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Common
Valley
Make 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 in. 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 2x2 along the “roof” edge of the plywood.
Lay Out the Dormer on the Floor Before You Cut
Before you can cut the hole for the dormer, you must know exactly where the side walls will be built. Mark the dormer wall layout on the attic floor and make sure the dormer walls are square with the exterior wall of the house. Use a plumb bob to transfer the inside edge of the walls to the underside of the roof. Drive nails up through the roof at the corners. Return to the roof, remove shingles in the area and snap chalk lines between the protruding nail points. Saw along the lines to open the roof.
The side walls will rest on the new rafters and the front wall will rest on the attic floor. Using the lines on the floor that indicate the front wall of the dormer, mark the old rafters for cutting. Then reinforce the opening in the roof by adding rafters along each side and the opposite side of the roof if necessary (Figure A). Pay close attention to fastening requirements. We anchored the bottom of the tripled 5-1/2 in. laminated veneer lumber (LVL) rafters to the building with the special metal straps specified by the architect. Make sure the new rafters line up with the edge of the roof sheathing (Figure A).
Roof Safety
Working on roofs is dangerous. 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.)
Use metal roofing brackets and 2x10 planks to provide solid footing. Follow the installation guidelines included with the brackets ($8 each from 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.
Plumb and Square the Front Wall
After seemingly endless planning and preparation, you’ll finally see real progress when the walls go up. Install a 2x6 header across the opening to support the cut-off rafters, then fit the front wall between the new rafters. Take a little extra care to make sure the top of the wall is level and brace it perfectly vertical (plumb) and square.
Complete the wall framing by building the side walls. Take care not to bow the top plate when angle-nailing studs.
Frame the Roof Following These Steps
You don’t have to understand all the technicalities of rafter cutting to build the dormer roof.
Make the full-scale drawing to the dimensions and roof slope in your plan (Figure B) and use it to measure the exact height of the ridge (temporary post) and the lengths and angles on the common rafters. If the slope of your dormer roof is the same as that of the old roof, you can use the roof jig technique shown to measure the slope and transfer it to the new rafters.
Measure the length of the valley rafters and cut them using Figure C as an angle guide. Don’t worry if you miscut a few boards. You can use them later for the shorter valley jack rafters.
Attach the rafters to the ridge by nailing through the ridge into the rafter with three 16d nails. Then toenail the opposing rafter to the ridge with six 8d common nails, three on each side. Use three 8d toenails to attach the rafter to the wall plate. Fill in the gable end studs after the rafters are complete.
Ask your building inspector whether you need additional seismic and high wind anchors.
Building Paper and Flashing Are What Really Keep the Water Out
It’s a great feeling to have the structure closed in and ready to finish, but now’s not the time to rush the job. Careful attention to leakproof flashings and roofing details (Figure A) 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.
Get the Framing 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 framing has been OK’d by the building inspector, you can proceed with the electrical rough-in, insulation, drywall and woodwork.
Finally, keep in mind 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.
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