12/12 roof pitch over a 30 ft span
| Rafter length15 ft half-span × 1.414 | 21.21ft |
|---|---|
| With a 12″ overhangwhat you actually cut | 22.63ft |
| Ridge height above the wall | 15ft |
| Pitch factor√(1 + (12/12)²) — multiply footprint by this for roof area | 1.414 |
| Angleset the mitre saw to this | 45° |
| As a percentage12/12 is 100%, not 12% | 100% |
| Roof area per 100 ft² of footprint | 141ft² |
| Asphalt shinglesingle layer of underlayment is enough | Standard |
| Walkableroof jacks or scaffolding | No |
- Pitch
- 12 /12
- Span
- 30 ft
Three ways of saying the same slope
12/12 means 12 inches of rise for every 12 inches of run. As an angle that is 45°. As a percentage it is 100%.
People quote one and mean another constantly. 12/12 is not 12%, and it is not 12° either.
What you actually cut
The rafter is the hypotenuse, so it is always longer than the half-span. Over a 30 ft span the half-run is 15 ft and the rafter is 21.21 ft — plus the overhang, giving 22.63 ft to cut.
The ridge sits 15 ft above the wall plate.
The number worth keeping
The pitch factor is 1.414. Multiply any footprint by it to get roof area: 100 ft² on the ground is 141 ft² of roof at this pitch.
That single number is the difference between ordering enough shingle and ordering the footprint.
Where the pitch decides the material
Below 2/12, asphalt shingles are not permitted at all — water moves too slowly and wind drives it back under the courses. That roof needs a low-slope membrane.
Between 2/12 and 4/12, shingles are allowed but the IRC requires two layers of underlayment (R905.2.7). It is the most commonly skipped requirement on shallow roofs and the one that surfaces as a leak two winters later.
At 4/12 and above, a single layer is enough.
And where it decides the day rate
Above 8/12 most people cannot stand on a roof. Roof jacks, planks or scaffolding change the cost of the job more than any material choice does.
Notes
- **Three ways of saying pitch, and they are not the same number.** 12/12 means 12 inches of rise per 12 inches of run. As an angle that is 45°. As a percentage it is 100%. People routinely quote one and mean another, and on a roof that mistake is expensive.
- The rafter is the hypotenuse, so it is always longer than the half-span: 15 ft of run becomes 21.21 ft of rafter, plus 1.41 ft for a 12″ overhang. Cut to 22.63 ft and trim on the roof rather than the other way round.
- At 12/12 a single layer of underlayment satisfies the code, and water sheds fast enough that wind-driven rain is not the concern it is on shallower roofs.
- The pitch factor 1.414 is the number to keep: multiply any footprint by it to get roof area. A 100 ft² footprint is 141 ft² of roof at this pitch.
- At 12/12 this is not a walkable roof for most people, and that changes the economics of doing the work yourself more than any material price does.
Nearby sizes
Read more
- Roof Pitch Multiplier Chart: The One Number That Turns Footprint Into RoofOne multiplication separates the area of the ground from the area of the roof. Skip it and every material order on the job is short.
- Minimum Roof Pitch for Asphalt Shingles: 2/12, With a Condition Most People SkipThe number everyone remembers is 2/12. The condition attached to it is the one that shows up as a leak two winters later.
- How to Measure Roof Pitch: Three Methods, and Which One to TrustPitch is rise over run, always over twelve. It is not a percentage and it is not an angle, and all three get quoted as if they were the same thing.
- Cost to Replace a Roof by Material: Price per Square Against Years of ServiceThe cheapest roof and the cheapest roof per year are rarely the same product, and which one you want depends on how long you are staying.