How to Measure a Roof
Two reliable ways to get roof area: the footprint method, done from the ground, and the plane-by-plane method for roofs with different sections. Both end in squares you can order from.
What you need
- A 100 ft tape (or a laser measure) and a helper
- Your roof pitch (see how to find it)
- A sketch of the roof from above, even a rough one
Method 1: footprint × pitch (from the ground)
- Measure the length and width of the house at the walls.
- Add the overhang on each side. Measure one overhang from the wall to the fascia; it is usually 12 to 24 inches.
- Multiply length × width (with overhangs) for the footprint. Add separate rectangles for garages, porches and wings.
- Multiply the footprint by the pitch multiplier.
This works for gable roofs, hip roofs and combinations, as long as every part has the same pitch. The roof area calculator does it in one step.
Method 2: plane by plane
For mixed pitches, or when you can safely get on the roof, measure each plane. For a rectangular plane, the area is the eave length times the slope length (eave to ridge along the roof). If you only know the horizontal run, multiply it by the multiplier first. Triangular hip ends are half of base × slope height.
Multi-section roof area calculator
Measure the edges too
Accessories are ordered by the linear foot, so write these down while you are at it:
| Edge | What goes there | Calculator |
|---|---|---|
| Eaves | Drip edge, starter strip, ice & water barrier, gutters | Drip edge & ridge cap |
| Rakes | Drip edge (and starter on many systems) | Drip edge & ridge cap |
| Ridges and hips | Ridge cap shingles, ridge vent | Drip edge & ridge cap |
| Valleys | Ice & water barrier, valley metal | Underlayment |
Tips
- Round each measurement up to the next half foot; small errors add up across a roof.
- Do not subtract chimneys or skylights smaller than about 4 × 4 ft. The cutting waste around them uses up the saved area.
- Satellite measurement reports are accurate for complex roofs but cost money. For simple shapes, the footprint method is within a few percent.
Stay off roofs that are steep, wet or more than one story unless you have fall protection. The footprint method gets you an accurate number from the ground.
Frequently asked questions
How do I measure my roof from the ground?
Measure the outside length and width of the house, add the overhangs, multiply for the footprint, then multiply by the pitch multiplier for your roof pitch.
Should I subtract skylights and chimneys?
Only large openings. Small penetrations create extra cutting waste that roughly cancels the saved area, so most estimators ignore them.
How accurate is the footprint method?
For roofs with a single pitch it is mathematically exact, so accuracy comes down to your tape measurements. Mixed pitches need section-by-section measurement.
Last reviewed 2026-10-10. Results are estimates for planning; confirm quantities with your supplier and requirements with your local building code and the manufacturer's instructions.