Roof Pitch Calculator · Free calculator

12/12 Roof Pitch Calculator

A 12/12 roof rises 12" for every 12" of run — an angle of 45.00° and a rafter multiplier of 1.414. It's a steep (needs roof jacks / harness) roof. Rise and rafter lengths for common runs are below.

Quick answer

Angle: 45.00°
Slope factor: 1.414
Grade: 100.0%
Rafter per ft of run: 1' 5"

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Roof pitch

12/12

45.0° · 100.0% grade

Angle

45.00°

Slope factor

1.414

Grade

100.0%

Rafter / ft run

16.97in

12/12 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'	4' 0"	5' 7-7/8"
6'	6' 0"	8' 5-13/16"
8'	8' 0"	11' 3-3/4"
10'	10' 0"	14' 1-11/16"
12'	12' 0"	16' 11-5/8"
14'	14' 0"	19' 9-9/16"
16'	16' 0"	22' 7-1/2"

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(12 ÷ 12) = 45.00°
Slope factor = √(12² + 12²) ÷ 12 = √288 ÷ 12 = 1.4142
Grade = 12 ÷ 12 × 100 = 100.0%
Rafter length = run × 1.4142

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 12/12 roof pitch?

45.00 degrees (arctan 12/12). The plumb cut on a rafter is 45.0° and the seat cut is 45.0°.

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

1.4142. Multiply the run (in feet or inches) by it to get the rafter length: a 12 ft run gives 16' 11-5/8".

Is a 12/12 roof steep?

A 12/12 roof (45.0°) is steep (needs roof jacks / harness).

What is 12/12 pitch as a percentage?

100.00% grade — the roof rises 12 inches per foot, or 1.000 ft per ft.