How Horus measures a roof
From a street address to roof squares in under a minute — and exactly where the numbers come from.
1. We find the building
When you enter an address, Horus converts it to coordinates and finds the nearest building in high-resolution aerial imagery. Before you pay, we show you a satellite image with that building outlined. If the pin landed on the neighbor’s house, click the correct roof and we re-run the search.
This step is free. If there is no high-resolution imagery for that roof, we tell you and you are not charged.
2. We measure the roof surface
Horus uses two independent sources for every roof:
- A 3D model of the roof planes. Each plane comes with its pitch, the direction it faces and its true sloped area, overhangs included. We add a small allowance — 1% on simple roofs, 5% on complex ones — for the small facets and edges that aerial models under-detect.
- The roof outline from the imagery. A pixel-level mask of the whole roof. We use it to measure the perimeter, draw the dimensioned outline sketch and catch gross errors such as the wrong building.
- The height of every eave. The 3D surface model also gives the height of each level roof edge above the ground next to it. One-story eaves sit around 9–12 ft and two-story eaves around 18–24 ft, so the report shows the number of stories and the eave height. Homes with both (a two-story house with a one-story garage or porch) show as 1–2 stories.
How accurate is it?
We test Horus against professional measurement reports from EagleView and GAF. Across the 8 residential roofs tested so far:
| Median difference in total roof area | 1.9% |
| Roofs within 2.5% of the professional total | 6 of 8 |
| Predominant pitch matched | 6 of 8 (the other two within 2/12) |
| Number of stories | 6 of 6 test homes where it was known (1-story, 2-story and mixed) |
| Larger misses | 2 roofs (10–18%): one partly under trees, one with 11-year-old imagery — both now flagged automatically |
Roof edge lengths are less precise than area: the eaves-and-rakes estimate is typically within about 15%, and can run short on two-story homes. We will keep publishing results as we test more roofs.
3. We build the report
The report includes the total roof area and squares, a pitch breakdown, a numbered map of every plane with its pitch and area, the shingle area versus low-slope area, the number of stories and eave height, the roof perimeter for drip edge, and a waste table with shingle bundles and underlayment rolls. Suggested waste is based on how many planes the roof has.
See a complete sample report →
What affects accuracy
- Imagery date. Every report shows when the aerial imagery was captured, and we tell you before you pay when it’s more than five years old. Additions or re-roofs after that date are not reflected.
- Trees. Branches over the roof can hide parts of it. Horus checks for tall vegetation along the roof edge and warns you in the report when it finds it.
- Very complex roofs. Many small planes, dormers and turrets are harder to separate from above.
- Imagery resolution. We only sell reports for roofs with high (≈10 cm per pixel) or standard (≈25 cm per pixel) imagery — never lower.
What’s not included (yet)
Instant reports do not include ridge, hip and valley lengths. Separating every edge reliably from aerial data alone is not accurate enough yet to sell as an instant result, so we don’t. The perimeter (eaves and rakes) is included.