How roofing is measured
Shingles are sold by the square: enough to cover 100 sq ft of roof. Most shingles come three bundles to a square, so one bundle covers about 33 sq ft. Some heavy designer shingles use four.
The roof is bigger than the house. A sloped roof covers more area than the floor beneath it, so multiply the footprint (including the overhangs) by a pitch multiplier:
| Pitch | Angle | Multiplier |
|---|---|---|
| 3/12 | 14° | 1.031 |
| 4/12 | 18.4° | 1.054 |
| 6/12 | 26.6° | 1.118 |
| 8/12 | 33.7° | 1.202 |
| 10/12 | 39.8° | 1.302 |
| 12/12 | 45° | 1.414 |
The multiplier is √(1 + (rise ÷ 12)²). On a hip roof with all slopes at the same pitch, the same multiplier applies to the whole footprint.
Worked example: a 40 × 30 ft gable roof at 6/12
40 × 30 = 1,200 sq ft of footprint. × 1.118 = 1,342 sq ft of roof. Add 10% for cuts and starter courses: 1,476 sq ft, or 14.8 squares. At 3 bundles per square that is 44.3, so buy 45 bundles.
How much extra to add
Use about 10% on a simple gable roof and 15% on hip roofs or roofs with valleys and dormers. Every hip and valley means angled cuts, and the offcuts are rarely reusable.
The rest of the materials
- Underlayment goes over the whole deck. Synthetic rolls commonly cover 10 squares before laps. #15 felt covers about 4 squares and #30 about 2.
- Starter strip runs along the eaves, and often the rakes, so the first course is sealed.
- Ridge cap covers the ridge and any hips. Coverage per bundle varies widely by product.
- Drip edge protects the deck edges. It is sold in 10 ft pieces that overlap a few inches.
- Nails: manufacturers usually specify 4 nails per shingle, or 6 in high-wind areas. Check the wrapper.
Many areas require a permit for a re-roof and limit how many layers can stay on the roof. Check before you start.