How roof square footage is calculated
A roof is bigger than the floor under it for two reasons: it hangs past the walls, and it is tilted.
Roof area = plan area x pitch factor
Squares = roof area / 100
Example: a 48 x 28 ft house with 12 inch overhangs has a 50 x 30 = 1,500 sq ft plan. At 6/12 (factor 1.118) the roof is 1,677 sq ft, or 16.8 squares before waste.
Pitch factor by pitch
| Pitch | Factor | 1,500 sq ft plan becomes |
|---|---|---|
| 3/12 | 1.031 | 1,546 sq ft |
| 4/12 | 1.054 | 1,581 sq ft |
| 5/12 | 1.083 | 1,625 sq ft |
| 6/12 | 1.118 | 1,677 sq ft |
| 7/12 | 1.158 | 1,737 sq ft |
| 8/12 | 1.202 | 1,803 sq ft |
| 9/12 | 1.250 | 1,875 sq ft |
| 10/12 | 1.302 | 1,953 sq ft |
| 12/12 | 1.414 | 2,121 sq ft |
The steeper the roof, the bigger the gap between footprint and real area. At 12/12 it is 41% more.
Want a pro to confirm these numbers?
Numbers look right? A licensed roofer can measure on site and give you an exact price.
How much waste to add
- 5%: shed roofs and simple metal panel jobs.
- 10%: plain gable roofs.
- 15%: hip roofs, where every hip is cut on both sides.
- 20%: roofs with valleys, dormers and turrets.
Ridge cap and starter strip are separate. The roof shingle calculator counts them.
Measuring a complicated roof
Break the roof into simple footprints: main house, garage, porch. Measure each flat from outside the walls, then add them with their own pitch. Satellite measuring tools and aerial reports are fine for estimates. For an exact count, a roofer measures each plane on site.
Sources and further reading
- Asphalt Roofing Manufacturers Association (ARMA)
- 2021 International Residential Code, Chapter 9: Roof Assemblies
- National Roofing Contractors Association (NRCA)
Frequently asked questions
How many square feet is a roofing square?
100 square feet. A 2,000 sq ft roof is 20 squares.
How big is the roof on a 2,000 sq ft house?
For a single-story home with a 6/12 pitch and 1 ft overhangs, roughly 2,400 to 2,500 sq ft. A two-story home with the same living area has about half the footprint, so a much smaller roof.
Do hip roofs have more area than gable roofs?
Not if the pitch and footprint match. Both have area equal to plan area times the pitch factor. Hip roofs need more waste and more ridge cap, which is why they cost more.
Should I include the overhang?
Yes. The roof covers the overhang, so shingles, underlayment and decking do too. Leaving it out undercounts by 5% to 10%.