Roof Pitch Multiplier Table
(Roof Pitch Factor Table)
A roof pitch multiplier, also called a roof pitch factor, is a number that is multiplied by the area covered by a sloped roof to produce the area of the actual surface of the roof. For roof slopes expressed as “X-in-12” (rise-in-run), the roof pitch multiplier is determined by finding the square root of ((rise/run)² + 1).
Divide the rise by the run (the run is 12). Square the result. Add 1. Find the square root of the result.
The slope of the roof provides the rise and the run to be plugged into the equation. A roof pitch of 4-in-12 (4:12) has a rise of 4 and a run of 12.
The roof pitch factor is also used to calculate the length of common rafters. To find the total length of a common rafter, consider the run to be the horizontal distance covered by the whole rafter, from the end of the rafter tail (the part of the rafter which extends past the outside of the wall) to the near side of the ridge board.
To find the location of the heel cut, the multiplier is multiplied by the “effective run” of the rafter. The effective run is the horizontal distance covered by the rafter from the near side of the ridge board, where the head cut will be, to the outside of the wall plate, where the heel cut will be.
Always measure for every rafter, even being off ⅛-inch can make a big difference if the error gets multiplied as you go along.
The following table provides the roof pitch multiplier for roofs of various slopes. For a more detailed explanation of the roof pitch multiplier, see "How to Find the Area of a Sloped Roof".
For help converting any slope from one way of expressing it to another, see Roof Slope Expressed as Rise-in-Run, Degrees, and Percentage.