Making Exterior Changes: Building a Dormer

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Building a dormer can convert a small, dark attic into a bright and spacious living area. Adding one or more windows improves light and ventilation. The usable floor space is also significantly increased. Depending on the exact size and shape of the dormer, the increase may be as much as 30 to 40 percent. If you compare the cost to a complete addition, a new dormer is an appealing project indeed.

Planning the Project

But adding a dormer also demands careful planning and skillful carpentry. In many instances you would be wise to seek professional assistance. If you have relatively little building experience, think seriously about hiring a contractor for at least part of the job. Perhaps you can work alongside and learn as you go. Or have the contractor handle the more difficult aspects such as cut ting into the roof and framing the shell. Then finish the job yourself: employing many of the techniques de scribed in other sections of this guide.

There are also compelling reasons for consulting a designer at the outset of the project. First the dormer size should be planned carefully to maximize the available living space. The optimum size reflects a delicate balance between your needs and your budget. A designer can help you get the most for your improvement dollars.

Second since the addition will dramatically alter the exterior appearance of the house, good design is essential. The dormer should match the style of the existing architecture. Generally the same type of windows, siding, and roofing should be used, and any details such as overhangs and fascia should be repeated. Most important, the dormer should blend smoothly with the rest of the house, complementing the overall design. In fact, the dormer shouldn’t appear to be an addition at all, but part of the original construction. If the proportion or location is wrong, the dormer will always look tacked on.

Dormers take two basic shapes: gable and shed. The gable dormer is generally smaller, adding light but not much floor space. It’s also more difficult to build since it requires joining two sloping surfaces into the existing roof line. Often two or more gable dormers are necessary to balance the exterior proportions of the house.

The shed dormer has a single sloping roof line that connects with the house at the ridge beam or farther down the slope of the roof. The front of the dormer can rest directly over the exterior wall below, or be set back several feet. The latter design generally results in a more attractive exterior appearance.

The advantages of the shed dormer are twofold: it’s easier to build, and it adds more usable living space with maximum headroom. This section shows how to frame and enclose a shed dormer that’s set back from the ridge and eave. The framing is basically the same for a ridge-to-eave dormer; only the details in connecting to the house are different.

To plan the new dormer, begin by determining the slope of the existing roof. To do this you must be familiar with some basic roofing terminology. The span is the width of the house and the run is half the width. The rise is the vertical distance between the cap plate and the ridge beam. The slope expresses the ratio between the rise and the run. For example, if the rise of your roof is 10 feet and the run is 20 feet, the ratio is 10 to 20, but the slope is expressed as 6 in 12, which means the roof rises o inches for 12 inches of run.

You can use either of two techniques to find the slope of the roof. The first is to measure the actual rise and span of the house. The second and easier method is to mark off 12 inches along the top of a carpenter’s level. Position one end of the level against a rafter; then measure the rise at the 12-inch mark. For example, if the rise is 8 inches, the slope is 8 in 12. Choose a straight rafter.

Next use the span and the slope to draw an accurate end view of the roof and attic on ¼-inch graph paper. If the attic is unfinished, determine the desirable ceiling height. Sketch the approximate shape of the dormer along the side elevation of the house. For a flat ceiling, plan to add ceiling joists. For a sloping ceiling, the slope of the dormer rafters deter mines the slope of the ceiling.

Besides considerations of exterior appearance and usable interior space, the possible size and shape of the new dormer are affected by a third factor—the roofing material. Check the building code for the minimum slope allowed for various types of roofing. These requirements vary depending on the climate, especially in locations with heavy rain or snow loads. As a rule the minimum slope for wood shingles and shakes is 4 in 12. For asphalt shingles the slope must be at least 3 in 12, although some codes allow a lower slope if the shingles are self-sealing and the underlayment of building pa per is doubled. For roll roofing the slope can be as low as 1 in 12. If the dormer is to the rear of the house, you may decide that roll roofing, although generally unattractive, is satisfactory. If it’s high enough, the roof may not be visible from the ground. If the dormer faces the street, however, you should match the roofing on the rest of the house.


Gable Dormer; Shed Dormer

Installing the Dormer

Once you’ve worked out a satisfactory plan, you’re ready to begin the actual construction of your new dormer. If the attic is unfinished, you will probably need to reinforce the floor system by adding new joists between the old. Then nail down a 5 ply wood subfloor to provide a sound work surface for the rest of the job.


Planning a Shed Dormer: Method I of finding the roof slope: Multiply the rise by 12 and divide by the run (e.g., 9 1/3 x 12 ÷ 14 = 8). The slope is 8 in 12. Method 2: Mark off 12” on a carpenter’s level, Hold one end against the rafter and measure vertically at the mark to find the slope. Double header; Double top plate; Sill; Cut roof rafters; Subfloor;

Begin from inside the attic by doubling the rafters on either side of the proposed opening. The new rafters should be the same size as the old and extend from the ridge board to the cap plate. Use a bevel gauge to mark the angles at both ends. Nail to the existing rafters with 10d nails staggered every 12 inches. Drive three 10d nails through the ridge board into the ends of the new rafters,

Mark the opening along the roof sheathing. At the four corners, drill guide holes or drive 10d nails through the sheathing and roofing. From the outside, using the protruding nails, snap chalk lines to outline the opening. Remove the roofing material following the procedures shown above, going beyond the chalk lines 10 to 12 inches. If the roofing is in good condition, salvage as much as you can. It can be reused on the dormer for a perfect match. Otherwise rip it off and remove it from the roof so you don’t slip on loose pieces.

Once the roofing material is removed, cut away the building paper with a utility knife. Snap new chalk lines. Set a circular saw or reciprocating saw to the depth of the sheathing and cut along the lines, Pry the pieces carefully from the rafters with a wrecking bar or flat bar. The sheathing can also be reused.

For a set back dormer you should brace the rafters before they are cut. (If there are kneewalls already in place, the lower bracing isn’t necessary.) Nail 2 by 4s to the bottom of the rafters, just above and below the opening. Nail 2-by-4 sole plates to the subfloor; then wedge studs between the two plates under every rafter.

Use a bevel gauge or level to mark the rafters for a plumb cut. Have a helper support the rafter as you cut both ends with a handsaw or reciprocating saw. Remove the cut portions and set them on the subfloor. (Don’t drop them, or the ceiling surface downstairs may be jarred loose.) Nail a 3-inch joist hanger to the double rafters at each corner of the opening. Cut a header the same size as the rafters and place in the hanger. Nail through the header into the ends of each cut rafter with two or three 10d nails. Install the second header and face nail to the first with 10d nails every 12 inches in a staggered pattern. Install the double headers at the bottom of the opening in the same way With this done you can remove the temporary bracing.

Frame the front wall for the dormer on the floor of the attic, following these procedures. To frame the window opening, see above. The length of the wall is the distance between the double rafters. But in this instance make the top plate long enough to ex tend beyond the end studs 3 1/2 inches on each side. Stand the framework in place. Brace temporarily with a diagonal 2 by 4 tacked to a block nailed to the subfloor. Alter plumbing the wall, nail through the sole plate into the joists. Toenail the studs to the header and face nail the end studs to the double rafters.

Build corner posts for the wall with two 2 by 4s and scraps of 3 plywood in between, The height of the posts extends from the roof sheathing at the corner of the opening to the top plate. Cut the bottom of the posts at the angle of the roofline. (Use the bevel gauge or stand the post against the rafter and mark the angle with a pencil.) Plumb the posts and toenail through the sheathing into the rafters below. Then nail the end studs of the front wall and the top plate to the posts. Nail a cap plate over the length of the wall.

You’re now ready to fit and install the rafters. These should be the same size and spacing as the house rafters. (Use the cut rafters if they’re long enough. If you use new lumber, be sure to place the crown, or high side, up.) Mark the correct spacing along the cap plate and header.

To mark the plumb cut at the end of the rafters, either of the techniques can be used. If you know the exact slope of the dormer roof, use the framing square to mark off the rise on the tongue and the run on the blade. Drawing a pencil along the edge of the tongue gives you the cutting angle. If this isn’t possible, have a helper hold the end of the rafter so the bottom edge aligns with the inner edge of the cap plate. At the other end hold a piece of scrap wood along the top of the rafter until it connects with the header. Position a level against the rafter next to the header; then mark along the side with a pencil. Cut the rafter and check it for fit.

If the plan calls for the rafters to overhang the front wall of the dormer, reposition the rafter as before and mark the outer end for a birds mouth —a notch cut into the rafter to provide a level surface for the rafter to rest or bear on a wall. If there’s no overhang, mark the rafter for a heel cut along the bottom and plumb cut at the front edge. Once the first rafter is properly cut, use it as a template for cutting all the other inside rafters.

The outside rafters will be placed over the double rafters instead of against the header. Thus the end angle is different. To determine this angle, place one end of the trimmer rafter alongside the double rafters, with the up per end against the header and the lower end on the cap plate. Then mark the side of the rafter by drawing a pencil along the edge of the sheathing. Transfer this angle and make the cut.

To install the rafters, secure their top ends to the header with joist hangers. Toenail their lower ends to the cap plate or use framing anchors. For the outer rafters, drive 10d nails through the bottom edge into the double rafters.

With the rafters in place, frame the gable ends of the shed. Mark a sole plate to fit between the corner post and the outside rafter. Bevel the ends for a snug fit. Nail through the sheathing into the double rafters with 16d nails. Mark stud locations along the sole plate, 16 inches on center. Hold each 2-by-4 stud vertically in its place with a level and mark where it touches the rafter. The top of the stud can be notched to accept the outside rafter or cut at an angle.

If the plan calls for ceiling joists, install these as shown here. Sheath the exterior walls as shown below. You can sheath the roof with salvaged materials or use new 1/2-inch sheathing grade plywood or 1-by-4 boards. Apply new shingles, being sure to flash the seams between roof and sidewall or roof and chimney with step flashing.

At this point the exterior of the dormer is ready for finishing with roofing and siding. (Information on selecting and installing various types of roofing and siding is covered in our guide Basic Carpentry Techniques.) To complete the interior framing of the attic, see this page.


Building a Dormer

Drill holes or drive large nails through the four corners of the proposed dormer to mark the corners on the roof. Install reinforcing on each side of the proposed dormer. Install a plywood subfloor if the attic is unfinished. Reinforcing rafter; Face nail rafter to the ridge beam. Ridge beam. Toenail into cap plate.


Transfer angle of header to rafters with a bevel square. Cut the sheathing on the marks and pry it off. Find the outside and snap chalk lines from hole to hole. Transfer angles of headers from your drawing to the rafters with a bevel square. Rafter of proposed dormer.


Build temporary 2-by-4 bracing to support the rafters when you cut them. Distance between double rafters; Build a stud wall for the front of the dormer. Rough opening for window; Double top plate extends 3½ beyond the width of the opening. End cut to match roof angle.
Nail joist hangers to the double rafters at each corner of the opening. Set one header in the hangers and nail in place. Nail a duplicate header over the first to make a double header.


Mark the plumb line on the end of the rafter with a framing square, or align the bottom edge of the rafter with the inside edge of the cap plate and use scrap wood to align the top of the rafter with the header. Mark the angle end of the rafter (dashed line) and transfer it to the corner of the rafter with a level square (solid line). Marked angle; Double rafter;

Tail cut — same angle plumb cut; Front stud wall; Mark and cut the birdsmouth and tail cut, using the same angles and method as for marking and cuffing the plumb cut. Install the rafters, using the first as a model. Fit and install gable and studs between the sole plate and end rafter. Notch the top of each stud to fit around the rafter. Fit and install a sole plate between the corner posts and the end rafter.

Thursday, September 2, 2010 2:24 PST