Roof Pitch Calculator · Free calculator

1/12 Roof Pitch Calculator

A 1/12 roof rises 1" for every 12" of run — an angle of 4.76° and a rafter multiplier of 1.003. It's a flat / low-slope (membrane roofing only) roof. Rise and rafter lengths for common runs are below.

Quick answer

Angle: 4.76°
Slope factor: 1.003
Grade: 8.3%
Rafter per ft of run: 1' 0-1/16"

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Roof pitch

1/12

4.8° · 8.3% grade

Angle

4.76°

Slope factor

1.003

Grade

8.3%

Rafter / ft run

12.04in

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

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(1 ÷ 12) = 4.76°
Slope factor = √(12² + 1²) ÷ 12 = √145 ÷ 12 = 1.0035
Grade = 1 ÷ 12 × 100 = 8.3%
Rafter length = run × 1.0035

Trade pro tips

Below 4/12, asphalt shingles need a double layer of underlayment (IRC R905.2.2); below 2/12 use membrane roofing.
Up to about 6/12 most roofers can walk the roof without jacks.
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 1/12 roof pitch?

4.76 degrees (arctan 1/12). The plumb cut on a rafter is 4.8° and the seat cut is 85.2°.

What is the rafter multiplier for a 1/12 pitch?

1.0035. Multiply the run (in feet or inches) by it to get the rafter length: a 12 ft run gives 12' 0-1/2".

Is a 1/12 roof steep?

A 1/12 roof (4.8°) is flat / low-slope (membrane roofing only).

What is 1/12 pitch as a percentage?

8.33% grade — the roof rises 1 inches per foot, or 0.083 ft per ft.