How to Cut a Dormer Into Your Roof

Have an architect design your dormer for you to make the best of the available space and the structure of your roof. The architect will create a design on a blueprint that’s easy to follow, and is in accordance with all local building codes. You’ll need to take the plans to your local building inspections department for approval and to receive a permit before you can begin building your dormer, otherwise you risk having fees assessed.

Rent a safety harness from a home improvement store or equipment rental shop and place it on following the manufacturer’s instructions to avoid falls. Put on a pair of heavy work gloves, safety goggles and ear protectors while working.

Mark the layout of the dormer walls onto the floor of your attic using a piece of chalk and a straightedge. Follow the architect’s plans to set the location precisely. Hold a plumb bob against the roof above each of the corners of the dormer walls. Mark the location of the corners against the roof, then drive a 16d nail through each mark with a hammer to create a mark that’s visible from the rooftop.

Place an extension ladder against the side of the house where you intend to install the dormer. Climb up to the roof and move to the proposed dormer area.

Locate the four nails driven through the roof to mark the dormer's location. Remove all shingles and roofing paper from the marked area. Use a small prybar to pull up the nails holding the shingles in place and set the shingles aside in case you can put them to use on the dormer roof. Cut through the roofing paper with a utility knife and pull it from the roof surface.

Snap a chalk line onto the rooftop to mark the inside edges of the dormer for the hole you need to cut. Connect the corners marked by the nails with the chalk line. Use a carpenter’s square at the corners of the opening lines to make certain the corners are 90 degrees.

Set the cutting depth of a circular saw to that of the thickness of the roof's plywood sheathing. Cut through the chalked outline of your dormer, beginning along the bottom edge, then extending upwards on both sides before cutting along the top. Use the prybar to pull the plywood from the roof.

Go back into the attic and mark the location of the front dormer wall lines on any rafter contained within the opening using a carpenter’s level placed flush on the chalked front wall line and held vertically against the rafter. Mark the location of the wall onto the rafter, then place a lower mark on the bottom of the rafter that’s the width of the planned header running along the bottom of the front wall for the dormer.

Brace the roof along the peak to either side of the rafters marked for removal using 2-by-6-inch planks cut to the height of the roof. Place the planks to either side of space that will be created by cutting the rafters. This temporary brace will help to relieve the weight of the roof above the dormer area after cutting the rafters until you have any new rafters in place for permanent support.

Use a reciprocating saw with a metal cutting blade to cut the rafters in the opening along the lower marked line, then along the top of the rafters running along the line where the rafter connects to the peaked roof. The metal blade will cut smoothly through the nails holding the top of the rafter in place, completing the cut for your dormer opening.