The single biggest error in roof estimating is using the plan area. A roof is sloped, so its true surface area is always bigger than its footprint, and the steeper the pitch, the bigger the difference.
The pitch factor
Pitch factor = √(1 + (rise ÷ run)²). Multiply the plan area (including overhangs) by this factor to get the true roof area.
| Pitch | Degrees | Factor |
|---|---|---|
| 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 |
For hip roofs where every slope has the same pitch, the same factor applies. A hip roof over a given footprint has the same area as a gable. The edges are what differ: hips instead of rakes.
Squares and bundles
Roofers count in squares: 100 sq ft of roof. Most shingles come three bundles to a square. Add 10% waste for simple gables and 15% or more for hips, valleys and dormers.
The edges
Measure eaves, rakes, ridge, hips and valleys separately. They drive:
- Starter strip along the eaves and rakes
- Drip edge
- Hip and ridge cap
- Ice and water shield at eaves and valleys
- Ventilation at eaves and ridge
Underlayment
Synthetic underlayment comes in rolls covering about 10 squares. Felt (#15) covers about 4 squares per roll. Below 4/12 most shingle makers require double coverage.
Check the minimum pitch
Asphalt shingles need at least 2/12. Use metal or membrane systems below that.
The roofing calculator works all of this out for gable, hip and mono-pitch roofs.
This guide is general information for estimating. It is not engineering, legal or tax advice. Check local codes and your inspector for your project.