How the roofing estimate works
- Footprint. Measure the outside length and width of the house and add the overhang on each side.
- Pitch factor. A sloped roof has more surface than the flat area under it. Multiply the footprint by the pitch factor.
- Waste. Add 10 percent for a simple gable roof and 15 percent or more for hips, valleys and dormers.
- Squares. Divide by 100. A roofing square is 100 sq ft.
squares = (L + 2 × overhang) × (W + 2 × overhang) × pitch factor × (1 + waste) ÷ 100
Worked example
A 40 × 30 ft house with 1 ft overhangs has a 42 × 32 ft footprint: 1,344 sq ft. At a 6/12 pitch the factor is 1.118, giving 1,503 sq ft of roof. With 10 percent waste that is 1,653 sq ft, or 16.5 squares. At 3 bundles per square, buy 50 bundles.
Pitch factors
| Pitch | Angle | Factor |
|---|---|---|
| 3/12 | 14.0° | 1.031 |
| 4/12 | 18.4° | 1.054 |
| 5/12 | 22.6° | 1.083 |
| 6/12 | 26.6° | 1.118 |
| 8/12 | 33.7° | 1.202 |
| 10/12 | 39.8° | 1.302 |
| 12/12 | 45.0° | 1.414 |
The factor is √(1 + (rise ÷ 12)²). For a hip roof with the same pitch on every side, the surface area is the same as a gable roof over the same footprint.
Bundles per square
Most standard three-tab and architectural asphalt shingles come three bundles to a square. Some heavier designer shingles need four or five. The wrapper states the coverage. You also need starter strips along the eaves and ridge cap shingles along ridges and hips, which are sold separately by length.
Measure safely
You do not need to climb on the roof to estimate it. Measure the house walls from the ground and identify the pitch from plans, from the attic, or with a pitch gauge app from a ladder. Roofing work at height is dangerous; for the job itself, use proper fall protection or a licensed roofer.
Frequently asked questions
What is a roofing square?
A roofing square is 100 square feet of roof surface. Roofers price and order materials by the square.
How many bundles of shingles are in a square?
Usually three for standard asphalt shingles. Check the wrapper, because some heavier shingles need four or five.
How do I find my roof pitch?
Pitch is the rise in inches over 12 inches of horizontal run. Measure 12 inches level from the roof surface in the attic, then measure straight up to the underside of the roof.
How much waste should I add for roofing?
About 10 percent for a simple gable roof and 15 percent or more for roofs with hips, valleys and dormers.
Last updated November 1, 2026.