How to Frame Rafters

Using the architectural plans, determine the height of the desired peak. At that rafter location, find the measurement of the width of the building. Divide the width by 2 to find the center point.

Apply the Pythagorean Theorem to find the length of the rafter to the outside of the wall. If you are using a ridge beam, subtract half of the ridge beam's width from the resulting length.

Using a speed square, set the desired pitch at the peak; for example, set the speed square for a 5/12 pitch. Measure from this line out to the distance determined in step 2. Using the speed square, draw the same pitch line, which gives the rafter length to the outside of the wall.

Measure up from the bottom of the rafter 2 inches and place the speed square with the right angle up, at the 2-inch mark, squared to the bottom of the rafter at the pitch line. Draw the right angle -- this will be the seat cut or birdsmouth in which the rafter will set on the wall.

Measuring down from the rafter from the birdsmouth and squaring from the seat cut, measure out the desired distance for the overhang. This is the long point measurement for the rafter.

Using your speed square, draw your pitch line to your long point, following the same pitch as used in the previous steps. This is the overall rafter length.

Cut the rafter at the peak line. Cut the birdsmouth at the 2-inch mark. Cut the overall rafter length.