How to Cut Angles for a Gambrel Roof

Mark off the ends where the rafters will join each other with 22.5 degree angles. To find the 22.5 degree angle, lay a framing square on the end of the rafter with its point hanging off the side of the rafter. To create a 45 degree angle, you would match the numbers on the framing square that are directly above the edge of the rafter. To create a 22.5 degree angle, make the number that is over the edge of the rafter on the long edge of the framing square twice as much as the number on the short edge of the framing square. If the number over the edge of the rafter on the long edge reads 12, and on the short edge reads 6, the short edge will be crossing the rafter at a 22.5 degree angle.

Cut the ends of two rafters at 22.5 degrees using a circular saw.

Join the ends of these rafters together. Note that the angle of the joint these have created is 45 degrees.

Measure out from the joint you just made to determine the required lengths of the top rafters for the gambrel roof. Cut another 22.5 degree angle at the other ends of these rafters. Be sure that the second 22.5 degree angle is in the opposite direction from the first, so that the outside of the rafter is longer than the inside dimension.

Cut two more 22.5 degree angles on two more rafters. These will each be attached to opposite sides of the first two rafters you joined, creating a four sided gambrel rafter joined by three 45 degree joints.