Roof Pitch Multiplier Table
(Roof Pitch Factor Table)
Roof Online Staff
Scroll down to see the chart.
Use Standard Pitch
You will need to know the slope of the roof in the standard pitch format “X-in-12” for the rest of this page to make sense.
What is a Roof Pitch Multiplier?
A roof pitch multiplier, also called a roof pitch factor, is a number that you use to find the actual surface area of a pitched roof.
You multiply the roof pitch multiplier by the area, or footprint, covered by a sloped roof to produce the actual surface area of the roof.
Roof Pitch Multiplier Formula
Once you know your roof slope expressed as “X-in-12” (rise-in-run), the roof pitch multiplier is determined by finding the square root of ((rise/run)² + 1).
Remember that 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.
So you divide the rise by the run (the run is always 12 and the rise depends on your particular roof).
Square the result.
Find the square root of the result.
Using the Pitch Multiplier for Common Rafters
The roof pitch factor is also used to calculate the length of common rafters.
To find the length of a common rafter, determine its span from the bottom of the end of the rafter tail to the top of the ridge cut at the ridge board. Once you know the actual horizontal distance it will cover, use the formula above.
Multiply the roof pitch multiplier by the run of the rafter (the horizontal distance covered by the rafter). That will give you the proper rafter length.
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.
Notes on the Roof Pitch Multiplier Chart
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".