How to Frame a Rafter Roof for an Addition

Determine the width and pitch, or slope angle, you want for your roof. Measure the addition from side wall to side wall. The pitch can vary from very shallow to steep. A common pitch for a gable roof is six inches by 12 inches, meaning the roof rises six inches for every foot of run, or distance between the eave and peak. Measure the length of the addition and divide it by 24 to get the number of rafters needed. You will space the rafters 24 inches apart.

Make a pattern rafter from a 2-by-4 or 2-by-6 board, depending on the total width the rafters will cover. A 2-by-4 will work for most small buildings. Start with a board that covers half the span. For a 24-foot wide addition, the span should measure 24 feet and the run should measure 12 feet. Put the right angle end of a framing square on the bottom of the test rafter, with the six inch mark on the short side of the square and the 12 foot mark on the top edge. That will give you a top angle, or plumb cut for the rafter.

Determine the rafter length with a table on the big part of the framing square. For a 6/12 pitch It will show 13.42, meaning a rafter must be 13.42 inches for every foot of run. For a 12-foot run, that equates to 161.04 inches. Mark that distance (ignore the .04, which is about 1/8 inch) on the bottom of the pattern to indicate where the side wall will be, then add whatever overhang is wanted (usually about 18 inches). That would make the total rafter length 179 inches or about 15 feet.

Cut a triangular notch, called a birds mouth, at the wall mark. Cut one inch deep into the bottom of the rafter, measure down four inches and cut the triangle from the top of the inch to the bottom of the rafter. This will fit securely against the 2-by-4 cap board on the wall. Test fit that rafter to make sure all dimensions are right, then cut the bottom of the rafter to the desired length, with an angle opposite that of the top.

Put two rafters on opposite sides on the back end wall and nail the bottoms temporarily into the wall caps on each side. Brace them plumb with 2-by-4s nailed to the rafters and to stakes in the ground behind the wall. Let the angle ends rest against each other. Move to the other end of the roof and repeat with two more rafters.

Slide a 2-by-6 inch ridge board between the end rafters from underneath, so it presses against it. Nail the rafters on each end to this ridge board, making sure rafters are plumb and the ridge board is at the top of the rafters and level. Then set other pairs of rafters, 24 inches apart, the length of the roof. Once all rafters are in place, double check plumb and level and nail all rafters firmly to the wall caps and the ridge board.

Finish the roof framing with gable studs and rafter ties. Gable studs are 2-by-4 boards cut to fit between the end wall cap and the ridge board and nailed in place. Rafter ties are 2-by-4 boards nailed horizontally between rafters about a third of a way down from the peak.