Roof Pitch Calculator · Free calculator

11/12 Roof Pitch Calculator

A 11/12 roof rises 11" for every 12" of run — an angle of 42.51° and a rafter multiplier of 1.357. It's a steep (needs roof jacks / harness) roof. Rise and rafter lengths for common runs are below.

Quick answer

Angle: 42.51°
Slope factor: 1.357
Grade: 91.7%
Rafter per ft of run: 1' 4-1/4"

▍Roof Pitch Calculator

LIVE

Roof pitch

11/12

42.5° · 91.7% grade

Angle

42.51°

Slope factor

1.357

Grade

91.7%

Rafter / ft run

16.28in

11/12 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' 8"	5' 5-1/8"
6'	5' 6"	8' 1-11/16"
8'	7' 4"	10' 10-1/4"
10'	9' 2"	13' 6-13/16"
12'	11' 0"	16' 3-3/8"
14'	12' 10"	18' 11-7/8"
16'	14' 8"	21' 8-7/16"

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(11 ÷ 12) = 42.51°
Slope factor = √(12² + 11²) ÷ 12 = √265 ÷ 12 = 1.3566
Grade = 11 ÷ 12 × 100 = 91.7%
Rafter length = run × 1.3566

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 11/12 roof pitch?

42.51 degrees (arctan 11/12). The plumb cut on a rafter is 42.5° and the seat cut is 47.5°.

What is the rafter multiplier for a 11/12 pitch?

1.3566. Multiply the run (in feet or inches) by it to get the rafter length: a 12 ft run gives 16' 3-3/8".

Is a 11/12 roof steep?

A 11/12 roof (42.5°) is steep (needs roof jacks / harness).

What is 11/12 pitch as a percentage?

91.67% grade — the roof rises 11 inches per foot, or 0.917 ft per ft.