Roofing squares and waste factor
Three numbers hide inside one. If a customer argues with the total, you need to know which of the three they are actually arguing with.
A roofing square is 100 square feet. Not 100 square feet of ground: 100 square feet of roof surface, slope included. The distinction is worth several hundred dollars on an ordinary house.
Pitch is applied before waste
A roof is not flat. Its footprint is smaller than its actual surface, and the gap depends on the pitch. The slope factor is the square root of (rise squared plus 144), divided by 12.
| Pitch | Slope factor |
|---|---|
| 4/12 | 1.0541 |
| 6/12 | 1.1180 |
| 8/12 | 1.2019 |
| 12/12 | 1.4142 |
In practice: 1,000 square feet of footprint at 6/12 is 1,118 square feet of roof, or 11.18 squares. The same 1,000 square feet at 12/12 is 14.14. Same house from above, four squares apart.
Waste comes after that
Waste covers cuts, valleys, rakes and damage. Ten per cent is the common figure on a simple roof, and more where there are valleys and hips to cut into. It applies to the roof area, not the footprint: 21.97 squares plus 10% is 24.17 squares.
The order matters. Pitch first, waste second. Applying waste to the footprint under-orders on any pitch above flat, and the error grows with the pitch.
What you should be able to show
- The measured area, in squares, before waste.
- The pitch used, and on which facets.
- The waste percentage, stated rather than folded into the total.
- The ordered total, which should be reachable from the three above.
A customer who argues with a number almost always argues with the total. If they can see the three numbers underneath it, the conversation is about which one is wrong instead of about whether you are honest.
Common questions
Is a square the same everywhere?
Yes: 100 square feet of roof surface. It is an ordering unit, not a ground measurement.
Is 10% always the right waste factor?
No. It is the common figure on a simple roof. A roof with many valleys, hips or penetrations needs more, and that is a roofer's judgement rather than a constant.
Why does pitch change the quantity?
Because what you cover is the sloped surface, not the building's shadow on the ground. The steeper it is, the wider the gap: 5% at 4/12, 41% at 12/12.
Closlyo returns the area, the pitch and the arithmetic behind each facet, so you can check the waste factor instead of trusting it.
Updated 2026-09-23