3 Ways to Express Roof Slope • Pitch, Degrees & Percentage
(Roof Slope Equivalents with Conversion Formulas)
By Jack Gray, Roof Online Editor • Updated October 8, 2022
Table of Contents
- Ways to Express Roof Slope
- Note: Correct Format for Stating a Standard Roof Pitch
- Useful Roof Slope Tools
- About This Table
- Table: Equivalent Ways to Express Roof Slope
- How to Convert from One Way to Express Roof Slope to Another
- Formula: Convert Standard Roof Pitch to Degrees
- Formula: Convert Standard Roof Pitch to Percentage
- Formula: Convert Roof Slope in Degrees to Percentage
- Formula: Convert Roof Slope in Degrees to Standard Roof Pitch
- Formula: Convert Roof Slope in Percentage to Degrees
- Formula: Convert Roof Slope in Percentage to Standard Roof Pitch
Ways to Express Roof Slope
There are three basic ways to express roof slope. Roof slope can be given in degrees, as a percentage, or as a rise-in-run ratio, otherwise known as standard roof pitch.
When you express roof slope in degrees, you are stating the degrees of the angle formed by the surface of the roof and the (imaginary) horizontal plane that intersects the roof.
In most countries around the world, people express roof slope in degrees rather than standard roof pitch. In the United States, however, this is very uncommon, much like the use of the metric system.
When you express roof slope in percentage, you are stating the percentage of the run (the horizontal distance covered by a section of the roof) that the rise of the roof (the vertical distance covered in the same roof section) is.
The use of percentage to express roof slope is very uncommon everywhere, and is typically only used when discussing the slope of low-slope (“flat”) roofs for purposes of structural engineering, plumbing, and roof drainage system design.
A 50% roof slope would have a pitch of 6-in-12. 100% would be 12-in-12. Very steep roofs can have slope percentages greater than 100%. The slope percentage of a 16:12 roof would be 133.33%
When you express roof slope in standard roof pitch, you are stating the roof slope as a ratio of rise in run. Traditionally, the run is always 12, because there are 12 inches in a foot. Standard roof pitch tells you how many inches the roof rises for every foot it runs horizontally.
You don’t have to be using inches for this to work. A roof that rises 6 inches per foot of run has the same slope as a roof that rises 6 centimeters per 12 centimeters of run. The ratio is the same.
Note: Correct Format for Stating a Standard Roof Pitch
When people use standard roof pitch to express roof slope, there tends to be a lot of confusion about how to say it or write it.
When referring to a specific roof pitch, the proper way to describe the pitch is in the form “X-in-12”. If you’re talking about a roof that rises 4 units for every 12 units it runs horizontally, you would say that the roof has a “4 in 12 pitch”.
When writing out a specific roof pitch, the accepted alternative to this, particularly in roofing industry literature, is to use a colon instead of the word “in”. This style takes the form “X:12” or “X : 12”, so for a 4 in 12 pitch, you would write “4:12 pitch”.
The use of a slash, as in 4/12, is not normally used by roofing professionals, even though you see it almost everywhere else. This is to avoid confusion: the slash normally means “divided by”, and that is not what you are trying to say when stating a roof pitch.
Useful Roof Slope Tools
If you’re not sure what the slope of your roof is and you want to determine that in either degrees or pitch, we recommend this slope finder on Amazon. It’s very inexpensive and very accurate.
If you want to know the slope of anything to an amazing degree of accuracy and you like cool new tools, you should check out this digital level.
It may be way too expensive for what you need, but this is what professionals use. It will tell you the slope of your roof in degrees, rise/run, or percentage, and automatically convert from one to the other.
In other words, it will do everything this article explains how to do.
Also, if you’re using this table, you might like to get yourself a construction calculator. This one is very good.
About This Table
The following table shows how the same common roof slopes can be expressed using rise-in-run ratios (standard pitch), degrees, and percentages.
See below, after the table, for directions on how to convert any one way to express roof slope to another. Even roof slopes not shown in the table. It’s just a little math.
Also see our page Roof Pitch to Degrees • Degrees to Roof Pitch which has conversion charts that convert slopes from 1 to 72 degrees into standard roof pitch (X-in-12) and roof pitches from ⅛-in-12 to 36½-in-12 into degrees.
Table: Equivalent Ways to Express Roof Slope
(Standard Roof Pitch)
How to Convert from One Way to Express Roof Slope to Another
To make things easier, use a good calculator that can perform the trigonometric functions.
Click the “2nd” button in the top left of the calculator field to find the “atan” (arctangent) button. Enter the number (0.54167 in the following example) and click the “atan” button and then the “=” button.
Formula: Convert Standard Roof Pitch to Degrees
Standard Roof Pitch to Degrees: Find the arctangent of (rise/run).
For a 6-1/2 inch in 12 inch slope (6-1/2 = 6.5),
arctan(6.5 divided by 12)
arctan(0.54167) = 28.44° slope.
Formula: Convert Standard Roof Pitch to Percentage
Standard Roof Pitch to Percentage: Divide the rise by the run and multiply by 100.
For a 3/8 inch in 12 inch slope (3/8 = 0.375),
0.375 divided by 12 = 0.03125
0.03125 x 100 = 3.125
3/8 in 12 slope = 3.125% slope.
Formula: Convert Roof Slope in Degrees to Percentage
Degrees to Percentage: Find the tangent of the degree value. Multiply by 100.
For a slope of 25°,
tan(25) = 0.4663
0.4663 x 100 = 46.63
25° slope = slope of 46.63%
Formula: Convert Roof Slope in Degrees to Standard Roof Pitch
Degrees to Standard Roof Pitch: Find the tangent of the degree value. Multiply the tangent by the run (which is always 12 in standard roof pitch). That gives you the rise. Put them together as rise-in-run.
For a slope of 35°,
tan(35) = 0.700
0.7 x 12 = 8.4
giving you a slope of 8.4-in-12 or
8-2/5 inches in 12.
Formula: Convert Roof Slope in Percentage to Degrees
Percentage to Degrees: Divide the percentage number by 100. Find the arctangent of the resulting value.
For a slope of 55%,
55 divided by 100 = 0.55
arctan(0.55) = 28.81
55% slope is equal to a slope of 28.81°.
Formula: Convert Roof Slope in Percentage to Standard Roof Pitch
Percentage to Standard Roof Pitch: Divide the percentage number by 100. Multiply the result by the run (which is always 12 in standard roof pitch). That gives you the rise. Put them together as rise-in-run.
For a slope of 25%,
25 divided by 100 = 0.25
0.25 x 12 = 3
giving you a slope of 3-in-12.
About the Author
Jack Gray is a principal roof consultant and vice president at the Moriarty Corporation, an award-winning building enclosure consultant firm founded in 1967. He is also the editor of the Roof Online website.
Mr. Gray has been involved in the roofing industry for over 25 years, with training and practical experience in roof safety, roof inspection, roof installation, roof condition assessment, estimating, roof design & specification, quality assurance, roof maintenance & repair, and roof asset management.
He was awarded the Registered Roof Observer (RRO) professional credential in 2009.
He also served as an infantry paratrooper in the 82nd Airborne Division and attended Cornell University. Read full bio.