Roof Pitch Calculator · Free calculator

8/12 Roof Pitch Calculator

A 8/12 roof rises 8" for every 12" of run — an angle of 33.69° and a rafter multiplier of 1.202. It's a conventional (walkable up to ~6/12) roof. Rise and rafter lengths for common runs are below.

Quick answer

Angle: 33.69°
Slope factor: 1.202
Grade: 66.7%
Rafter per ft of run: 1' 2-7/16"

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Roof pitch

8/12

33.7° · 66.7% grade

Angle

33.69°

Slope factor

1.202

Grade

66.7%

Rafter / ft run

14.42in

8/12 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'	2' 8"	4' 9-11/16"
6'	4' 0"	7' 2-9/16"
8'	5' 4"	9' 7-3/8"
10'	6' 8"	12' 0-1/4"
12'	8' 0"	14' 5-1/16"
14'	9' 4"	16' 9-15/16"
16'	10' 8"	19' 2-3/4"

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(8 ÷ 12) = 33.69°
Slope factor = √(12² + 8²) ÷ 12 = √208 ÷ 12 = 1.2019
Grade = 8 ÷ 12 × 100 = 66.7%
Rafter length = run × 1.2019

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 8/12 roof pitch?

33.69 degrees (arctan 8/12). The plumb cut on a rafter is 33.7° and the seat cut is 56.3°.

What is the rafter multiplier for a 8/12 pitch?

1.2019. Multiply the run (in feet or inches) by it to get the rafter length: a 12 ft run gives 14' 5-1/16".

Is a 8/12 roof steep?

A 8/12 roof (33.7°) is conventional (walkable up to ~6/12).

What is 8/12 pitch as a percentage?

66.67% grade — the roof rises 8 inches per foot, or 0.667 ft per ft.