How to Frame a Roof Between Two Existing Roofs

Measure the dimensions of the walls of your addition along with the ultimate height of your new roof. For a shed roof, you need to have the back wall higher than the front wall to create an angle. Shed roofs are easy to set up and connect with other roofs and don't have the drainage problems of flat roofs.

Use the measurements to order rafters from a hardware store for a shed roof. The rafters serve as the framework for your roof and can be ordered from most lumber stores, though you need to place the order two to three weeks in advance. Although you can make your rafters by hand, these pieces are essential for the integrity of the roof, so having them precision cut by the lumber store can make things far easier. Your rafters need to overhang the front and back wall somewhat to properly shield them from water.

Mark the point on your first rafter where it will contact both walls. This may be easier if you position the rafter on the wall first to determine exactly where it contacts the walls and the pitch or angle of the rafter.

Draw a perpendicular line down the sides of the rafter at these two connection points and hang plumb lines down from these points.

Repeat Steps 3 and 4 for the remaining rafters.

After verifying all of the rafters are aligned properly with each other, affix them to the walls using roofing nails. The length of the nail needed depends on how thick your rafters are.

Run a piece of string from the peak of your roof to the point on each of the other two roofs where they will connect with your new roof. The point where your new roof connects with an old roof is the ridge point.

Snap chalk lines from the ridge point to the place where the new walls intersect the new roof. These lines are your valley lines.

Measure the valley lines and cut two boards of that length with 45-degree angles at each end. Nail these boards in along the valley lines.

Measure the distance between the ridge points and the peak of your new roof and cut a board of that length, ensuring the end of the board has the same angle as your old roof. Nail this board to the peak of your new roof and the ridge points.