Formula
How this estimate works
roof area = footprint area x pitch multiplier; area with waste = roof area x (1 + waste percent)
A 40 ft by 30 ft footprint at 6:12 pitch with 10% waste is about 1,475 square feet, or 14.75 roofing squares.
Estimate roof surface area
Start with the flat roof footprint, then multiply by the pitch multiplier to estimate sloped surface area. Add waste for cuts, overlaps, and layout.
Complex roof shapes
Simple footprint estimates work best for basic gable or shed roofs. Hips, valleys, dormers, overhangs, and multi-plane roofs should be measured in sections.
Assumptions used
- Footprint area is the flat horizontal area covered by the roof.
- Pitch multiplier estimates the increase from slope.
- Waste is applied after the sloped roof area is calculated.
Before you order materials
- Measure each roof plane separately for complex roofs.
- Include overhangs if they need roofing material.
- Use the result as a planning estimate before supplier takeoff.
Frequently asked questions
How do I calculate roof area from footprint?
Multiply the flat roof footprint by the roof pitch multiplier, then add waste if you are estimating materials.
What is a roofing square?
One roofing square equals 100 square feet of roof area and is commonly used for shingles and other roofing materials.
Does this include overhangs?
Only if the overhangs are included in the footprint dimensions you enter. Otherwise, measure and add them separately.