A gabled roof looks great from the outside and adds space and light inside.
Time Multiple Days
ComplexityAdvanced
Cost$501-1000
Introduction
Looking for a simple expansion that'll give you more space in your home? A new dormer in a cramped, underused attic might just be the ticket. It'll create the additional headroom you need to make a comfortable living space--an extra bedroom, studio or playroom. And a dormer window provides natural light, fresh air and perhaps some dramatic treetop views. Dormers look great from the outside, too, adding variety to a plain front and improving your home's curb appeal, not to mention the resale value. We'll show you the major steps to follow when building a dormer, including how to cut and reframe your existing roof, assemble the dormer walls and frame your new roof.Tools Required
- 2-ft. and 4-ft. levels
- Circular saw
- Drill/driver - cordless
- Extension ladder
- Framing square
- Hammer
- Reciprocating saw
- Safety glasses
- Stapler
- Tape measure
- Tool belt
Is Framing a Gable Dormer for Me?
Adding a gabled dormer roof creates additional space and lets the sunlight shine in.
Building a roof dormer is an ambitious and challenging project. You should have a few remodeling projects under your belt before tackling a task this big and complex. If you’re a little unsure of your rough-framing or roofing skills, hire a carpenter to help with this phase of the roof dormer project. The gable dormers we built are 6 feet wide, about 9 feet from the floor to the peak, and set back from the front wall of the house by about 2 feet. Each dormer adds about 30 sq. ft. of headroom.
Projects this large and complex are expensive, especially when adding elements like windows. Plan on a big-time commitment as well. Set aside at least four days for the basic framing. Once the roof’s open, you’ve got to keep at it until the dormer is weathertight. Then, you can relax a little as you finish the exterior siding and trim. Plan on spending seven or eight more weekends finishing the interior and completing the exterior painting.
Building a dormer requires a tool belt loaded with all the basic carpentry tools, plus a few extras—a circular saw, reciprocating saw, framing square and 2- and 4-ft. levels. You’ll also need good equipment for working up high: an extension ladder, roofing brackets and scaffolding. We also recommend you rent or buy a personal fall arrest system consisting of a full-body harness, lanyard, rope, roof anchor and all the hardware and instructions.
Project step-by-step (19)
Step 1
Roof Safety
Working on roofs is dangerous. Before beginning this project, review this tips to minimize your risk:
- Rent or buy a personal fall arrest system. Purchase for $250 or rent for about $35 a day. (Rentals can be difficult to find.)
- To provide solid footing, use metal roofing brackets and 2×10 planks. Follow the installation guidelines included with the brackets (available at any large home center).
- Sawdust and debris make the roof slippery, so sweep it off often. Wear flexible, soft-soled shoes to increase traction.
- Never work on a wet roof.
Step 2
Work out dormer details on paper
Figure A: Dormer Anatomy
The size of the new triple rafters and common rafters will vary with the size of the roof and dormer and local conditions. Have an architect or structural engineer size them.
Like any other major remodeling project, your dormer project will progress more smoothly if you work out the details on paper or a full-size layout on your garage floor (Figure B), before beginning construction. Dormers are complex, and not every house is suitable. To make sure the dormers work, we recommend hiring an architect who specializes in residential construction or remodeling. The architect will help you consider issues such as roof slope, interior headroom, exterior appearance, structural strength, roof condition and cost. If the dormer is feasible, the architect will draw up plans that include all the dimensions and special structural details. Use the plans to obtain a building permit from your local building inspections department.
After the building department has approved the plans, take them to a full-service lumberyard. A salesperson will assemble a materials list and cost estimate and help you order windows and special items. Before you cut the hole in the old roof, have all of your materials delivered and rent a trash container or trailer for the debris. Buy a mesh-reinforced plastic tarp large enough to cover the entire dormer area. Stretch one edge of the tarp so it laps over the roof ridge and secure it with 2x4s screwed into the roof sheathing. Roll the tarp back, and it will be ready to pull over the roof dormer at night or in the event of a surprise rainstorm.
Step 3
Remove shingles and sheathing board
Remove the shingles from the area of the new dormer. Snap chalk lines for the inside edge of the dormer side walls, set your saw to cut through the roof sheathing and saw from bottom to top on both sides. Leave the overhang sheathing uncut. Pry off the old sheathing boards.
Step 4
Make sure everything is plumb
Using dimensions from your plan, draw lines on the floor to indicate the location of the front dormer wall. Plumb up from the outside face of the wall line and mark the rafters. Deduct an additional 1-1/2 inches to allow space for the new header and cut off the rafters with a circular saw.
Step 5
Install the new rafters
Cut the new triple rafters to the same length and angles as the old ones and slide them under the roof sheathing. Nail through the ridge board into the ends with three 16d common nails. Nail them together with pairs of 16d nails every 16 inches. Toenail the bottoms to the plate with two 16d nails, then add metal strap.
Step 6
Put up the front wall
Install the lower header with joist hangers (Figure A) and assemble the front wall on the attic floor. Stand the front wall and align it with the marks on the floor. Shim under the bottom plate to level it if necessary, then nail it into the floor joists with 16d nails. Plumb the corners with a level and brace them with 2x4s.
Step 7
Install the top plate
Add a corner stud, then cut the roof slope angle on the end of a 2×4 for the side wall’s top plate. Put the plate in position, level it and mark its length. Cut it and nail it to the corner stud and roof, ensuring it remains level.
Step 8
Install the wall studs
Fit and install the bottom plate. Then measure, cut and nail in the wall studs every 16 in. with two 16d nails at the top and bottom. Make sure they’re plumb. Add a second top plate overlapping the corners.
Step 9
Make a Simple Rafter Jig
If the slope of your dormer roof is the same as your old roof, hold a scrap of plywood against the old roof boards and draw a level line on it. Make a rafter-cutting jig by measuring 12 inches along the “level” line and using a square to draw a perpendicular line up to the “roof” edge of the board. Cut along these two lines and screw a scrap of 2×2 along the “roof” edge of the plywood.
Stretch a string across the top of the rafters, aligned with the inside of the side wall, and make a mark where this line intersects the top of the side rafters. Now, measure from this point up to the intersection of the ridge board and header to find the length of the valley rafters. Cut them according to Figure D.
Nail up both valley rafters. Use a straightedge to ensure they’re aligned with the tops of the common rafters. Then measure, cut and nail the valley jack rafters every 24 inches on center. Cut valley jack rafters to the same angle as the common rafters, but bevel the bottom cut 45 degrees.
Step 10
Cut and install the rafters
Eliminate guesswork—make a full-size drawing of the gable wall on the floor of your garage or attic. Begin with a baseline and snap a center line perpendicular to it. Use the 3-4-5 triangle method to make the 90-degree angle. Then, add the 2×4 side walls and the roof slope using a square. Snap lines for the rafters, then draw in the ridge and soffit detail. Cut a common rafter for a pattern and lay it on the drawing to check for fit. Measure and cut the temporary post that supports the ridge.
Figure B: Rafter design
Step 11
Place the ridge board
Cut a 2×4 to temporarily support the new 2×8 ridge at its correct height (see Figure B above). Hold the ridge board level and mark the side rafters at the top of the ridge board. Cut a 2×8 header to fit across the opening and attach it to the new rafters at that height with metal joist hangers.
Step 12
Attach the ridge board to the header
Cut the ridge board to length and notch it according to your soffit details. Then, for extra strength, nail it to the center of the header using a metal joist hanger. Cut and install the common rafters spaced 24 inches on center (Figures B and C).
Step 13
Rafter cuts
Construct a simple jig for marking the common rafters. For the two valley rafters, use the same dimension for the “plumb” side but extend the “level” side to 17 inches (17 inches works for every roof slope). The top plumb cut on valley rafters has a 45-degree bevel on each side. The bottom plumb cut has a single 45-degree bevel.
Figure C: Common Rafter Cuts
Figure D: Valley Rafter Cuts
Step 14
Sheathe the walls and roof dormer
Add the subfascia and 2×4 overhangs (Figure A). Sheathe the walls and roof with plywood. Drive 8d nails every 6 in. along the edges of the sheathing and every 8 inches along all other studs. Complete the soffit and overhang details according to the plan (Figure A).
Step 15
Install roof trim
Fit the roof trim and nail it on (Figure A). Complete the underside of the overhang and soffits, and add a roof edge molding to match your house. Install step flashing and reroof along the dormer edges.
Step 16
Shingle the roof
Cover the valley and roof edges with a 36-inch wide strip of ice-and-water barrier. Then, install a preformed galvanized metal valley (or use other methods approved by your building inspector). Shingle the roof from the bottom up according to the instructions on the shingle packaging.
Step 17
Install the windows and siding
Flash and install the windows. Then staple No. 30 building paper to the sides, lapping top sheets over bottom sheets. Nail the corner boards into place and add the siding. Prime and paint.
Step 18
Install the sheathing
Fill in the gable end studs after the rafters are complete. Then, install the roof sheathing. Having the structure closed in and ready to finish is a great feeling, but now’s not the time to rush the job. Careful attention to leakproof flashings and roofing details at this point in the job will save you unbelievable headaches later on. Think like water. Start at the bottom and work up with felt, shingles and flashings. Always overlap the lower piece.
Finish the exterior of your dormer with materials that match or complement those on your house.
Step 19
Finish the interior framing
Nail up the rafter ties, making sure to keep them level and aligned with each other. Add blocking and furring strips as needed to bring all of the interior framing to the same level, ready for drywall.
Once the dormer is weathertight and you can sleep without worrying about the pitter-patter of rain, move inside. Add rafter ties to strengthen the roof and to provide space for insulation and ventilation. When the building inspector has OK’d the framing, you can proceed with the electrical rough-in, insulation, drywall and woodwork.
Finally, remember that even with the best planning, you’ll occasionally hit a roadblock. Take a break. You’ll figure something out and probably end up with a few good stories to tell your grandchildren about your dormer-building adventure.
Originally Published: November 29, 2019
