Worked example: a 36 × 24 ft gable roof
Hypothetical educational example from plan dimensions: 36 by 24 feet, 1 foot of overhang, a 6/12 gable, and the calculator’s 10% gable waste. It is not a price and not an instruction to walk on the roof.
Inputs
| Style | gable |
|---|---|
| Building | 36 ft along the ridge × 24 ft |
| Overhang | 1 ft |
| Pitch | 6/12 |
| Waste | blank, so 10% |
| Bundles per square | 3, the common asphalt packaging, not every product |
| New decking | no |
Arithmetic
- Plan with overhangs = 38 × 26 = 988 sq ft.
- Pitch factor = √(1 + 0.5²) = 1.118. Roof area ≈ 1,105 sq ft.
- Squares = 1,105 × 1.10 ÷ 100 = 12.15. Bundles = 12.15 × 3 = 36.45, which rounds up to 37.
- Underlayment at the ProArmor net of 929 sq ft needs 2 rolls. Ridge length is 38 ft.
Required, waste and purchase
- Required
- 12.15 squares of field shingles before the bundle roundup, and 38 ft of ridge cap.
- Purchased
- 37 bundles at 3 per square. Two underlayment rolls at 929 sq ft net. Ridge cap is 2 bundles at 20, 25 or 33 ft per bundle for this 38 ft ridge, so the roundup hides the product difference on this particular roof. Starter uses the site constant of about 105 ft per bundle (2 bundles here). That 105 ft figure was not re-verified against a named starter product on 2026-11-09. Ice-and-water uses the site constant of 200 sq ft per roll and comes out to 2 rolls for one course along 76 ft of eaves.
- Units
- US feet, roofing squares (100 sq ft) and bundles.
Assumptions
- Three bundles per square matches GAF’s statement for most standard asphalt shingles. Some GAF products are four or five bundles per square.
- 929 sq ft is Owens Corning ProArmor with a 3 inch overlap, not every synthetic roll.
What this example does not decide
- Not a bid, a tear-off plan, or fall-protection instructions.
- Slopes under 4/12 can need a different underlayment build-up. This example is 6/12 and does not apply that rule.
Where to go next
- Roof area calculator — Run these inputs
- Shingle calculator — When you already know eaves and ridge lengths
- Roof pitch calculator — If 6/12 is not yet known
- Roofing topic — Pitch factor and product-specific accessories
- Roofing squares explained — How to measure planes
Questions
Why is the roof larger than 36 × 24?
36 × 24 is the building. One foot of overhang on each side makes the plan 38 × 26, and the 6/12 pitch multiplies that plan by about 1.118.
Does every ridge bundle cover 33 feet?
No. GAF’s reference guide lists about 20, 25 and 33.3 linear feet for different caps. This 38 ft ridge still rounds to 2 bundles at each of those coverages. A longer ridge would not.
Sources
- GAF – How many bundles of shingles are in a square — accessed 2026-11-09: “For most standard asphalt shingles (3-tab and architectural), there are three bundles per square. A roofing square is equal to 100 square feet of roof area.”
- Owens Corning – ProArmor synthetic roof underlayment — accessed 2026-11-09: “Coverage (with 3-in. overlap) 929 sq. ft.”