1/12 pitch at a glance
| Angle | 4.76° |
|---|---|
| Slope | 8.3% (1 in of rise per foot of run) |
| Pitch multiplier | 1.0035 |
| Hip and valley factor | 1.4167 per foot of common run |
| Type | Low slope |
A 1/12 roof gives a nearly flat look, common on porch roofs, carports and lean-to sheds. Its surface is 0.3% larger than the flat area it covers, so a 2,000 sq ft footprint needs about 2,007 sq ft of roofing.
Rafter length and ridge height by building width
On a simple gable at 1/12, a 24 ft wide building needs a 12 ft 0 1/2 in rafter measured from the outside of the wall to the center of the ridge, and a 32 ft wide one needs 16 ft 0 5/8 in. For 1/12, 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 | 0 ft 10 in | 10 ft 0 3/8 in |
| 24 ft | 12 ft | 1 ft 0 in | 12 ft 0 1/2 in |
| 28 ft | 14 ft | 1 ft 2 in | 14 ft 0 5/8 in |
| 32 ft | 16 ft | 1 ft 4 in | 16 ft 0 5/8 in |
| 36 ft | 18 ft | 1 ft 6 in | 18 ft 0 3/4 in |
Roof area, squares and shingle bundles
At 1/12 the multiplier is 1.003, so every 1,000 sq ft of footprint becomes 1,003 sq ft of roof. The order column for 1/12 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 | 1,204 sq ft | 12.0 | 13.2 | 40 |
| 1,600 sq ft | 1,606 sq ft | 16.1 | 17.7 | 53 |
| 2,000 sq ft | 2,007 sq ft | 20.1 | 22.1 | 67 |
| 2,400 sq ft | 2,408 sq ft | 24.1 | 26.5 | 80 |
What roofing can go on a 1/12 roof?
Allowed: Built-up, modified bitumen and single-ply membrane; Mineral-surfaced roll roofing.
Too low for: Asphalt shingles (needs 2/12); Clay and concrete tile (needs 2.5/12); Metal shingles (needs 3/12); Wood shingles (needs 3/12); Slate (needs 4/12).
At 1/12, 2 of the 7 roofing types in the code table are allowed outright. At 1/12 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 1/12 roof?
At 1/12: 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 1/12
Level a board against a rafter in the attic, mark 12 inches along it and measure straight down: 1 inches means 1/12. A phone level that reads 4.8 degrees says the same thing, and the roof pitch calculator converts either reading.
A worked example: a 28 by 40 ft house at 1/12
Half the width is 14 ft, so the ridge sits 1 ft 2 in above the top plate and each rafter is 14 ft 0 5/8 in long before overhang. The roof surface is 28 × 40 × 1.003 = 1,124 sq ft, which is 11.2 squares. On a 40 ft long 1/12 roof, rafters at 24 inches on center come to 21 per side.
Cutting rafters at 1/12
Each foot of horizontal run adds 12.04 inches of rafter length. On a speed square, put the pivot on the board edge and swing it until the 1 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 4.8 degrees off square, and the horizontal seat cut is 85.2 degrees off square.
Attic headroom at 1/12
Never reaches 5 ft of headroom in a 28 ft wide house, because the ridge is only 1 ft 2 in above the top plate. Treat the space as storage or not at all.
1/12 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 | this pitch |
| 2/12 | 9.5° | 1.014 | 2,028 sq ft | +21 sq ft |
| 3/12 | 14.0° | 1.031 | 2,062 sq ft | +55 sq ft |
Other pitches: 2/12 pitch, or the full roof pitch chart.
Frequently asked questions
What angle is a 1/12 roof pitch?
4.76 degrees. The slope is 8.3%.
What is the multiplier for a 1/12 pitch?
1.0035. Multiply the flat footprint by 1.003 to get the roof surface area.
Can you put shingles on a 1/12 roof?
Asphalt shingles cannot go on a 1/12 roof under the International Residential Code. Below 2/12 a membrane roof is used instead.
Is a 1/12 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)