How to Calculate Rafter Lengths

Adjoining rafters on pitched roofs comprise the upper sides of opposing right triangles.

As a do-it-yourself builder, you can expand your knowledge of basic roof construction by calculating the length of the rafters for a particular roof section or room. While it's possible to do the calculations by applying mathematics of triangle geometry, professionals generally use a method that saves time and ensures the rafters are cut to the correct lengths.

1. Determine the direction the rafters rise from opposite outside walls to the ridge. Typically, this is shown on the roof construction plan in the approved building plans. Alternatively, measure the room or section of the building each way to determine which outside walls are longer than the walls at each end. For example, if a room or section of a building is 20 feet long by 14 feet wide, the lower ends of opposing pairs of rafters attach perpendicular at the 20-foot-long walls, and the upper ends rise to a ridge above the center, or 7 feet from those outside walls.

2. Determine the pitch of the roof, which is the angle that rafters rise from the outside walls to the ridge. Roof pitch is designated by a set of numbers, such as 4/12 or 6/12. The first number represent the amount -- in feet -- that a rafter rises. The second number is the benchmark for typical pitched roof calculations. For example, a rafter installed at a 4/12 pitch rises 4 feet for every 12 feet from an outside wall.

3. Refer to the measurements of the shorter opposing walls of the room or building. In this example, the shorter walls are 14 feet long. Divide this in half and the result is 7 feet. Using the appropriate roof-pitch table in a rafter cutting guide -- such as 4/12 -- determine the length of each rafter based on the correlating measurement shown for 7 feet.

4. Add the measurement for the overhang -- the eaves -- on the outside wall to the result obtained in the rafter guide to determine the overall rafter length.