9/12 Pitch Roof on a 30 Foot Span
A steeper 9/12 house roof with a longer eave overhang.
Formula SSC-2026.1
9.00 / 12 · 36.87°
Pitch angle
36.87°
Multiplier 1.2500
Common rafter
20 ft 6 9/16 in
Tail 1 ft 10 1/2 in
Cuts
36.87° plumb
53.13° seat
Roof area
1,767 ft²
17.67 squares
Shingle bundles
59
3 bundles per square + 10% waste
Sheathing / felt
61 sheets
5 underlayment rolls
Delivery weight
7,076 lb
3,210 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 | 17.67 squares | 10% | 59 bundles | USD | $2,478.00 |
| 4x8 CDX sheathing | 55.2 sheets | 10% | 61 sheets | USD | $1,708.00 |
| Underlayment rolls | 1,767 ft² | overlap allowance | 5 rolls | USD | $275.00 |
| Ridge cap bundles | 43.0 ft ridge | — | 2 bundles | 25 ft per bundle | — |
| Pitch | Degrees | Multiplier | Rafter | Squares | Bundles | Sheets |
|---|---|---|---|---|---|---|
| 6.0/12 | 26.57° | 1.118 | 18 ft 4 9/16 in | 15.80 | 53 | 55 |
| 9.0/12 | 36.87° | 1.250 | 20 ft 6 9/16 in | 17.67 | 59 | 61 |
| 12.0/12 | 45.00° | 1.414 | 23 ft 2 15/16 in | 19.99 | 66 | 69 |
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.
Contractor sign-off
Project: 9/12 Pitch Roof on a 30 Foot Span
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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
Nine in twelve is about 36.87 degrees. The span is 30 ft, so each rafter run is 15 ft minus half the ridge. The overhang is 1.5 ft, which adds a longer tail and a larger rake allowance. Bundles, sheets, and underlayment all move with that extra slope length. The seat cut is 90 degrees minus the pitch angle. Birdsmouth depth for a 3.5 in wall plate is a field cut on top of these angles.
Code reference
IRC R802 and R905. Overhang length also affects uplift and lookouts, which are not sized here.
Field questions
- What is the seat-cut angle on a 9/12?
- It is 90 degrees minus the pitch angle. The pitch angle is the arctangent of 9/12, about 36.87 degrees, so the seat is about 53.13 degrees.
- Why does an 18 inch overhang add so much area?
- It lengthens every rafter and it is also applied at both rakes. Both effects are in the area.
- Are ridge bundles included?
- Gable roofs add ridge-cap bundles at 25 ft of ridge per bundle. A shed roof reports zero.