Roof Pitch Calculator · Free calculator

9/12 Roof Pitch Calculator

A 9/12 roof rises 9" for every 12" of run — an angle of 36.87° and a rafter multiplier of 1.250. It's a steep (needs roof jacks / harness) roof. Rise and rafter lengths for common runs are below.

Quick answer

Angle: 36.87°
Slope factor: 1.250
Grade: 75.0%
Rafter per ft of run: 1' 3"

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Roof pitch

9/12

36.9° · 75.0% grade

Angle

36.87°

Slope factor

1.250

Grade

75.0%

Rafter / ft run

15.00in

9/12 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' 0"	5' 0"
6'	4' 6"	7' 6"
8'	6' 0"	10' 0"
10'	7' 6"	12' 6"
12'	9' 0"	15' 0"
14'	10' 6"	17' 6"
16'	12' 0"	20' 0"

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(9 ÷ 12) = 36.87°
Slope factor = √(12² + 9²) ÷ 12 = √225 ÷ 12 = 1.2500
Grade = 9 ÷ 12 × 100 = 75.0%
Rafter length = run × 1.2500

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 9/12 roof pitch?

36.87 degrees (arctan 9/12). The plumb cut on a rafter is 36.9° and the seat cut is 53.1°.

What is the rafter multiplier for a 9/12 pitch?

1.2500. Multiply the run (in feet or inches) by it to get the rafter length: a 12 ft run gives 15' 0".

Is a 9/12 roof steep?

A 9/12 roof (36.9°) is steep (needs roof jacks / harness).

What is 9/12 pitch as a percentage?

75.00% grade — the roof rises 9 inches per foot, or 0.750 ft per ft.