applied on top of it. The boards must be rigid enough to hold the occasional weight of workers on the roof. A roofing concept rarely seen in small residential buildings places the rigid insulation (usually in the form of polystyrene foam board) above the roofing membrane. The light panels must be weighted down by a heavy layer of gravel to keep them from blowing away and to protect them from the sun’s decaying rays. This sometimes adds considerably to the weight of the roof.

Roofing Membranes

The roofing membrane is the waterproof skin that protects the inside of the house from water penetration. This skin, which must seal the house completely, has to be flexible so as to expand and contract with the thermal expansion and contraction of the other structural materials, and must be able to withstand the freezing effects of winter as well as the blistering heat of the sun. Most of the membranes described below aren't made to stand up to regular pedestrian traffic. If you want to use your roof as a deck, you will have to add a traffic deck above the membrane.

The traditional roofing membrane is a “built- up” roof consisting of multiple ( three to five) layers of asphalt-impregnated felt, with hot mop pings of coal-tar bitumen between the plies of felt. A five-ply built-up roof has a dead load of about 5 psf. Some designers add a layer of aggregate to the surface to protect the membrane from the sun.

Recent technological developments have produced new materials with the required characteristics listed above. Cold compounds of asphalt and other materials may be used in place of the hot bitumens in built-up roofing. These mastics are usually brushed on. Recently sheeting materials have been used and are known as single-ply roofing. These highly elastic membranes are made of polyvinyl chloride, synthetic rubber (neoprene or ethylene propylene diene monomer), and other sheeting. Modified bitumen sheets have seams that are joined by welding either with a blow torch or chemically. Some of these sheeting materials require aggregate protection and some don't .

If you want to use the roof as a play deck, you will have to install a decking material over the roofing, being very careful not to rip the membrane. One system used is an open-jointed paving block that's laid over the membrane. Water is allowed to fall between the blocks down to the drain. These blocks are particularly valuable if you must repair the roof at any future date, since they are easy to move and stack. The blocks add considerably to the dead load of the entire structure and their weight must be considered in any design calculations. Many roofing material companies will not guarantee their roofs unless their instructions for traffic protection are adhered to strictly.

Adding a Small Overhang or Porch Extension to a Flat Roof

Illustration 3 shows you how to frame an over hang for a new roof. If you want to add a cantilever overhang to an existing roof (say, to protect a door), you will have to cut back into the existing roof (see the section on framing a cantilever in Section 21). If it proves to be impractical to restructure the roof to provide for a cantilever, rethink your concept. It may be more economical to use the house to support one side of the extended roof, and columns to support the far side. For flashing details (to ensure that the intersection of the roofs is watertight).

Penetrating an Existing Flat Roof for a Skylight

A number of things must be taken into consideration when adding a skylight to a flat roof. The first is a structural consideration: how is the opening to be restructured? For the most part, an opening for a small skylight (let us say 2’ X 4’) should be no more complicated to frame than any opening in a floor system. The “hole” must be reinforced with doubled headers and trimmers as detailed in illustrations.

Cutting through the vapor barrier, insulation, and roofing membrane is a bit trickier. If the existing roofing is a built-up membrane, it can be cut through and the skylight added (use the kind that's self-flashing). Mop on additional layers of built-up roofing around the entire area. If the existing roofing is a single-ply membrane, consult the manufacturer before you even contemplate cutting through the membrane. Each material has its own adhesives and idiosyncrasies.

The vapor barrier and insulation should present no problem if they are the traditional between- the-joists kind. Just be sure that the vapor barrier is continuous and completely covers the insulation. For a roof with insulation boards above the structural deck, purchase a self-flashing skylight with a relatively high curb. Consult with both the manufacturer of the roofing system and the manufacturer of the skylight before making the installation.

PITCHED ROOFS

The pitched roof is ubiquitous in most parts of the country. Originally imported from Northern Europe to New England because of its ability to quickly shed rainwater and snow, the pitched roof can be seen in houses all over the United States. If your existing house has a pitched roof, it's likely that your new extension or new porch should have a roof with a similar slope, for stylistic if not purely technical reasons.

Technically, the angle of roof slope determines the amount of waterproofing required on the roof. The steeper the pitch, the quicker rain and snow are discarded, the less waterproofing is required. In addition, a steep roof can be designed for a smaller live load (mostly since snow loads will not accumulate). The slope of the roof can be analyzed by comparing the rise to the run.

If the run of the rafter is 10’ horizontally and the rise is 5’ vertically, then the slope is 1 to 2 or, expressed in inches, 6” to 12” (or about a 27° slope).

If you are planning a new extension to an existing house, the correct angle of slope for the roof should be determined more on the basis of aesthetics than on snow loads. If you want the extension to blend in with the existing house, it may be best to slope the new roofs at the same angle as the existing ones. You don’t, of course, have to make the new roof at the same pitch as the existing roof, as long as the new roof is proportionately in keeping with the existing roofline. It always helps to construct a model of the existing house and the extensions.

There are two different kinds of pitched roofs: one with an attic and one without. The difference between them involves more than the availability of storage space. The roof with an attic is structured very differently from one without an attic. If you are thinking about removing the attic in your house to create a cathedral ceiling effect, you may have to reconfigure the house structurally.

The Pitched Roof with an Attic

The pitched roof with an attic is usually constructed using lightweight structural members, 2” wide, to create a stable triangular structural element. The rafters support the weight of the roof and are supported in turn at the sills on either side of the house. The rafters lean in on one an other and in that way transfer their loads like two people leaning back to back. The central ridge holds the rafters in line but isn't really structural since it does not take on the weight of the rafters. This leaning-in arrangement creates a lateral (sideways) thrust on the walls pushing them out ward, which is counteracted by the ceiling joists that act as ties to hold the building together. If, during the course of renovating, you remove the ties, it's likely that the roof will collapse.

Designing a Light-Framed Pitched Roof with an Attic

To structure the roof, draw a plan and section of the roof and ceiling at ½” scale and the plan of the top floor of the extension (showing the double plates of the load-bearing walls and partitions). A section through the designed roof is drawn and its exact pitch is measured. This section helps determine the actual length of the piece of lumber to be used as a rafter (the span of the rafter plus the overhang). Ceiling joists are selected using Table. (This table is based on a live load of 30 psf, which is the design live load for an attic to be used for living. If the attic is to be used for light storage only, a design live load of 20 psf can be used.) The ceiling joists are usually framed with the same spacing as the rafters, to simplify framing and nailing.

The sizing of the rafters is dependent on the amount of annual snowfall and the pitch of the roof. The steeper the roof, the lower the design load. Tables B and C cover the design of roofs with pitches of over and under 3 in 12, respectively, on the assumption of a 20 psf and a 30 psf live load. Check with your building department to determine the snow live load for your area and the reduction coefficient for the amount of your roof pitch. Tables can be used as guide lines if the design loads conform to (or are less than) the ones provided for in our tables. The procedure for rafter selection is exactly the same as that outlined for joists in Section 21. In this case, however, the “span” is the horizontal projected length of the rafter between supports (that is, between the plates on one side and the ridge on the other.

The ridge itself is usually constructed of a 2”-thick section that's about 2” deeper than the rafter sections to simplify framing. Since it's non- structural, the span of the ridge isn't an important consideration. The roof is sheathed in ¾” plywood (exterior grade), which adds rigidity to the structure.

The triangular walls at the ends of the gable are constructed out of studs in much the same way as the wall sections outlined here. This attic space may be insulated in one of two ways. The space between the ceiling joists can be stuffed with insulation or, in the case of a heated attic, by insulating the spaces between the rafters and in between the studs of the end walls. It is important to install large louvers with powerful fans in the end walls of the attic to dissipate the heat that can build up under the roof in the summer.

*These tables are supplied for preliminary design purposes only and aren't to be used for final selection of rafters or ridge beams. Be advised that the lumber referred to in the tables is “stress-graded.” The lumber available to you in lumberyards isn't stress-graded. Therefore, when using the tables in this guide, assume that the lumber is no. 3 grade to get your preliminary sizing. Most municipalities adhere to strict building codes and use their own guidelines.

TABLE: JOISTS -- 30 PSF LIVE LOAD (to be used for preliminary design only)

Species | Grade | Joist size (inches) | Joist spacing (inches) | Maximum allowable span (feet-inches)

The Pitched Roof without an Attic

The pitched roof without an attic can be structured as a light-frame or (more likely) can be designed for plank-and-beam construction. (It is possible to mix the two systems. Often a house has a platform-frame floor system, a combination of stud walls and columns, and a plank-and-beam roof.) If you are using a light-frame roof, you will have to design tie beams or collar beams at intervals to counteract the lateral thrust. Plank- and -beam (also called post-and-beam) differs from light-frame construction in that it consists of structural members that are heavier than studs and joists and are spaced at greater intervals than the usual 16” on center. Generally the rafters are about 3” (or 4”) by 10” or more and are spaced 4’ to 6’ apart. The planks are about 2” thick and span between the rafters. Because the structural members are heavier than in light-frame construction, YOU will have to use timber connectors instead of nails.

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TABLE: RAFTERS, HIGH SLOPE (3 IN 12) WITH NO FINISHED CEILING—20 PSF LIVE LOAD, 7 PSF DEAD

Coast Sitka spruce - select structural

Douglas fir-larch - select structural (north)

no. 1 and appearance

Eastern hemlock - select structural tamarack (north)

no. 1 and appearance no. 2

Hem-fir (north) - select structural no. 1 and appearance no. 2 no. 3

Ponderosa pine - select structural no. 1 and appearance no. 2

Spruce-pine-fir - select structural

Western white pine - select structural no. 1 and appearance no. 2 no. 3 (to be used for preliminary design only)

Rafter sizes (inches)

Rafter spacing (inches)

Maximum allowable span (feet-inches)

Species Grade 2X4 2x6 2x8 2X10

Coast Sitka spruce - select structural no. 1 and appearance no. 2 no. 3

Douglas fir-larch (north)

Eastern hemlock- tamarack (north) - select structural appearance

Hem-fir (north) - select structural no. 1 and appearance no. 2 no. 3

Ponderosa pine - select structural no. 1 and appearance no. 2 no. 3

Red pine - select structural no. 1 and appearance no. 2 no. 3 - select structural no. 1 and appearance no. 2 no. 3

Western cedars (north) - select structural no. 1 and appearance no. 2 no. 3

Western white pine - select structural no. 1 and appearance no. 2 no. 3

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The major structural feature of the plank-and- beam roof is that the ridge is actually a beam and is designed to take on and transfer the loads from the rafters. The cross-sectional dimensions of the ridge beam, which must be calculated as with any other beam, are determined by its span and loading. If the ridge is designed as a beam, it bears the full weight of the load of the rafters. This load is transferred to the columns and there should be no roof-generated lateral thrust exerted on the walls. When designing the ridge beam, remember that it must be supported fully by columns or buttresses, either free-standing or buried in the walls. Even so, if the ridge beam is to be more than 15’ long (supported or unsupported), to ensure stability two or more collars are used to tie the roof back.

If you plan to construct a new extension to the house with a plank-and-beam roof, you may want to take advantage of the soaring effect of a cathedral ceiling. If you plan to nail a gypsum- board ceiling to the bottom of the rafters, you can insulate the space between planks and ceiling with batts of insulation. As it happens, you may wish to expose the handsome structural elements, the planks and rafters. If so, have rigid insulation installed above the planks. It should be noted that some architects consider the 1½” or more of wood in the thickness of the planks to be enough insulation. This may be adequate if your winters and summers aren't too intense, but be sure to consult the energy conservation codes.

Designing a Plank-and-Beam Roof

Draw a framing plan of the roof. Use a large scale such as 1 1/2” = 1’-0”. Make sure that the ends of the ridge beams are properly supported by free standing columns or columns buried in the end walls. Limit the span of the ridge beams to no more than 16’. Keep the “run” of the rafters (the horizontal distance between supports) short, under 14’. Also make sure that there are no uninterrupted, large expanses of glass under the load- bearing wall where the rafters come down. See the “Framing Plan” illustration.

Determine the slope of the roof by drawing a section through the plan. The “span” of the rafter can be determined by scaling the drawing or by using the right-triangle method of a 2 + b 2 = c 2 The span of the rafters (the actual length of the section between supports) should be under 18’. See the “Ridge Beam Sections for Plank and Beam Construction” illustration under Table.

Decide on the rafter spacing. Tongue-and- groove planks 2” thick are good for spans of up to 8’. We have designed our charts for either 6’ or 8’ spacings. If your room is 16’ long, you can have two spacings (one line of rafters) at 8’ apart. Determine the stress capabilities of the lumber you are using by consulting the chart in the instructions for the use of Table.

Grade:

  • Red pine
  • Coast Sitka spruce; select structural
  • Douglas fir-larch; select structural
  • Eastern hemlock-select structural
  • tamarack (north); no. 1 and appearance
  • Hem-fir (north); select structural no. 1 and appearance no. 2 no. 3
  • Ponderosa pine; select structural no. 1 and appearance no. 2 no. 3
  • select structural no. 1 and appearance no. 2 no. 3
  • Spruce-pine-fir; select structural no. 1 and appearance no. 2 no. 3
  • Western cedars; select structural
  • Western white pine: select structural no. 1 and appearance no. 2 no. 3

TABLE: ROOF BEAMS (RAFTERS) FOR PLANK-AND-BEAM CONSTRUCTION (to be used for preliminary design only)

For L.L. of 20 psf or less

For L.L. of 20—30 psf Stress grade

The above table is designed to simplify the selection of beams or rafters to be used in flat-roof framing (post-and-beam) or for pitched-beam roofs (as rafters).

SPAN : Considered to be from support to support. If the roof is flat, it's the horizontal distance between supports. If the roof is pitched, it's the actual length of the section between supports.

SPACING: Either 6’-0” or 8’-0” is the dimension between the beams (rafters). It is the span of the planks.

L. L.: Either 20 psf or less, or between 20 and 30

psf should cover most code requirements.

The fiber stress in bending for the particular wood being designed for.

2/3 X 6 indicates a built-up section using two

3 X 6 sections.

Douglas fir no. 1 select

no. 2 construction

no. 3 standard

no. 1 select structural

prime structural

common structural

utility structural f = 950

Deflections were not considered in compiling this table. Since there are hardly any stiff wall-finishing materials (for example, plaster) in common use today, deflections of only fractions of an inch would not be harmful to the building or its inhabitants.

Determine the live load requirements by checking the building code for your locality. Table D is based on L.L. of 20—30 psf, which should satisfy the requirements for most areas of the country. (Areas in the northern regions of the United States may require a greater live load as a reflection of the area’s greater snow accumulation. Verify.)

With the input from all of the steps above, use Table D to determine the size of the rafters.

Use Table for the preliminary selection of a ridge beam (if your live load isn't more than 30 psf). This table has been greatly simplified for the purposes of this guide and is keyed to Table.

TABLE: RIDGE-BEAM SECTIONS (to be used for preliminary design only)

FOR PLANK-AND-BEAM CONSTRUCTION

For ridge spans of either 12’ or 16’ | For L.L. of 20 psf

Ridge span = 12’ | Ridge span = 16’

Span of rafter

The simplification of the charts and tables makes the design procedure appear to be less complex than it actually is. Once again, since the structural design of the house is so important to the future safety of its occupants, you must consult with an architect or structural engineer before constructing a roof. In most areas it's required by law. If your spacings, length of ridge beam, quality of lumber, or snow loads don’t conform to our tables, you will not be able to use them even for preliminary design purposes.

Roofing shingles, insulation, condensation control, and flashing are covered in here.

[Designing or Modifying the Roof]

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