Subtract half the ridge board thickness from each rafter when you cut it.
How the roof pitch calculator works
Pitch is rise over run. The angle in degrees is the arctangent of the pitch divided by 12, and the pitch multiplier is the square root of one plus the pitch over 12, squared. The multiplier tells you how much longer the sloped surface is than its flat footprint: 1.054 for 4/12, 1.118 for 6/12 and 1.414 for 12/12.
The common rafter covers half the building width plus the overhang, so its length is that horizontal run times the multiplier. The roof surface is the footprint, including the overhang on every side, times the same multiplier. For a hip roof with the same pitch on all sides this gives the same area as a gable, but the extra cuts at the hips mean more waste, so RoofMath switches to 15% when you choose a hip.
Roof pitch chart
| Pitch | Degrees | Grade | Multiplier |
|---|---|---|---|
| 1/12 | 4.8° | 8% | 1.003 |
| 2/12 | 9.5° | 17% | 1.014 |
| 3/12 | 14.0° | 25% | 1.031 |
| 4/12 | 18.4° | 33% | 1.054 |
| 5/12 | 22.6° | 42% | 1.083 |
| 6/12 | 26.6° | 50% | 1.118 |
| 7/12 | 30.3° | 58% | 1.158 |
| 8/12 | 33.7° | 67% | 1.202 |
| 9/12 | 36.9° | 75% | 1.250 |
| 10/12 | 39.8° | 83% | 1.302 |
| 11/12 | 42.5° | 92% | 1.357 |
| 12/12 | 45.0° | 100% | 1.414 |
| 13/12 | 47.3° | 108% | 1.474 |
| 14/12 | 49.4° | 117% | 1.537 |
| 15/12 | 51.3° | 125% | 1.601 |
| 16/12 | 53.1° | 133% | 1.667 |
| 17/12 | 54.8° | 142% | 1.734 |
| 18/12 | 56.3° | 150% | 1.803 |
| 19/12 | 57.7° | 158% | 1.873 |
| 20/12 | 59.0° | 167% | 1.944 |
| 21/12 | 60.3° | 175% | 2.016 |
| 22/12 | 61.4° | 183% | 2.088 |
| 23/12 | 62.4° | 192% | 2.162 |
| 24/12 | 63.4° | 200% | 2.236 |
Ordering materials
Shingles are sold in bundles, usually three to a square. Order whole bundles and add starter strips for the eaves and rakes and ridge cap shingles for the ridge and hips. Underlayment, drip edge and flashing are measured separately. A roofer's measurement will always be the safest basis for an order. Everything is calculated in your browser.
Frequently asked questions
How do I calculate rafter length?
Rafter length = (half the span + overhang) × √(1 + (pitch ÷ 12)²).
What is the pitch multiplier?
The ratio of sloped length to horizontal run; 1.118 for a 6/12 roof.
What lumber size do I need?
Buy the next standard length above the rafter length, and check span tables for depth.
Is RoofMath free?
Yes, with no sign-up or ads.
Are my measurements saved?
Yes, in this browser only.