How to measure a roof (without climbing it)
Three ways to measure a roof — on the roof, from the ground and from aerial imagery — and the math that turns measurements into squares.
Method 1: On the roof
Measure the length and width of every plane with a tape, compute each plane’s area, and add them up. It is the most direct method but also the slowest and the most dangerous. Only do it with proper fall protection, never on a wet or steep roof, and remember that hips and valleys make planes into triangles and trapezoids.
Method 2: From the ground
- Measure the footprint. Walk the outside walls and sketch the shape. Add the overhang on each side — measure it at a gable end or estimate 12–18 inches.
- Find the pitch. From a ladder at the eave, hold a level horizontally against the roof, mark 12 inches along the level, and measure straight down to the roof surface. That number is the rise: 6 inches means 6/12.
- Apply the multiplier. Footprint area × the pitch multiplier = roof surface. A 6/12 roof uses 1.118. See the pitch chart.
- Convert to squares. Divide by 100 and add waste.
Method 3: From aerial imagery
High-resolution aerial imagery and 3D surface models let software measure each roof plane from above: its area, pitch and the direction it faces. This is how professional roof reports are produced, and it is what Horus does in under a minute for $15.
Aerial measurement avoids ladders entirely, but it depends on the imagery: trees over the roof and changes made after the imagery date are not captured. Every Horus report shows the imagery date for that reason.
Which method should you use?
| Method | Time | Risk | Best for |
|---|---|---|---|
| On the roof | 1–3 hours | High | Final check before ordering |
| From the ground | 30–60 min | Low | Simple rectangular roofs |
| Aerial report | About a minute | None | Quotes and material planning |