Reference data
Roof pitch, angle and multiplier
Every pitch from 1:12 to 18:12 as an angle in degrees and as the factor that turns footprint into roof area.
Written and edited by Pablo Ruiz Quintero
Last reviewed 2026-09-16
What is the area multiplier for a 6:12 roof pitch?
1.118. A 6:12 roof has 11.8% more surface than the ground it covers, so a 1,200 ft² footprint carries 1,342 ft² of roof. That factor is the most useful number in roofing and the one most often skipped.
| Pitch | Angle | Area multiplier | Extra area | Shingles |
|---|---|---|---|---|
| 1:12 | 4.8° | 1.0035 | +0% | Not permitted |
| 2:12 | 9.5° | 1.0138 | +1% | Doubled underlayment |
| 3:12 | 14.0° | 1.0308 | +3% | Doubled underlayment |
| 4:12 | 18.4° | 1.0541 | +5% | Standard |
| 5:12 | 22.6° | 1.0833 | +8% | Standard |
| 6:12 | 26.6° | 1.1180 | +12% | Standard |
| 7:12 | 30.3° | 1.1577 | +16% | Standard |
| 8:12 | 33.7° | 1.2019 | +20% | Standard |
| 9:12 | 36.9° | 1.2500 | +25% | Standard |
| 10:12 | 39.8° | 1.3017 | +30% | Standard |
| 11:12 | 42.5° | 1.3566 | +36% | Standard |
| 12:12 | 45.0° | 1.4142 | +41% | Standard |
| 13:12 | 47.3° | 1.4743 | +47% | Standard |
| 14:12 | 49.4° | 1.5366 | +54% | Standard |
| 15:12 | 51.3° | 1.6008 | +60% | Standard |
| 16:12 | 53.1° | 1.6667 | +67% | Standard |
| 17:12 | 54.8° | 1.7341 | +73% | Standard |
| 18:12 | 56.3° | 1.8028 | +80% | Standard |
Where the multiplier comes from
It is Pythagoras, not a lookup table someone compiled. A roof slope is the hypotenuse of a right triangle: the horizontal run is 12, the vertical rise is the first number of the pitch, and the slope length is the square root of the sum of their squares. Divide that by 12 and you have the factor.
Which means it is exact, it never changes, and you can derive it yourself on a phone if you ever doubt the table: √(6² + 12²) ÷ 12 = √180 ÷ 12 = 1.1180.
The same number, three ways
A roofer says 6:12, an architect writes 26.6°, and a truss supplier may call it a 1:2 slope. They are the same roof. Pitch is the one to use on a job site because it is the one you can measure with a level and a tape; degrees are for drawings.
The slope limits are not preferences
Below 2:12 asphalt shingles are not permitted at all — water moves too slowly and finds its way under the laps, so the roof needs a membrane system. Between 2:12 and 4:12 they are allowed with a doubled underlayment, which means ordering twice the rolls. Those are code requirements, and a calculator that ignores them will happily quantify a roof that cannot be built.
Using it
The roofing calculator applies this factor automatically: give it the building footprint, the overhang and the pitch, and it returns the roof area in squares, the bundles, every accessory, and what the old roof weighs on the way to the skip.
Send this to whoever is paying
The link carries your measurements, so whoever opens it sees the same numbers you are looking at — and you can come back to it yourself tomorrow with the delivery note in front of you.
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About the author
Pablo Ruiz Quintero — Editor
Editor of MaterialsEstimator. Decides which documents count as a source, what every default value is, and where the line sits between what the reader parses and what the engines compute.
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