Measuring overhangs and eaves from aerial imagery
The overhang is roof. Why you trace the eave line, not the walls, how an overhang moves area, perimeter and squares, and how to find the fascia line from above.
When you measure from aerial imagery, trace the roof edge at the drip edge, not the wall line. A top-down image shows the overhang as part of the roof, so it is already in the trace; the mistakes start when wall dimensions from plans, property records or a tape on the siding get mixed into a roof measurement.
Roof edge versus wall line
From straight above, the walls hide under the overhang. You see the outer edge of the roofing, which sits outside the wall by the overhang depth at eaves and rakes.
House plans, assessor records and your tape on the siding are wall dimensions: they describe the building, not the roof. The trace on the image is the roof edge, and that's what you shingle. Start from wall numbers and you have to add the overhang on every side yourself, which is where squares go missing.
Why drip edge and starter follow the eave line
Drip edge, starter strip, gutter and the ice and water course at the eave all run along the fascia, at the outer edge of the overhang. None of them run along the wall. Trace the walls and every one of those lengths comes up short, along with the field.
Rake overhangs matter too. They push the gable ends outward, which makes each eave longer by the rake overhang at both ends. A deep eave overhang adds area but not eave length. A rake overhang adds both.
How an overhang changes area, perimeter and squares
Take a gable ranch with walls of 56 × 28 ft. The ridge runs the long way, so the eaves are on the 56 ft sides and the gables on the 28 ft ends. Say the eaves overhang 18 in (1.5 ft) and the rakes 12 in (1 ft). Pitch is 5/12, factor 1.083.
| Measure | Wall line | Roof edge |
|---|---|---|
| Plan dimensions | 56 × 28 ft | 58 × 31 ft |
| Plan area | 1,568 sq ft | 1,798 sq ft |
| True area at 5/12 | 1,698 sq ft | 1,947 sq ft |
| Squares | 17.0 | 19.5 |
The arithmetic:
- Roof-edge dimensions: 56 + 1 + 1 = 58 ft long (a rake overhang at each end), 28 + 1.5 + 1.5 = 31 ft wide (an eave overhang on each side).
- Plan area: 56 × 28 = 1,568 sq ft at the walls; 58 × 31 = 1,798 sq ft at the edge. That's 230 sq ft more, about 14.7%.
- True area: 1,568 × 1.083 = 1,698 sq ft; 1,798 × 1.083 = 1,947 sq ft.
- The gap: 1,947 − 1,698 = 249 sq ft, or 2.5 squares, before waste. At 3 bundles per square that's about 8 bundles you'd never order.
The edges move too:
- Perimeter in plan: 2 × (56 + 28) = 168 ft at the walls; 2 × (58 + 31) = 178 ft at the edge.
- Eaves: 2 × 56 = 112 ft against 2 × 58 = 116 ft.
- Rakes: each covers half the width in plan, 14 ft against 15.5 ft. Sloped at 5/12 that's 15.2 ft against 16.8 ft, so four rakes total 60.6 ft against 67.1 ft.
- Drip edge: 112 + 60.6 = 172.6 ft against 116 + 67.1 = 183.1 ft, about 10.5 ft short.
Ten feet of drip edge is one stick. The 2.5 squares are the expensive part, and waste won't cover them: waste is for cuts, starter and cap, not for roof you never measured.
Spotting the overhang in top-down imagery
On a clear image you are looking for the line where the roofing stops:
- The fascia line. A thin, straight line at the outer edge of the shingles, often a different tone from the field.
- The shadow band. On the sun side, the overhang throws a dark strip down the wall or onto the ground. The roof edge is the outer edge of the roofing, not the outer edge of that shadow.
- Gutters. The outer lip of a gutter sits a few inches past the drip edge. Trace the shingle edge, not the gutter.
- Lean. Imagery not shot straight down can show one wall face beside the roof. That's siding, not overhang.
Rakes are harder: a tight one barely shows. Gables are usually symmetrical, so use the clear rake as the reference for the other. If shadows or trees hide an edge, project it from the visible parts of the same line. Our guide to how accurate satellite roof measurements are covers those error sources in more detail.
Typical overhang depths
Depth varies with style, region and the age of the house. Many homes land somewhere around 12 to 24 inches at the eaves, but tight rakes of a few inches and deep eaves past 2 ft both turn up. Treat any number as a range until you've measured.
You only need the depth when you start from wall dimensions. A satellite trace draws the edge directly. To get the depth itself, tape from siding to fascia on site.
Where you trip
- Using the wall footprint. Assessor and plan dimensions are walls. Add the overhang on every side, or trace the edge instead.
- Tracing the shadow. The edge of the shadow band is not the edge of the roof.
- Tracing the gutter lip. It adds a few inches all the way round.
- Forgetting rake overhangs. They lengthen every eave and add area at both gable ends.
How to measure a roof from satellite imagery runs the full method, and measure your first roof in RoofStruct walks the screens: trace at the edge, then label each line as eave or rake under "What is this edge?". It's free, unlimited self-serve measuring and PDF reports, no card. For pitch, the pitch finder works from a street-level photo.
FAQ
Do I measure a roof to the wall or the edge of the overhang?
To the edge of the overhang. That's the roof you shingle, and drip edge, starter and gutter run along it.
How do I find the overhang on a satellite image?
Look for the fascia line at the outer edge of the shingles and the shadow band the overhang casts on the sun side. Trace the edge of the roofing, not the shadow or the gutter lip.
How much does an overhang add to a roof measurement?
It depends on the footprint and the depth. On a 56 × 28 ft ranch with 18 in eaves and 12 in rakes, plan area grows from 1,568 to 1,798 sq ft, about 2.5 squares at 5/12.
How deep is a typical roof overhang?
There isn't one standard. Many houses fall somewhere around 12 to 24 inches at the eaves, with rakes often shallower.