What does a pitch like 6/12 mean?
In roofing, pitch is usually stated as a ratio of inches of rise per 12 inches of horizontal run. A 6/12 roof climbs 6 inches for every foot you move toward the ridge. Strictly, building references separate pitch, which uses the full span, from slope, which uses the run, but contractors almost always say pitch and mean rise per 12.
A 12/12 roof rises 12 inches per foot and sits at 45 degrees. A 6/12 roof sits at about 26.6 degrees.
Where does the pitch multiplier come from?
The roof surface along the slope is the hypotenuse of a right triangle. The run is 12 and the rise is the pitch number. The slope length per foot of run is the square root of (12 squared plus rise squared), and dividing by 12 gives the multiplier.
For 6/12: the square root of (144 + 36) is 13.416, and 13.416 / 12 = 1.118. Multiply any flat plane area by 1.118 and you have the true surface of a 6/12 plane.
- 4/12: 1.054 (18.4 degrees)
- 5/12: 1.083 (22.6 degrees)
- 6/12: 1.118 (26.6 degrees)
- 7/12: 1.158 (30.3 degrees)
- 8/12: 1.202 (33.7 degrees)
- 9/12: 1.250 (36.9 degrees)
- 10/12: 1.302 (39.8 degrees)
- 12/12: 1.414 (45.0 degrees)
How do you measure the pitch of a roof?
From the attic or a safe ladder position, hold a level horizontal against the underside of a rafter or on the roof surface. Mark 12 inches along the level from the point where it touches the roof, then measure straight down or up from the 12 inch mark to the roof. That distance in inches is the rise per 12.
A measurement report or a phone pitch gauge gives the same answer without climbing. Always read each roof section separately, because additions and porch roofs often have a different pitch than the main roof.
How much does pitch change the order?
Take a 1,800 square foot footprint at 1418 Ridge Rd with an 8/12 pitch. Roof surface: 1,800 x 1.202 = 2,163.6 sq ft, or 21.6 squares. If the estimator forgot the multiplier, the takeoff would be 1,800 / 100 = 18 squares, which is 3.6 squares short, about 17 percent below the real area (3.6 / 21.6 = 16.7 percent).
That shortfall comes straight out of material and labor. On a steep roof the error is worse, which is why the multiplier belongs in every footprint-based estimate.
Does pitch affect more than area?
Yes. Steeper roofs usually take more labor per square because crews need roof jacks, toe boards or fall protection and move more slowly. Low slope roofs have their own limits, because shingle manufacturers set a minimum slope and special underlayment rules for their products.
Read the installation instructions for the exact product you are quoting, and check local code, instead of relying on one number from memory.
Step by step
- Place a level. Hold a 12 inch level horizontal against the rafter underside or the roof surface.
- Measure the rise. At the 12 inch mark, measure the vertical distance to the roof. That number is the pitch, for example 6 for 6/12.
- Look up or compute the multiplier. Use the table or take the square root of (1 + (rise / 12) squared).
- Multiply the footprint. Multiply each roof section's flat footprint by its multiplier to get true surface area.
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Sources: Vertigraph: How to Calculate Roofing Squares (pitch multiplier table); Wikipedia: Roof pitch (pitch and slope terms)