3/12 pitch at a glance
| Angle | 14.04° |
|---|---|
| Slope | 25.0% (3 in of rise per foot of run) |
| Pitch multiplier | 1.0308 |
| Hip and valley factor | 1.4361 per foot of common run |
| Type | Low pitch |
A 3/12 roof gives a low, wide profile, common on ranch houses, porch roofs and metal buildings. Its surface is 3.1% larger than the flat area it covers, so a 2,000 sq ft footprint needs about 2,062 sq ft of roofing.
Rafter length and ridge height by building width
On a simple gable at 3/12, a 24 ft wide building needs a 12 ft 4 3/8 in rafter measured from the outside of the wall to the center of the ridge, and a 32 ft wide one needs 16 ft 5 7/8 in. For 3/12, add your overhang and take off half the ridge board when you cut. Other spans are in the rafter length calculator.
| Building width | Run | Ridge rise | Rafter length |
|---|---|---|---|
| 20 ft | 10 ft | 2 ft 6 in | 10 ft 3 3/4 in |
| 24 ft | 12 ft | 3 ft 0 in | 12 ft 4 3/8 in |
| 28 ft | 14 ft | 3 ft 6 in | 14 ft 5 1/8 in |
| 32 ft | 16 ft | 4 ft 0 in | 16 ft 5 7/8 in |
| 36 ft | 18 ft | 4 ft 6 in | 18 ft 6 5/8 in |
Roof area, squares and shingle bundles
At 3/12 the multiplier is 1.031, so every 1,000 sq ft of footprint becomes 1,031 sq ft of roof. The order column for 3/12 adds 10% for waste and counts 3 bundles per square; include the eaves in your footprint or let the roofing squares calculator do it.
| Footprint | Roof area | Squares | Squares + 10% | Bundles |
|---|---|---|---|---|
| 1,200 sq ft | 1,237 sq ft | 12.4 | 13.6 | 41 |
| 1,600 sq ft | 1,649 sq ft | 16.5 | 18.1 | 55 |
| 2,000 sq ft | 2,062 sq ft | 20.6 | 22.7 | 69 |
| 2,400 sq ft | 2,474 sq ft | 24.7 | 27.2 | 82 |
What roofing can go on a 3/12 roof?
Allowed: Built-up, modified bitumen and single-ply membrane; Mineral-surfaced roll roofing; Metal shingles.
Allowed with conditions: Asphalt shingles, with double underlayment below 4/12; Clay and concrete tile, with double underlayment below 4/12; Wood shingles, with shorter exposure below 4/12.
Too low for: Slate (needs 4/12).
At 3/12, 3 of the 7 roofing types in the code table are allowed outright and 3 more with conditions. At 3/12 they follow the minimum slopes in the International Residential Code; local code and the maker's installation instructions can be stricter.
Can you walk on a 3/12 roof?
At 3/12: OSHA classes this as a low-slope roof (4/12 or less). Most people can walk it in soft-soled shoes in dry weather, though any roof edge is still a fall hazard.
How to check that your roof is 3/12
Level a board against a rafter in the attic, mark 12 inches along it and measure straight down: 3 inches means 3/12. A phone level that reads 14.0 degrees says the same thing, and the roof pitch calculator converts either reading.
A worked example: a 28 by 40 ft house at 3/12
Half the width is 14 ft, so the ridge sits 3 ft 6 in above the top plate and each rafter is 14 ft 5 1/8 in long before overhang. The roof surface is 28 × 40 × 1.031 = 1,154 sq ft, which is 11.5 squares. On a 40 ft long 3/12 roof, rafters at 24 inches on center come to 21 per side.
Cutting rafters at 3/12
Each foot of horizontal run adds 12.37 inches of rafter length. On a speed square, put the pivot on the board edge and swing it until the 3 mark on the common scale lines up with the same edge; the pivot edge now scribes the plumb cut. On a miter saw, the plumb cut is set 14.0 degrees off square, and the horizontal seat cut is 76.0 degrees off square.
Attic headroom at 3/12
Never reaches 5 ft of headroom in a 28 ft wide house, because the ridge is only 3 ft 6 in above the top plate. Treat the space as storage or not at all.
3/12 compared with the pitches next to it
The table uses the same 2,000 sq ft footprint for every row, so the last column shows the extra roofing a steeper pitch costs you.
| Pitch | Angle | Multiplier | Roof area | Difference |
|---|---|---|---|---|
| 1/12 | 4.8° | 1.003 | 2,007 sq ft | -55 sq ft |
| 2/12 | 9.5° | 1.014 | 2,028 sq ft | -34 sq ft |
| 3/12 | 14.0° | 1.031 | 2,062 sq ft | this pitch |
| 4/12 | 18.4° | 1.054 | 2,108 sq ft | +47 sq ft |
| 5/12 | 22.6° | 1.083 | 2,167 sq ft | +105 sq ft |
Other pitches: 2/12 pitch and 4/12 pitch, or the full roof pitch chart.
Frequently asked questions
What angle is a 3/12 roof pitch?
14.04 degrees. The slope is 25.0%.
What is the multiplier for a 3/12 pitch?
1.0308. Multiply the flat footprint by 1.031 to get the roof surface area.
Can you put shingles on a 3/12 roof?
Asphalt shingles can, with double underlayment, go on a 3/12 roof under the International Residential Code.
Is a 3/12 roof walkable?
OSHA classes this as a low-slope roof (4/12 or less).
Sources
- Cornell LII: 29 CFR 1926.500, definitions of low-slope and steep roofs
- LSU AgCenter: Code-Compliant Roofing Installations (PDF)