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Figure A: Dormer Anatomy
Note: The size of the new triple rafters and common rafters will vary with the roof and dormer size and local conditions. Have an architect or structural engineer size them.
Make Your Mistakes on Paper Where They’re a Whole Lot Easier to Fix!
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. Expect to pay $500 to $2,000 for this service. Use the plans to obtain a building permit from your local building inspections department.
After the plans have been approved by the building department, take them to a full-service lumberyard. A salesperson will put together 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 at night or in the event of a surprise rainstorm.
Unlock the Mysteries of Rafter Cutting
Figure B: Rafter Design
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 2x4 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 C: 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 in. (17 in. 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.
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