Roof Pitch Calculator · Free calculator

10/12 Roof Pitch Calculator

A 10/12 roof rises 10" for every 12" of run — an angle of 39.81° and a rafter multiplier of 1.302. It's a steep (needs roof jacks / harness) roof. Rise and rafter lengths for common runs are below.

Quick answer

Angle: 39.81°
Slope factor: 1.302
Grade: 83.3%
Rafter per ft of run: 1' 3-5/8"

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Roof pitch

10/12

39.8° · 83.3% grade

Angle

39.81°

Slope factor

1.302

Grade

83.3%

Rafter / ft run

15.62in

10/12 rise and rafter length by run

Run = half the building span. Rafter length is ridge to building line, no overhang.

Run	Total rise	Rafter length
4'	3' 4"	5' 2-1/2"
6'	5' 0"	7' 9-3/4"
8'	6' 8"	10' 4-15/16"
10'	8' 4"	13' 0-3/16"
12'	10' 0"	15' 7-7/16"
14'	11' 8"	18' 2-11/16"
16'	13' 4"	20' 9-15/16"

Roof pitch comparison chart

Pitch	Angle	Slope factor	Grade	Class
1/12	4.76°	1.003	8.3%	flat / low-slope
2/12	9.46°	1.014	16.7%	low-slope
3/12	14.04°	1.031	25.0%	low-slope
4/12	18.43°	1.054	33.3%	conventional
5/12	22.62°	1.083	41.7%	conventional
6/12	26.57°	1.118	50.0%	conventional
7/12	30.26°	1.158	58.3%	conventional
8/12	33.69°	1.202	66.7%	conventional
9/12	36.87°	1.250	75.0%	steep
10/12	39.81°	1.302	83.3%	steep
11/12	42.51°	1.357	91.7%	steep
12/12	45.00°	1.414	100.0%	steep

How the math works

Angle = arctan(rise ÷ run) = arctan(10 ÷ 12) = 39.81°
Slope factor = √(12² + 10²) ÷ 12 = √244 ÷ 12 = 1.3017
Grade = 10 ÷ 12 × 100 = 83.3%
Rafter length = run × 1.3017

Trade pro tips

Asphalt shingles are fine at this pitch with standard underlayment.
At 8/12 and steeper, use roof jacks and a fall-arrest harness.
Measure pitch in the attic: level 12" off a rafter and measure straight up to its bottom edge.

Frequently asked questions

What angle is a 10/12 roof pitch?

39.81 degrees (arctan 10/12). The plumb cut on a rafter is 39.8° and the seat cut is 50.2°.

What is the rafter multiplier for a 10/12 pitch?

1.3017. Multiply the run (in feet or inches) by it to get the rafter length: a 12 ft run gives 15' 7-7/16".

Is a 10/12 roof steep?

A 10/12 roof (39.8°) is steep (needs roof jacks / harness).

What is 10/12 pitch as a percentage?

83.33% grade — the roof rises 10 inches per foot, or 0.833 ft per ft.