How it works
Roof pitch is how many inches the roof rises for every 12 inches of level run. If you measure 4 inches of rise over 8 inches of run, that's 6/12. The angle is the arctangent of rise ÷ run.
The pitch multiplier is the length of the sloped roof for each foot of level run. Rafter length = (half the span + overhang) × multiplier. Roof area for a simple gable = 2 × rafter length × building length. Roofers sell by the square, which is 100 sq ft. Add 10 to 15% for waste, hips and valleys.
Pitch = rise ÷ run × 12 · Angle = atan(rise ÷ run) · Multiplier = √(1 + (rise ÷ run)²)
Roof pitch chart
| Pitch | Angle | Slope | Multiplier |
|---|---|---|---|
| 1/12 | 4.76° | 8.3% | 1.003 |
| 2/12 | 9.46° | 16.7% | 1.014 |
| 3/12 | 14.04° | 25.0% | 1.031 |
| 4/12 | 18.43° | 33.3% | 1.054 |
| 5/12 | 22.62° | 41.7% | 1.083 |
| 6/12 | 26.57° | 50.0% | 1.118 |
| 7/12 | 30.26° | 58.3% | 1.158 |
| 8/12 | 33.69° | 66.7% | 1.202 |
| 9/12 | 36.87° | 75.0% | 1.250 |
| 10/12 | 39.81° | 83.3% | 1.302 |
| 11/12 | 42.51° | 91.7% | 1.357 |
| 12/12 | 45.00° | 100.0% | 1.414 |
Worked example
A 6/12 roof is 26.57° (50% slope), multiplier 1.118. On a 30 ft wide, 40 ft long house with a 12-inch overhang, each rafter is (15 + 1) × 1.118 = 17 ft 10¾ in, and the roof is about 1,431 sq ft, or 14.3 squares, before waste.
Frequently asked questions
How do I measure roof pitch?
Hold a level against the roof (or a rafter in the attic), mark 12 inches along the level, and measure straight up from that mark to the roof. That number over 12 is the pitch.
What is a 4/12 pitch in degrees?
18.43°. A 6/12 is 26.57°, 8/12 is 33.69°, and 12/12 is 45°.
What is the most common roof pitch?
On US houses, 4/12 to 9/12. Below 3/12 is a low-slope roof that needs special roofing; above 9/12 is steep and harder to walk on.
How many squares is my roof?
Divide the roof area in square feet by 100. A 1,500 sq ft roof is 15 squares. Buy about 10% extra for cuts and waste.