What you are measuring
Roofers price and order material by roof surface area — the sloped area of every roof plane — not by the house's floor area. Two numbers get you most of the way: the building footprint (the area under the roof seen from above, including overhangs) and the pitch (how many inches the roof rises per 12 inches of run). Surface area is footprint × slope factor.
Method 1: from the ground (footprint × slope factor)
- Measure the outside walls and add the overhang on each side (usually 12–24 in). A 50 × 30 ft house with 1 ft overhangs is 52 × 32 ft = 1,664 sq ft of footprint.
- Find the pitch. Hold a level horizontally against a rake board or a gable end, measure 12 in along it and the vertical distance down to the roof edge. 6 in means 6/12. A pitch app or a pitch gauge works too.
- Multiply by the slope factor for that pitch: 4/12 → 1.054, 6/12 → 1.118, 8/12 → 1.202, 12/12 → 1.414. 1,664 × 1.118 = 1,860 sq ft.
- Divide by 100 for roofing squares: 18.6 squares, before waste.
Good for: a first estimate on a simple gable or hip roof with one pitch. Weak on: roofs with several pitches, dormers, porches or upper and lower roofs — every extra plane is a chance to miss area.
Method 2: on the roof, plane by plane
- Sketch the roof and number every plane (each flat face — a simple gable has 2, a hip roof 4, real houses often 8–20).
- Measure each plane along the slope: rectangles are length × width, triangles (hip ends) are ½ × base × height, trapezoids are ½ × (top + bottom) × height.
- Add the planes. Measure ridges, hips, valleys, eaves and rakes too — they drive ridge cap, starter strip, drip edge and valley material.
Good for: the final bid and the material order. Costs: time on the roof and a fall risk; steep or wet roofs are a reason not to.
Method 3: by address, from an aerial survey
Public agencies fly airborne LiDAR — laser scanners that record the 3D position of points on the ground and on rooftops, typically several points per square meter. From the points that land on a roof, software fits the roof planes and reads their pitch directly, then computes surface area and edge lengths. Roofari does this for free by address; the full report reconstructs every facet in 3D.
LiDAR measures height, so pitch comes from the roof itself rather than being inferred from a flat photo. Its limits are honest ones: the survey has a date (a roof changed since then is not seen), dense trees hide parts of a roof, and very small features can fall between points. More on LiDAR vs. photo measurement →
Which method should you use?
| Method | Time | Best for | Watch out for |
|---|---|---|---|
| Ground estimate | 15–30 min | Simple roofs, ballpark | Several pitches, dormers |
| On the roof | 1–3 h | Final bid and order | Safety, steep roofs |
| Aerial LiDAR by address | Seconds | First estimate, many leads | Survey date, trees |
Many contractors combine them: an aerial number to qualify the lead and quote a range, then a site visit to confirm before ordering.
Questions
How do I measure my roof without climbing on it?
Measure the footprint from the ground (outside walls plus overhangs), find the pitch from a gable end or rake, and multiply the footprint by the slope factor. Or enter the address on Roofari to read area and pitch from aerial LiDAR.
Is roof area the same as the house’s square footage?
No. Roof area is larger: it includes overhangs and follows the slope. A 2,000 sq ft single-story home with a 6/12 roof and 1 ft overhangs often has 2,400–2,600 sq ft of roof. Two-story homes have less roof than floor area.
How accurate is a roof measurement from the ground?
On a simple roof with one pitch, usually within 5–10% if overhangs are included. Complex roofs with several pitches and dormers are where ground estimates miss most.