How to measure a roof: squares, pitch and waste explained
Whether you're bidding a re-roof or checking a contractor's quote, every roof estimate starts with the same numbers. Here's how to calculate them by hand — and when it's faster to order a satellite report instead.
1. Measure the footprint
Measure the length and width of each section of the house, including overhangs. Multiply length × width for each rectangle and add them together. This is the flat (plan) area.
2. Find the pitch
Pitch is the rise in inches for every 12 inches of horizontal run. Hold a level 12″ out from the roof surface and measure straight down to the roof — a 6″ drop means 6/12.
3. Apply the pitch multiplier
Multiply the flat area by the multiplier for that pitch (table below). A 1,000 sq ft footprint at 6/12 becomes about 1,118 sq ft of actual sloped roof.
4. Convert to roofing squares
One roofing square = 100 sq ft. Divide the sloped area by 100. 1,118 sq ft ≈ 11.2 squares.
5. Measure edges
Record eaves (drip edge and starter), rakes, ridges and hips (ridge cap) and valleys (valley liner). These lengths drive accessory quantities.
6. Add waste
Add a waste allowance for cuts and starter courses. Simple gable roofs need less; roofs with many hips, valleys and dormers need more. Follow your shingle manufacturer's guidance.
Roof pitch multiplier table
Multiplier = √(1 + (rise ÷ 12)²). Multiply the flat footprint by this number.
| Pitch | Multiplier |
|---|---|
| 3/12 | 1.031 |
| 4/12 | 1.054 |
| 5/12 | 1.083 |
| 6/12 | 1.118 |
| 7/12 | 1.158 |
| 8/12 | 1.202 |
| 9/12 | 1.250 |
| 10/12 | 1.302 |
| 12/12 | 1.414 |
Steep vs flat sections
Many homes combine a pitched asphalt roof with a low-slope section (porches, additions). Measure them separately — flat sections use different materials such as rubber membrane and are priced differently.
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