Roof Pitch and Common Rafter Calculator
Measure a gable or shed roof from span, length, pitch, and eave overhang. The truss triangle scales as you move the pitch.
Formula SSC-2026.1
6.00 / 12 · 26.57°
Pitch angle
26.57°
Multiplier 1.1180
Common rafter
14 ft 5 9/16 in
Tail 1 ft 1 7/16 in
Cuts
26.57° plumb
63.43° seat
Roof area
1,215 ft²
12.15 squares
Shingle bundles
41
3 bundles per square + 10% waste
Sheathing / felt
42 sheets
4 underlayment rolls
Delivery weight
4,914 lb
2,229 kg · 70 lb bundles, 46 lb sheets, 28 lb rolls
Rise and run are scaled from the span, overhang, and pitch. The small triangle is the pitch ratio.
| Item | Net | Waste | Order | Unit price | Line total |
|---|---|---|---|---|---|
| Architectural shingle bundles | 12.15 squares | 10% | 41 bundles | USD | $1,722.00 |
| 4x8 CDX sheathing | 38.0 sheets | 10% | 42 sheets | USD | $1,176.00 |
| Underlayment rolls | 1,215 ft² | overlap allowance | 4 rolls | USD | $220.00 |
| Ridge cap bundles | 42.0 ft ridge | — | 2 bundles | 25 ft per bundle | — |
| Pitch | Degrees | Multiplier | Rafter | Squares | Bundles | Sheets |
|---|---|---|---|---|---|---|
| 4.0/12 | 18.43° | 1.054 | 13 ft 7 5/8 in | 11.46 | 38 | 40 |
| 6.0/12 | 26.57° | 1.118 | 14 ft 5 9/16 in | 12.15 | 41 | 42 |
| 8.0/12 | 33.69° | 1.202 | 15 ft 6 9/16 in | 13.06 | 44 | 45 |
| 12.0/12 | 45.00° | 1.414 | 18 ft 3 9/16 in | 15.37 | 51 | 53 |
Show the deterministic formula
Angle = arctan(rise ÷ 12). Multiplier = √(1 + (rise ÷ 12)²). Gable run = span ÷ 2 − ridge thickness ÷ 2. Rafter length = (run + horizontal overhang) × multiplier.
Plumb cut = pitch angle. Seat cut = 90° − pitch angle. Area uses both slopes on a gable and one slope on a shed, and it includes a rake overhang equal to the eave overhang.
Bundles = ceiling of squares × 3 × 1.10. Sheets = ceiling of area × 1.10 ÷ 32. Rolls use 360 ft² effective coverage. Delivery weight plans 70 lb per bundle, 46 lb per sheet, and 28 lb per roll, then the header converts that pound total at 1 lb = 0.45359237 kg.
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This sheet is an estimate for ordering and layout discussion. It is not a sealed design. Confirm the quantities with a licensed local professional and the authority having jurisdiction before you buy or build.
Engineering notes
Pitch angle is the arctangent of rise over 12. The pitch multiplier is the square root of 1 plus rise-over-12 squared. On a gable, the rafter run is half the span minus half the ridge thickness. The common rafter length multiplies run plus horizontal overhang by the multiplier. A shed uses the full span as the run and adds overhang at both the low eave and the high side.
Shingle bundles are 3 per square plus 10 percent waste. Sheathing is 4x8 sheets at 32 square feet with 10 percent waste. Underlayment rolls are counted at 360 square feet effective on a 400 square foot roll. Ridge cap is estimated at 25 feet per bundle on gables.
Code reference
IRC R802 for wood roof framing and R905 for roof coverings. Pitch, span, and overhang still have to satisfy local snow, wind, and tie-down rules. This page does not size rafters for load.
Field questions
- What is the difference between plumb cut and seat cut?
- The plumb cut matches the roof angle and is the vertical cut at the ridge. The seat cut is the complementary angle, 90 degrees minus the pitch angle, and it sits level on the wall plate.
- Does the rafter length include the tail?
- Yes. The reported common rafter length includes the body to the ridge face and the tail past the plate. The tail length is also listed on its own.
- Are the shingle bundles a manufacturer takeoff?
- They are a planning count at 3 bundles per 100 square feet plus 10 percent waste. Hip, valley, starter, and waste at cut edges can change the order.