12/12 pitch at a glance
| Angle | 45.00° |
|---|---|
| Slope | 100.0% (12 in of rise per foot of run) |
| Pitch multiplier | 1.4142 |
| Hip and valley factor | 1.7321 per foot of common run |
| Type | Steep |
A 12/12 roof gives a 45 degree roof, where the rise equals the run, typical of A-frames and steep gable fronts. Its surface is 41.4% larger than the flat area it covers, so a 2,000 sq ft footprint needs about 2,828 sq ft of roofing.
Rafter length and ridge height by building width
On a simple gable at 12/12, a 24 ft wide building needs a 16 ft 11 5/8 in rafter measured from the outside of the wall to the center of the ridge, and a 32 ft wide one needs 22 ft 7 1/2 in. For 12/12, 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 | 10 ft 0 in | 14 ft 1 3/4 in |
| 24 ft | 12 ft | 12 ft 0 in | 16 ft 11 5/8 in |
| 28 ft | 14 ft | 14 ft 0 in | 19 ft 9 5/8 in |
| 32 ft | 16 ft | 16 ft 0 in | 22 ft 7 1/2 in |
| 36 ft | 18 ft | 18 ft 0 in | 25 ft 5 1/2 in |
Roof area, squares and shingle bundles
At 12/12 the multiplier is 1.414, so every 1,000 sq ft of footprint becomes 1,414 sq ft of roof. The order column for 12/12 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 | 1,697 sq ft | 17.0 | 18.7 | 57 |
| 1,600 sq ft | 2,263 sq ft | 22.6 | 24.9 | 75 |
| 2,000 sq ft | 2,828 sq ft | 28.3 | 31.1 | 94 |
| 2,400 sq ft | 3,394 sq ft | 33.9 | 37.3 | 113 |
What roofing can go on a 12/12 roof?
Allowed: Asphalt shingles; Clay and concrete tile; Metal shingles; Wood shingles; Slate.
At 12/12, 5 of the 5 roofing types in the code table are allowed outright. At 12/12 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 12/12 roof?
At 12/12: This roof is too steep to walk safely. Work is done from roof jacks, staging or a lift with fall protection, and roofing labor is priced higher because every bundle has to be handled on the slope.
How to check that your roof is 12/12
Level a board against a rafter in the attic, mark 12 inches along it and measure straight down: 12 inches means 12/12. A phone level that reads 45.0 degrees says the same thing, and the roof pitch calculator converts either reading.
A worked example: a 28 by 40 ft house at 12/12
Half the width is 14 ft, so the ridge sits 14 ft 0 in above the top plate and each rafter is 19 ft 9 5/8 in long before overhang. The roof surface is 28 × 40 × 1.414 = 1,584 sq ft, which is 15.8 squares. On a 40 ft long 12/12 roof, rafters at 24 inches on center come to 21 per side.
Cutting rafters at 12/12
Each foot of horizontal run adds 16.97 inches of rafter length. On a speed square, put the pivot on the board edge and swing it until the 12 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 45.0 degrees off square, and the horizontal seat cut is 45.0 degrees off square.
Attic headroom at 12/12
At least 5 ft of headroom for 18.0 ft of the 28 ft width, measured from the top plate. A usable attic room needs more than that at 12/12, because the floor joists and ceiling take up some of the height.
12/12 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 |
|---|---|---|---|---|
| 10/12 | 39.8° | 1.302 | 2,603 sq ft | -225 sq ft |
| 11/12 | 42.5° | 1.357 | 2,713 sq ft | -115 sq ft |
| 12/12 | 45.0° | 1.414 | 2,828 sq ft | this pitch |
Other pitches: 11/12 pitch, or the full roof pitch chart.
Frequently asked questions
What angle is a 12/12 roof pitch?
45.00 degrees. The slope is 100.0%.
What is the multiplier for a 12/12 pitch?
1.4142. Multiply the flat footprint by 1.414 to get the roof surface area.
Can you put shingles on a 12/12 roof?
Asphalt shingles can go on a 12/12 roof under the International Residential Code.
Is a 12/12 roof walkable?
This roof is too steep to walk safely.
Sources
- Cornell LII: 29 CFR 1926.500, definitions of low-slope and steep roofs
- LSU AgCenter: Code-Compliant Roofing Installations (PDF)