How to Pitch a Roof Rafter

Use a framing square or a level to calculate the pitch on an existing roof. Put the wide blade of the square on top of the roof, 12 inches in from the edge, with the thin tongue against the outside edge; the distance between the blade and where the tongue touches the roof will be the pitch. Do a similar test on the under side of the rafter below the roof; measure 12 inches in from the edge and the distance to the rafter from that point. Use a level instead of a square; hold it level 12 inches in and measure the depth. Either test is basically forming a triangle and measuring the angle of the hypotenuse.

Calculate pitch angles on a rafter with a framing square. Put the square on one end of a 2-by-4-inch board, with the point or heel at the bottom of the board's 4-inch face and the thin tongue to the right. Align the 12-inch mark on the wide blade and the pitch mark on the thin tongue at the top of the board. For a 5/12 pitch, that would be the 5-inch mark on the tongue. That forms an angle at the end for a top or plumb cut to form a peak..

Figure one other element, a bearing-point notch called a birdsmouth, to insure a proper pitch on a rafter. Calculate the length of rafter needed using the "length of common rafter per foot of run" table on the square's blade. Look under the pitch mark to determine that difference. It will show 13 under the 5-inch mark, for instance, meaning the rafter must be 13 inches long for every foot of run; the run is half the width of the roof, the distance each rafter must span.

Multiply that difference times the run to get the point for the birdsmouth, a triangle which will fit exactly on top of the wall to form the proper pitch. Mark that distance on the bottom of the board from the plumb cut and draw a vertical line one inch up into the rafter board. Measure 3 1/2 inches back up the bottom of the board toward the top, then connect that point and the top of the 1-inch mark to make the triangle.