How Much to Add for Roof Pitch
A footprint measured from the ground or a plan is flat. The roof is not. Multiply by the pitch factor to get the real surface you have to cover.
Add for pitch calculator
The pitch multiplier
The multiplier is the length of a rafter for each foot of level run. Because every plane of a roof with one pitch tilts by the same amount, it works for gable, hip and most combination roofs alike: measure the outside footprint including overhangs, then multiply once. Roofs with mixed pitches need each section handled separately, which the measure a roof guide covers.
Pitch factor table
The table also gives each pitch in degrees, since many apps, saws and solar specs ask for the angle. For example, a 2/12 pitch is 9.46°, a 3/12 pitch is 14.04° and a 20/12 pitch is 59.04°. The pitch to degrees converter handles any value, including fractions like 2.5/12.
| Pitch | Degrees | Multiplier | Add for pitch | 1,000 sq ft footprint becomes |
|---|---|---|---|---|
| 1/12 | 4.76° | 1.0035 | 0.3% | 1,003 sq ft |
| 2/12 | 9.46° | 1.0138 | 1.4% | 1,014 sq ft |
| 3/12 | 14.04° | 1.0308 | 3.1% | 1,031 sq ft |
| 4/12 | 18.43° | 1.0541 | 5.4% | 1,054 sq ft |
| 5/12 | 22.62° | 1.0833 | 8.3% | 1,083 sq ft |
| 6/12 | 26.57° | 1.1180 | 11.8% | 1,118 sq ft |
| 7/12 | 30.26° | 1.1577 | 15.8% | 1,158 sq ft |
| 8/12 | 33.69° | 1.2019 | 20.2% | 1,202 sq ft |
| 9/12 | 36.87° | 1.2500 | 25.0% | 1,250 sq ft |
| 10/12 | 39.81° | 1.3017 | 30.2% | 1,302 sq ft |
| 11/12 | 42.51° | 1.3566 | 35.7% | 1,357 sq ft |
| 12/12 | 45.00° | 1.4142 | 41.4% | 1,414 sq ft |
| 13/12 | 47.29° | 1.4743 | 47.4% | 1,474 sq ft |
| 14/12 | 49.40° | 1.5366 | 53.7% | 1,537 sq ft |
| 15/12 | 51.34° | 1.6008 | 60.1% | 1,601 sq ft |
| 16/12 | 53.13° | 1.6667 | 66.7% | 1,667 sq ft |
| 17/12 | 54.78° | 1.7341 | 73.4% | 1,734 sq ft |
| 18/12 | 56.31° | 1.8028 | 80.3% | 1,803 sq ft |
| 19/12 | 57.72° | 1.8727 | 87.3% | 1,873 sq ft |
| 20/12 | 59.04° | 1.9437 | 94.4% | 1,944 sq ft |
| 21/12 | 60.26° | 2.0156 | 101.6% | 2,016 sq ft |
| 22/12 | 61.39° | 2.0883 | 108.8% | 2,088 sq ft |
| 23/12 | 62.45° | 2.1619 | 116.2% | 2,162 sq ft |
| 24/12 | 63.43° | 2.2361 | 123.6% | 2,236 sq ft |
Worked examples
Half-pitch multipliers and material limits
Pitches like 2.5/12 and 4.5/12 are common on additions and porches. Here is every half step, with the OSHA roof class and the pitch where each covering becomes code-legal.
| Pitch | Degrees | Slope % | Area multiplier | Roof class | Code minimum reached (IRC R905) |
|---|---|---|---|---|---|
| 0.5/12 | 2.39° | 4.2% | 1.0009 | Low-slope (OSHA) | Now allowed: Standing-seam metal panels; Built-up, modified bitumen, single-ply membrane; Lapped metal panels with lap sealant |
| 1/12 | 4.76° | 8.3% | 1.0035 | Low-slope (OSHA) | Now allowed: Mineral-surfaced roll roofing |
| 1.5/12 | 7.13° | 12.5% | 1.0078 | Low-slope (OSHA) | |
| 2/12 | 9.46° | 16.7% | 1.0138 | Low-slope (OSHA) | Now allowed: Asphalt shingles |
| 2.5/12 | 11.77° | 20.8% | 1.0215 | Low-slope (OSHA) | Now allowed: Clay and concrete tile |
| 3/12 | 14.04° | 25.0% | 1.0308 | Low-slope (OSHA) | Now allowed: Metal roof shingles; Lapped metal panels, no sealant; Wood shingles and shakes |
| 3.5/12 | 16.26° | 29.2% | 1.0417 | Low-slope (OSHA) | |
| 4/12 | 18.43° | 33.3% | 1.0541 | Low-slope (OSHA) | Now allowed: Slate |
| 4.5/12 | 20.56° | 37.5% | 1.0680 | Steep roof (OSHA) | |
| 5/12 | 22.62° | 41.7% | 1.0833 | Steep roof (OSHA) | |
| 5.5/12 | 24.62° | 45.8% | 1.1000 | Steep roof (OSHA) | |
| 6/12 | 26.57° | 50.0% | 1.1180 | Steep roof (OSHA) | |
| 6.5/12 | 28.44° | 54.2% | 1.1373 | Steep roof (OSHA) | |
| 7/12 | 30.26° | 58.3% | 1.1577 | Steep roof (OSHA) | |
| 7.5/12 | 32.01° | 62.5% | 1.1792 | Steep roof (OSHA) | |
| 8/12 | 33.69° | 66.7% | 1.2019 | Steep roof (OSHA) | |
| 8.5/12 | 35.31° | 70.8% | 1.2255 | Steep roof (OSHA) | |
| 9/12 | 36.87° | 75.0% | 1.2500 | Steep roof (OSHA) | |
| 9.5/12 | 38.37° | 79.2% | 1.2754 | Steep roof (OSHA) | |
| 10/12 | 39.81° | 83.3% | 1.3017 | Steep roof (OSHA) | |
| 10.5/12 | 41.19° | 87.5% | 1.3288 | Steep roof (OSHA) | |
| 11/12 | 42.51° | 91.7% | 1.3566 | Steep roof (OSHA) | |
| 11.5/12 | 43.78° | 95.8% | 1.3851 | Steep roof (OSHA) | |
| 12/12 | 45.00° | 100.0% | 1.4142 | Steep roof (OSHA) |
Minimum roof pitch by material
| Roof covering | Minimum pitch | Degrees | IRC section | Note |
|---|---|---|---|---|
| Standing-seam metal panels | 0.25/12 | 1.19° | R905.10.2 | — |
| Built-up, modified bitumen, single-ply membrane (TPO, EPDM, PVC) | 0.25/12 | 1.19° | R905.9, R905.11, R905.12 | Design slope for drainage |
| Lapped metal panels with lap sealant | 0.5/12 | 2.39° | R905.10.2 | — |
| Mineral-surfaced roll roofing | 1/12 | 4.76° | R905.5.2 | — |
| Asphalt shingles | 2/12 | 9.46° | R905.2.2 | Double underlayment from 2/12 up to 4/12 |
| Clay and concrete tile | 2.5/12 | 11.77° | R905.3.2 | Double underlayment from 2.5/12 up to 4/12 |
| Metal roof shingles | 3/12 | 14.04° | R905.4.2 | — |
| Lapped metal panels, no sealant (exposed-fastener) | 3/12 | 14.04° | R905.10.2 | — |
| Wood shingles and shakes | 3/12 | 14.04° | R905.7, R905.8 | Shorter exposure below 4/12 |
| Slate | 4/12 | 18.43° | R905.6.2 | — |
The multiplier gives the area to cover, not the amount to buy. Add waste on top: about 10% for simple gable roofs and 15% or more for hips and valleys. The waste factor calculator walks through it.
Sources
- International Code Council, International Residential Code (2021 and 2024), Section R905 minimum slopes, as summarized in IRC 2024 Chapter 9 (UpCodes) and the MyBuildingPermit residential re-roof checklist. Local amendments and manufacturer instructions can be stricter.
- OSHA, 29 CFR 1926.500(b): a low-slope roof is 4 in 12 or less; a steep roof is greater than 4 in 12. 1926.501(b)(13) requires fall protection for residential construction work 6 ft or more above a lower level.
Frequently asked questions
How many degrees is a 2/12 pitch?
A 2/12 pitch is 9.46 degrees. Its pitch multiplier is 1.0138, so it adds only about 1.4% to the footprint area.
How many degrees is a 3/12 pitch?
A 3/12 pitch is 14.04 degrees, with a multiplier of 1.0308 (about 3.1% added area).
What is 20/12 roof pitch in degrees?
A 20/12 pitch is 59.04 degrees. Its multiplier is 1.9437, so the roof surface is nearly double the footprint.
What is the pitch factor for a 6/12 roof?
The pitch factor (multiplier) for 6/12 is 1.118. Multiply the flat area by 1.118, which adds about 11.8%.
Do I use the pitch multiplier on a hip roof?
Yes. If every plane has the same pitch, the sloped area of a hip roof is the footprint times the same multiplier as a gable roof. Hips change the waste and the ridge-cap footage, not the area formula.
Does the multiplier include waste?
No. It gives the true surface area. Add a waste allowance separately when ordering material.
How do I find the footprint?
Measure the outside of the walls and add the overhang on every side, or measure the drip edge-to-drip edge dimensions from the ground. Building plans list the roof outline too.
Last reviewed 2026-10-16. Results are estimates for planning; confirm quantities with your supplier and requirements with your local building code and the manufacturer's instructions.