3/12 roof pitch over a 48 ft span
| Rafter length24 ft half-span × 1.031 | 24.74ft |
|---|---|
| With a 12″ overhangwhat you actually cut | 25.77ft |
| Ridge height above the wall | 6ft |
| Pitch factor√(1 + (3/12)²) — multiply footprint by this for roof area | 1.031 |
| Angleset the mitre saw to this | 14° |
| As a percentage3/12 is 25%, not 3% | 25% |
| Roof area per 100 ft² of footprint | 103ft² |
| Asphalt shingleIRC R905.2.7 — 2/12 to 4/12 requires two layers | Double underlayment |
| Walkableproper footwear and a harness | Yes, with care |
- Pitch
- 3 /12
- Span
- 48 ft
Three ways of saying the same slope
3/12 means 3 inches of rise for every 12 inches of run. As an angle that is 14°. As a percentage it is 25%.
People quote one and mean another constantly. 3/12 is not 3%, and it is not 3° either.
What you actually cut
The rafter is the hypotenuse, so it is always longer than the half-span. Over a 48 ft span the half-run is 24 ft and the rafter is 24.74 ft — plus the overhang, giving 25.77 ft to cut.
The ridge sits 6 ft above the wall plate.
The number worth keeping
The pitch factor is 1.031. Multiply any footprint by it to get roof area: 100 ft² on the ground is 103 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.** 3/12 means 3 inches of rise per 12 inches of run. As an angle that is 14°. As a percentage it is 25%. 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: 24 ft of run becomes 24.74 ft of rafter, plus 1.03 ft for a 12″ overhang. Cut to 25.77 ft and trim on the roof rather than the other way round.
- Between 2/12 and 4/12 the IRC requires **two layers of underlayment** under asphalt shingles (R905.2.7). It is the single most commonly skipped requirement on shallow roofs, and it is the one that shows up as a leak two winters later.
- The pitch factor 1.031 is the number to keep: multiply any footprint by it to get roof area. A 100 ft² footprint is 103 ft² of roof at this pitch.
- 3/12 is walkable with proper footwear and a harness. Wet, mossy or frosted, none of that holds.
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.
- Repair or Replace: The Four Tests That Decide, and the One That Overrides ThemMost roofs get repaired three times before they get replaced, and the three repairs cost more than the replacement would have.