The pitch of a roof determines the height of the ridge board that runs the length of the house. Carpenters measure roof pitch as inches of rise per 12-inches of run. A 1-in-12 roof pitch roof rises 1 inch for every 12 inches of run. Rafters are nailed to the ridge board and run down to overhang the exterior wall. Determining the height of the ridge board sets the stage for cutting the angles on the rafters and setting them at them correct height for the pitch of the roof.
Divide the width of the exterior house frame in inches by two. This provides the run for one side of the roof in inches. Divide the run by 12 inches using the calculator to find the run factor. The outside width of the wall framing is measured on the house frame and does not include the sheathing or siding added to the framing.
Multiply the roof pitch by the run factor. The result equals the total distance the roof rises. The bottom of the ridge board is set at this height assuming the ridge board and the roof rafters use the same size lumber, the most common way of building with rafters.
Calculate the top height of the ridge board. Add in the size of the dimensional lumber used for the ridge rafter -- 5 1/2-inches for 2-inch by 6-inch lumber, or 3 1/2-inches for 2-inch by 4-inch lumber. This gives the total centre height of the rafters.
Learning to use a framing square and a roofing square greatly simplifies roof construction. Building a scaled-down version of the roof is a good idea if you don't have roof-framing experience. You can measure the pitch of an existing roof with a framing square and a level. Place the square against the edge of the roof with one leg pointing up and the 12-inch mark of the horizontal leg aligned with the edge of the roof. Use the level to hold the horizontal leg level. The point at which the vertical leg aligns with the roof edge is the pitch.