Constructing a New Roof or Modifying an Existing One

This section is important for anyone who is doing any construction that involves the roof. Of course, if you are anticipating an addition to the house, you will want to study this section on roof construction. Even if you are only adding a sky light, you should be very familiar with the structural configuration of the roof so that you don’t accidentally dismantle it. In this section we will be discussing only pitched roofs; the structure of the flat roof is very similar to that of the floor system, and a re-reading of this section will be helpful. Please note that roof construction can be very dangerous; if you haven't done this kind of work before, call in a contractor.

Tools: Carpentry and demolition tools


• Ceiling joists: If your design requires them, quantity and dimensions can be taken from the framing plan.

• Rafter stock: To determine the length of the rafter stock, you can make a large-scale drawing of the rafter and its supports. Or you can use the formula provided in the section “Preparing the Rafter Stock for Cutting” or 3. Add to this figure the length of the overhang and about another 9” for cutting.

For the number of rafters required, multiply the length of the roof (the ridge length) x 3/4 (for 16” spacing; for 24” spacing use ‘/2) and add 1 for the end rafter. This gives you the number of rafters for one side of the roof; for the total number of rafters, double the figure. Always add 10 or 15 % for waste.

• Plywood sheathing: Thicknesses varying from 1/2” (for 12” or 16” spacing) to 3/4” (for 24” spacing). For the number of panels required, multiply 2 x (the length of the rafter + overhang) x the length of the ridge. In the case of plywood roof sheathing, you need order only about 5 % over the calculated amount required, since there is little waste.

• Ridge stock: 2” thick. For the depth of the ridge, add 2” to the depth of the rafters as determined by the design. The length of the ridge is equal to the length of the building + the gable overhang.

Nails: 3d, 4d, 5d, 10d

Roofing underlayment paper

• Asphalt strip shingles: 3 bundles per 100 square feet

• Metal roof flashing, as required

• Flexible roof flashing and cement, if required


The pitched roof and the shed roof can be constructed using light-frame or plank-and-beam construction. Generally pitched roofs have two sloping sides and require tie beams to hold the roof together (unless a ridge beam is used), whereas the shed roof may not need ties. The plank-and-beam roof is composed of heavier sections (spaced farther apart)’ and may require a crane for construction. Another major difference between light - frame and plank-and-beam construction is that the latter uses metal connectors rather than nails.

Laying Out the Rafters and Ceiling


Mark the top plates of the walls to receive the rafters using the method outlined in Section 29 on constructing the floors. If your design calls for attic ceiling joists, mark the plates for both the joists and the rafters. When laying out the rafters and joists, make sure that you have exactly the same markings on the opposite walls unless there is an intermediary support (such as a partition) between the two exterior walls. In that case, plan to use overlapping or butt joints for the ceiling joists, and mark the plates accordingly. Position the ceiling joists (if you have them as part of your layout) and toe-nail them to the plates using 10d nails.

Preparing the Rafter Stock for Cutting

To lay out the rafters you will have to know the slope of the roof (that is, the unit rise to the run) expressed as a ratio (for example, 5:12, 8:12) and the line length of the rafter expressed in feet and inches.

The ratio of the slope of the roof can be calculated by the method outlined in Section 23. The line length of the rafter can be determined by making a large-scale drawing of the assembly and measuring off the drawing, or applying the formula below.

It is a good practice to cut a few pieces of rafter, test them on the roof, adjust them if needed, and then cut all the remaining rafters, using the first as a pattern. On a framing square, measure (and mark with tape) the rise of the roof on the tongue (short end) and the run on the blade (long end). The line length of the rafter is the distance between the bird’s-mouth cut (which is the notch that “sits” the rafter onto the plates) and the cut for the ridge.

Choose a very straight and clean piece of rafter stock. Try to find a piece that does not have a camber, but if it does, position it so that the crown of the camber will be the top of the rafter. Position the framing square (as directed on point 1 and draw the ridge cut line. From point 1, along the top of the ridge, measure the line length of the rafter (this gives you point 2. Position the tongue mark (in this case 5) on point 2, and move the square until the 12 mark on the blade rests on the top of the rafter stock in the position. Draw a line down along the outside of the tongue, from point 2; this will be the vertical cut line for the bird’s mouth. Invert the square. Lay it as shown in fig. 8 and measure off 5½” for 2 X 6 exterior stud walls (or 3 1/2” for 2 x 4 walls). This is the final cut of the bird’s mouth, representing the width (5 1/2” or 3 1/2”) of the plates. Measure off the overhang as designed and draw the line for the overhang cut. Prepare five rafters, using the first as a pattern.

Preparing the Ridge

The ridge, as we have learned, serves little structural purpose, yet it's important that the pieces selected for the ridge be as straight as possible. The rafters that will butt against it are cut exactly to fit, and too much of a warp or camber in the ridge will throw off the assembly.

Set the ridge sections alongside the plates and lay out the rafter spacing on the ridge by transfer ring the marking directly from the plates. If you can't find a single length for the ridge, lay out the ridge pieces so that they will be joined to each other at the point of intersection with two rafters. Next, move the ridge section to the center of the building and rest it across the ceiling joists (or on a temporary platform supported by scaffolding) directly below where it will be when in its final position.

Raising the Rafters and Ridge

Cut five pieces of rafter stock (and not more until these have been tested for accuracy). The first four pieces will serve as testers and will be nailed temporarily in place to the ridge section. Al though it's possible to raise the roof by yourself using bracing and scaffolding, we don’t recommend it. It is much better practice to work with one or two helpers. Beginning at one gable end, temporarily nail a rafter to the plate while an assistant holds up the other end at the approximate future height of the ridge. Nail the second rafter to the opposite plate while the assistant holds the two rafters in place. Lift one end of the ridge section, and while a second helper supports the other end of the ridge piece, temporarily nail the rafters to it. Move some scaffolding to support the free end of the ridge temporarily. Move down about six rafter spacings and nail the two remaining rafters temporarily in place. The assembly should now be plumbed and braced, making sure that the ridge is perfectly level. To check the assembly for accuracy, measure the distance from the top of the ridge section to the top of the ceiling joists. If this conforms to the design height, and all else looks squarely cut and neat, you can proceed further.

Permanently nail the section already constructed and cut the rest of the rafters using the fifth rafter as a pattern for the rest. Nail the intervening rafters in place following the above procedure of first toe-nailing the rafter to the plate and then to the ridge section. Alternate the installation of the rafters so that only two or three rafters are nailed into place on one side of the ridge before you move to the other side and install the other half of the rafter pairs. Brace the assembly together by nailing a 2 X 4 as bracing to the top of the rafters. Since you will be climbing over the roof, bracing the framing isn't meaning less work. Nail the rafters securely to their adjoining ceiling joists. When the roof is complete, drop a plumb line at either gable end to make sure that the end gables are straight.

Use a straightedge to make sure that all of the rafters lie on an even plane. Use a taut line to check that the rafter ends (at the exterior walls) are in a straight line. If there is to be a fascia piece at the end of the rafters, plan for it.

Completing the Gable Wall Section

Lay out the stud spacing at the gable ends at 16” o.c. If there is to be a window or vent opening, lay out the trimmers as well, following the directions outlined in Section 30. Hold a piece of stud stock at the first stud space and mark the angle at which it intersects the end rafter. Repeat at each stud spacing. Cut the studs and nail them in place. (Care should be taken not to force the gable-end studs into place. The pressure might distort the end rafter.) Not all of the sections need to be custom-fitted as described. The distance between the first and second studs will be standard. If the second stud is 9” longer than the first, then the third stud will be 9” longer than the second, and so forth. The studs can then be laid out on the ground and cut in pairs. The window or vent opening is framed similarly to a conventional opening, as described in Section 30.

Framing Openings in a New Roof

Roof openings may be framed by measuring and cutting the rafters before installation in a way similar to the framing of openings in a floor. Before framing any openings, be sure that you drop a plumb line down to the house below to make sure that the hole you are framing is directly above the item (for example, the chimney) you are framing.

Erecting the Roof Sheathing

The plywood sheathing, which is installed with its long side perpendicular to the run of the rafters, provides a nailing surface for the roofing. Start applying the plywood sheathing on the lower part of the roof and work your way up to the ridge, cutting the boards where needed. The panels should be applied in an alternating pattern as shown in 14. As a safety precaution, scaffolding should be erected at the periphery of the house to facilitate the nailing of the lower row of sheathing.


Demolishing All or Part of the Roof

Roof demolition, like the demolition of a load- bearing wall, is potentially very dangerous. First, if you don’t do it correctly, the structure may collapse around you. In general, if you are going to remove any part of a pitched roof, read Section 23, on roof structure, first. Remember that most rafters are leaning against one another. If you cripple the rafters on half of a pitched roof with out providing for support, it's likely that the whole roof will cave in. If you are planning to cut into your roof, have an expert design the opening, plan the temporary shoring, and outline a quick demolition and reconstruction procedure.

Second, unless you have a very large crew of experienced demolishers (unlikely), the work will proceed very slowly. A flash rainstorm can cause a great deal of damage to your unprotected house. For these reasons we strongly advise you to hire a contractor to do the necessary work. If you are making a relatively small opening in the roof, however, you may be able to handle it yourself providing you know (in advance) what you are going to do, you work quickly, and you wrap the work in plastic sheets between workdays.

Cutting into a Sloped Roof for a Skylight or Chimney Opening

Many skylights come with installation instructions. Purchase a unit that has its own curb and has self-flashing. Be sure to have the unit on hand before disturbing the roof. Plan to position the skylight so that the fewest possible rafters will be cut. Buy long, narrow skylights and install the long side parallel to the rafters. If the skylight or chimney is less than 30” wide, you will have to cut into only one rafter.

First expose the structure on the underside of the roof. Strip off all of the roofing, flashing, insulation, and interior finish but don't yet remove anything structural. If there are electrical wires, drainage pipes, or any other services in the roof, remove or relocate them.

Double the rafters on either side of the future opening and temporarily shore up the rafter that will be cut both above and below the cut marks. Draw the outline of the exact size of the rough opening required on the underside of the sheathing and drill some critical holes through the roof to locate the opening on the upper side. Remove roofing material along the cut lines of the opening to a width of about a foot outside of the opening. If you have asphalt strip shingles in good condition, carefully remove the strips and save for reuse, so that the patching will match the existing roof. From the underside, cut and remove the rafter section(s). Install doubled 2 X 8 headers (if the opening is up to 48” wide) to the top and bottom sections of the crippled rafters and secure to both the crippled rafters and the doubled ones at the ends. If the opening is narrower than the rafter spacing, you may have to install some additional trimmers to fill in. Cut through the sheathing and continue to install the skylight and its flashing.

In addition to the general demolition and construction precautions listed in the introduction to Part Four, we include these:

• Don’t work on the roof on very windy days, as the materials and equipment have a tendency to blow off.

• Don’t store anything, even temporarily, on the roof. We know of an electrician who stepped on an un secured piece of plywood. He and the plywood fell from the roof together.

• Don’t work on the roof on extremely hot or cold days, as you may end up destroying the existing roof, which may be either soft or brittle.

• Use a very secure ladder, wear skid-resistant shoes, and be very careful.

• Since no two roofs are the same, you should consult with an engineer or architect before cutting into any structural members.

The plywood sheathing is by no means water tight, and if you aren't adding your roofing material immediately, you should protect the roof until it can be finished.

Installing the Waterproofing Underlayment

There are a variety of underlayments sold to cover the sheathing before the installation of the strip shingles. These products provide an added layer of moisture protection against wind-driven rain. The most commonly available is an asphalt-saturated felt. Use a material that will allow vapor to pass through to prevent the accumulation of condensate between sheathing and underlayment. Fifteen-pound saturated felt paper is considered a good choice in buildings with well-ventilated at tics. Plastic sheeting is a bad choice.

Prepare the sheathing to receive underlayment by making sure that there are no nailheads or splinters projecting, the sheathing is dry, clean, and free of dirt, and any knotholes are repaired by nailing metal flashing material over them. Underlayment material consists of asphalt-saturated felt or a similar material, which comes in rolls 36” wide and 72” or 144” long. The paper is laid in long horizontal rows beginning with one layer parallel to the edge of the roof and working the layers up to the ridge. The horizontal joints should overlap about 2”. Two people are usually required for the job. The underlayment is stapled to the sheathing every 12” and along the edges of the roof. Lap the felt over the ridge. A drip edge (purchased prefabricated) should be in stalled under the felt at the bottom edge of the roof and over the felt at the side edge. Install all flashing as directed.

Installing Asphalt Strip Shingles

Asphalt strip shingles are very easy and quick to install. The most common shingle is the square butt strip that's 36” long and 12” wide and has three “tabs” (phony shingles) with two cutouts between the tabs. The strips are laid up so that 5” is left exposed “to the weather.” The strips are sold twenty-seven to a bundle. Three bundles will cover 100 square feet of roof.

*Steep roofs finished with wood shingles often don't require felt at all because of the tendency of the shingles to quickly shed water. In climates with regular snowfalls the lower 5’ of roof should, however, receive the protection of some water proof underlayment to prevent what is called the ice dam effect. The ice dam is created by an accumulation of melting snow at the periphery of the roof a few days after a snowfall. The snow slides to the edge of the roof and melts. The temperatures fall and the melting snow turns to ice. If more snow should fall, this ice dam will prevent the new accumulation from sliding off the roof. When this second layer begins to melt, the water will be backed up behind the dam and may make its way through the shingles to the sheathing.

The Construction Process: ROOFING

Asphalt shingles require a full underlayment of saturated felt. Drip edges of 26-gauge galvanized steel are applied at the eaves and at the rake. Install the flashing that can be applied before the roofing. Some flashing details require the alternate application of roofing and flashing. The strips are applied in long horizontal lines. Chalk lines are snapped to keep the rows straight and parallel. (Begin the work by snapping one line for each course. As you get more proficient you will need only one line for every three courses.) Use 1 1/4” galvanized nails with barbed shanks and large heads. The shingle manufacturer will most likely recommend the type of nails that will best do the job. Nail according to the nailing pattern in 18 or the manufacturer’s instructions, if available. Cut strips with a linoleum knife.

To begin, start at the eaves somewhere in the center between the gable ends and work outward in horizontal lines. Lay a starter strip, which is a layer of strip shingles applied upside down, or a special strip sold for this purpose. The starter strip in effect closes the gaps made by the cutouts at the eaves. Lay this starter strip so that it barely covers the drip edge. Follow one of the patterns. Check often to make sure that your rows are straight and even. At the ridge use individual 10”-wide shingles, folded over the ridge and laid one over the other so that the nailing is covered.


The system that leads water from the roof to the ground is composed of gutters and downspouts. The gutters are trough-like canals that are attached to the eaves of the roof to channel the rainwater to downspouts, which take the water to the ground. The excess water might be deposited directly onto the grass or some paved area or might be taken down into a dry well.

Gutters are made of either wood (which should be lined), galvanized metal, copper, plastic, or aluminum and come prefabricated and shaped. Wood gutters are installed after the roof sheathing is applied and before the roof is shingled. The gutter is nailed directly to the fascia using galvanized nails. Be sure that the wood gutter is primed before it's erected. Roof gutters are set so that there is an incline toward the downspouts. The downspouts, usually made of metal, are connected to the gutters at some point where they will be least obvious. A hole is made in the gutter at the point of intersection and the sheet-metal sleeve (for the downspout) is fitted inside. Galvanized-iron and aluminum gutters and down- spouts are designed so that the component parts fit easily together.

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Since flashing material is applied to a building at different points in the construction process, we must discuss it as a separate issue. Flashing is an extremely important aspect of the roofing. Flashing material, which can be metal (such as copper or aluminum) or a flexible fabric, is placed between the intersection of two different materials, at the juxtaposition of two planes, or at any crucial area where moisture penetration may occur.

FLASHING AT THE RIDGE: Flashing at the ridge can be exposed or concealed. We recommend that you use the concealed flashing for aesthetic reasons. Use 16-ounce copper, .019” aluminum, 26-gauge stainless-steel, or 24-gauge galvanized-steel material. Use 10” lengths of flashing material and lap 4”. Shingles are applied up to the point where they reach the ridge. The flashing is applied and nailed over the shingles into the sheathing. Special ridge shingles are installed in an alternating pattern.

FLASHING THE EDGE OF THE ROOF: The edge of the roof, over the cornice, must be protected from a driving rain. The material to be used is the same as that used for ridge flashing. Details are shown in 1-2. If a gutter is to be installed.

FLASHING AROUND THE CHIMNEY: Chimney flashing is particularly difficult because the chimney and the house frame are structured to be discontinuous. There is likely to be some settling of the structural frame of the house, which will create gaps between the chimney and the roof. Therefore, two overlapping layers of flashing are used to pre vent the penetration of water while allowing for the independence of chimney from roof. One is attached to the roof itself, and the other is integrated into the construction of the chimney. The overlapping of the layers allows for some movement.

Before any flashing material is applied, the shingles are laid up the roof until they reach the bottom of the chimney opening. The base flashing, which is 90-pound mineral-surfaced roofing paper, is cut for the bottom section. The flashing should extend about 10” up the chimney and 10” along the roof. Cut the four pieces of roofing and cement securely to the roof as well as to each other. The flashing must be applied in the following order so that no rain can penetrate as water travels down the roof: bottom, sides, then top.

The flashing that's attached to the chimney is cap flashing, made up of metal flashing material that's set into the joints between the masonry when the bricks are being laid. The first of the pieces applied is the lower section. It is cut the width of the chimney plus enough more to wrap around the corners. The material is set 1/2” into the mortar joint and overlaps the base flashing. The side pieces must be stepped up the chimney in shorter pieces to conform to the slope of the roof. The back of the chimney presents a special problem because it provides a pocket for water and snow accumulation. For this reason, a small cricket or gable is built to keep snow and water from collecting. It is covered with flashing and carried under the shingles and cap flashing.

FLASHING AROUND A VENT: Use one piece of sleeve flashing to cover the vent pipe. The sleeve should lap 2” over the vent and flare a minimum of 6” at the base. Use cleats to hold the flashing sleeve in place.

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