How to Cut a Barn Style Roof

Make a basic gambrel-style roof with four equal length rafters, two on each side. Use a speed square to mark each end of each board at 22 1/2 degrees, but with angles opposite. Put the pivot point of the square at the top of a rafter board and line up 22 1/2 degrees on the table on the angled bottom of the square with the edge of the board. Move to the other end and repeat the process with the pivot point on the other side of the board. Mark the angles with a pencil.

Lay out four rafters the same way. Mark top and bottom cuts on each rafter and cut the angle with a circular saw. Set the 22 1/2-degree top cuts on two opposing rafters at the peak to form a 45-degree angle. Align the 22 1/2 degree bottom cut on the top rafter with the top cut on the bottom rafter, also at a 45-degree angle; the bottom cut of that rafter will sit on top of the wall.

Use a reverse pitch method to cut gambrel rafters of unequal length for a wide top slope and a short bottom edge. Get the pitch for each half of the roof, typically 7/12 for the top slope and 12/7 for the bottom, meaning the top rafter rises 7 inches per foot from the midway point to the peak and the bottom rafter slopes down 12 inches for every 7. Use the speed square to cut these pitches, too.

Put the pivot point of the square at what will be the top end of the top rafter and align the "common top cut" table on the square at 7. Move to the other end of the board and mark the bottom cut with the square on the other side of the board aligned with the 12 on the top cut table. Cut the bottom rafter the same way. Follow the same formula for any pitch of gambrel sides.