A 4/12 roof against 8/12
4/12 gives 18.55 squares. 8/12 gives 21.15 squares — a difference of 2.60 squares more (+14.0%).
The steeper roof covers more surface over the same footprint — and costs more to walk on.
| Input | 4/12 | 8/12 |
|---|---|---|
| Footprint area(sq ft) | 1,600 | 1,600 |
| Roof pitch(/12) | 4 | 8 |
| Waste factor(percent) | 10 | 10 |
| Roofing squares | 18.55 squares | 21.15 squares |
Every figure, both ways
| Step | 4/12 | 8/12 | Difference |
|---|---|---|---|
| Pitch multiplier | 1.05 | 1.2 | +0.15 |
| Actual roof area(sq ft) | 1,686.55 | 1,922.96 | +236.41 |
| Area with waste(sq ft) | 1,855.2 | 2,115.26 | +260.05 |
| Roofing squares(squares) | 18.55 | 21.15 | +2.6 |
| Bundles (three per square)(bundles) | 56 | 64 | +8 |
| Underlayment rolls (4 squares each)(rolls) | 5 | 6 | +1 |
Questions people actually ask
- Does raising roof pitch change the answer in proportion?
- No, and that is the point of putting them side by side. Roof pitch changes by 100% while roofing squares changes by 14%. The step table below shows which line breaks the proportion.
- What is held constant between the two columns?
- Everything except roof pitch: footprint area, waste factor. That is what makes the difference attributable — change two things at once and neither column tells you which one moved the answer.
- Can I compare my own two numbers?
- Yes, from the address bar: add ?a.pitch=… and &b.pitch=… to set each side, and a parameter without a prefix sets both at once. The page is computed per request, so any pair works — these two are only the ones worth a link.
Change either side by adding query parameters — ?a.pitch=… and b.pitch=…. A parameter without a prefix sets both sides at once. Source: Asphalt Roofing Manufacturers Association, National Roofing Contractors Association