4/12 vs 8/12 Gable Roof Material Estimator
The same 28 by 40 ft gable priced at 4/12, with 8/12 in the comparison table.
Formula SSC-2026.1
4.00 / 12 · 18.43°
Pitch angle
18.43°
Multiplier 1.0541
Common rafter
15 ft 8 15/16 in
Tail 1 ft 5/8 in
Cuts
18.43° plumb
71.57° seat
Roof area
1,323 ft²
13.23 squares
Shingle bundles
44
3 bundles per square + 10% waste
Sheathing / felt
46 sheets
4 underlayment rolls
Delivery weight
5,308 lb
2,408 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 | 13.23 squares | 10% | 44 bundles | USD | $1,848.00 |
| 4x8 CDX sheathing | 41.3 sheets | 10% | 46 sheets | USD | $1,288.00 |
| Underlayment rolls | 1,323 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 | 15 ft 8 15/16 in | 13.23 | 44 | 46 |
| 6.0/12 | 26.57° | 1.118 | 16 ft 8 7/16 in | 14.03 | 47 | 49 |
| 8.0/12 | 33.69° | 1.202 | 17 ft 11 7/16 in | 15.08 | 50 | 52 |
| 12.0/12 | 45.00° | 1.414 | 21 ft 1 1/2 in | 17.74 | 59 | 61 |
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: 4/12 vs 8/12 Gable Roof Material Estimator
Prepared by
Date
Verified by (PE / electrician / inspector)
License number
Signature
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
A steeper pitch lengthens the rafter and grows the roof area even though the building footprint stays 28 by 40 ft. The working pitch is 4/12. The table repeats 4/12 and 8/12, and also shows 6/12 and 12/12 so the jump is not a single step. Overhang is 1 ft and the ridge is 1.5 in. Order the steeper roof only after you also check tie-downs and the rafter size, which this quantity tool does not do.
Code reference
IRC R802 and R905. Pitch changes area and length only in this calculator. Structural capacity is a separate check.
Field questions
- Why does 8/12 need more shingles than 4/12?
- The slope length is the horizontal run times a larger multiplier. Both slopes of the gable get longer, so the squares go up.
- Is the building length included twice?
- No. Each slope is rafter length times the building length plus rake overhangs. A gable has two slopes.
- What overhang is in the area?
- One foot at the eaves, and the same one foot is used as the rake overhang at each end of the ridge.