3/12 Roof Pitch: 14.0° Angle & Rafter Multiplier

Run	Total rise	Rafter length
4'	1' 0"	4' 1-1/2"
6'	1' 6"	6' 2-3/16"
8'	2' 0"	8' 2-15/16"
10'	2' 6"	10' 3-11/16"
12'	3' 0"	12' 4-7/16"
14'	3' 6"	14' 5-3/16"
16'	4' 0"	16' 5-15/16"

Run

Total rise

Rafter length

1' 0"

4' 1-1/2"

1' 6"

6' 2-3/16"

2' 0"

8' 2-15/16"

10'

2' 6"

10' 3-11/16"

12'

3' 0"

12' 4-7/16"

14'

3' 6"

14' 5-3/16"

16'

4' 0"

16' 5-15/16"

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(3 ÷ 12) = 14.04°
Slope factor = √(12² + 3²) ÷ 12 = √153 ÷ 12 = 1.0308
Grade = 3 ÷ 12 × 100 = 25.0%
Rafter length = run × 1.0308

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 3/12 roof pitch?

14.04 degrees (arctan 3/12). The plumb cut on a rafter is 14.0° and the seat cut is 76.0°.

What is the rafter multiplier for a 3/12 pitch?

1.0308. Multiply the run (in feet or inches) by it to get the rafter length: a 12 ft run gives 12' 4-7/16".

Is a 3/12 roof steep?

A 3/12 roof (14.0°) is low-slope (shingles need double underlayment).

What is 3/12 pitch as a percentage?

25.00% grade — the roof rises 3 inches per foot, or 0.250 ft per ft.

3/12 Roof Pitch Calculator

Quick answer

▍Roof Pitch Calculator

3/12 rise and rafter length by run