Measure the span of the common rafters. This is the width of the building.
Determine the run of the common rafter. This is calculated by dividing the width of the building by two.
Decide the pitch of the roof you want to build. A pitch of 3:6 means the roof rises 3 inches for every 6 inches of run (width).
Place the framing square on the wood at the top edge of the rafter. Align the 3-inch measurement on the short side of the square with the top edge of the rafter and the 6-inch measurement on the long side of the square with the top edge of the rafter.
Draw a line along the outside edge of the framing square with a pencil. This line indicates where to make a cut to build the rafter.