How to measure a roof from satellite imagery, step by step
A step-by-step satellite roof measurement: trace the facets, label every edge, set pitch, turn plan area into squares, export the PDF, and spot where the imagery lies.
To measure a roof from satellite imagery, pull up the address, trace the roof's outline at the drip edge, split it into facets, label every edge, set the pitch, and let the software turn plan area into true area and squares. The imagery gives you the footprint and the line lengths; pitch is the one number you bring from somewhere else.
The rest is the detail that keeps the number honest. The steps use RoofStruct's builder because that's what we make: free, unlimited self-serve measuring and PDF reports, no card. The method is the same in any tracing tool.
What you need before you start
- The address. A street address or a pin on the right building. Rural lots and new subdivisions sometimes land on the wrong house, so confirm the roof matches the job.
- A pitch, or a way to get one. Satellite imagery is shot from above. It shows where the ridge is, not how steep the planes are. You'll need a pitch from a site visit, the homeowner's paperwork, or a street-level photo.
- A waste number. Not for the measurement itself, but for the order that follows it. Our waste factor guide by roof type has the starting points.
If you've never opened the builder, measure your first roof in RoofStruct walks through the screens with every button named. This guide is the trade logic behind those screens.
Step 1: Find the address and frame the roof
Search the address and zoom until the whole roof fills the view with a little ground around it. Too far out and every corner you click lands a foot off.
Before you trace, read the roof. Find the main ridge, count the hips and valleys, and spot anything that breaks the field: dormers, chimneys, skylights, a lower roof over a garage or porch. A quick scan here saves you from tracing a gable as a hip and redoing it.
Check the shadows too. A hard shadow line usually marks a ridge or hip. It's also where you lose detail, which we'll come back to.
Step 2: Trace the outline and split it into facets
Pick "Draw Roof" and click each outer corner of the roof, then close the shape on the first point.
Trace the roof edge, not the walls. From above, satellite imagery shows the outer edge of the shingles at the drip edge, overhang included. Your drip edge, starter and gutter all run along that line, so that's the line you want. A wall-line trace undercounts every square on the job.
Here's how much an overhang matters. A 40 × 30 ft footprint measured at the walls is 1,200 sq ft of plan area. Add a 1 ft overhang all the way around and the roof is 42 × 32 ft, or 1,344 sq ft. That's 144 sq ft of plan area, about 1.4 squares, before pitch is applied. Trace the walls and you short the order by that much plus waste.
Then split the outline into facets: one closed shape per roof plane. A simple gable is two facets. A hip roof is four. A cross-gable with a garage wing might be eight or ten. Each facet gets its own pitch and its own area, which is the reason to split it at all. One big outline with one pitch is a guess dressed up as a measurement.
Snap the splits to the ridge, hips and valleys you can see in the image.
Step 3: Label every edge
Every edge of every facet is one of 12 edge types. Select an edge and the builder asks "What is this edge?". The common ones are right there: ridge, hip, valley, eave, rake. The rest sit under the extra types for walls, flashing and transitions.
Labels drive the material list, so this is where a measurement turns into an order:
| Edge type | What it feeds |
|---|---|
| Eave | Drip edge, starter strip, gutter, ice and water at the eave |
| Rake | Drip edge, starter on the rake if you run it there |
| Ridge and hip | Ridge cap |
| Valley | Valley metal or woven valley, ice and water |
Shared edges count once. The line between two facets is one valley or one hip, not two, so label it once and the length lands in one bucket.
The panel keeps score with a counter that reads "Edges labeled X of Y". Don't move on until X equals Y. An unlabeled 30 ft eave is 30 ft of drip edge and starter that never makes the order.
Step 4: Set the pitch
Pitch is the step satellite imagery can't do for you. From straight above, a 4/12 plane and a 10/12 plane with the same footprint look identical. Rise isn't in the picture.
You have three ways to get it:
- Measure it on site. A pitch gauge or a level and tape on a rake. Still the reference.
- Read it from a street-level photo. Line up a rake against a known rise and run. Our free pitch finder does the arithmetic once you've lined it up.
- Use the job file. Plans, a previous report, or the original permit set if the homeowner has it.
Set one predominant pitch for the whole roof, then override the facets that differ: the porch at 3/12, the dormer cheeks, the steeper front gable. That's where the facet split from step 2 pays off.
The math behind it is the pitch factor: true area = plan area × the square root of (1 + (rise ÷ 12)²).
| Pitch | Pitch factor | 1,000 sq ft plan becomes |
|---|---|---|
| 4/12 | 1.054 | 1,054 sq ft |
| 6/12 | 1.118 | 1,118 sq ft |
| 8/12 | 1.202 | 1,202 sq ft |
| 10/12 | 1.302 | 1,302 sq ft |
| 12/12 | 1.414 | 1,414 sq ft |
Pitch errors are expensive. Take 1,800 sq ft of plan area. At 6/12 it's 1,800 × 1.118 = 2,012 sq ft. Call it 5/12 by mistake and you get 1,800 × 1.083 = 1,950 sq ft, about 62 sq ft or 0.6 of a square short. Call it 8/12 and you get 1,800 × 1.202 = 2,164 sq ft, about 1.5 squares over. One pitch step costs more than most tracing errors.
Step 5: Read the area, squares and lengths
Once every facet has a pitch and every edge has a label, the totals mean something. Here's a full worked example for a plain gable ranch, every number one you can check by hand:
- Plan area: 1,800 sq ft (traced at the drip edge, overhang included).
- Pitch: 6/12 on both planes, factor 1.118.
- True area: 1,800 × 1.118 = 2,012 sq ft.
- Squares: 2,012 ÷ 100 = 20.1 squares.
- With 10% waste: 20.1 × 1.10 = 22.1 squares.
- Bundles at 3 per square: 22.1 × 3 = 66.3, so order 67 bundles.
The lengths come straight off the labels: total eave and rake for drip edge and starter, ridge plus hips for cap, valleys for metal and ice and water. A hip roof with zero hip length means a label went wrong.
Step 6: Check it, then export the PDF
Before you export, look for the two warnings that mean the area is wrong:
- Overlaps. If two facets cover the same patch of roof, the builder tells you that area is "counted twice". Usually it's a split line drawn past a corner. Fix the shape, don't ignore the warning: a 40 sq ft overlap is 40 sq ft of shingles you'll pay for and never install.
- Unpitched facets. A facet with no pitch has its area "counted flat". A 300 sq ft plan facet left flat on an 8/12 roof shows 300 sq ft instead of 300 × 1.202 = 361 sq ft, about 0.6 of a square short.
Then confirm every edge is labeled, and export. On the Free plan the full PDF carries RoofStruct branding. Send it to a supplier or attach it to a quote.
Where satellite imagery fails
These are limits of shooting a roof from above, not of any one tool. Know them before you quote off a trace.
Shadows. A long shadow can swallow a valley, a hip on the shaded side, or a cricket behind a chimney. If you can't see a line, don't invent it. Trace what you can see, then check the hidden side against a street-level photo or your own photo.
Trees. Branches over an eave hide the true edge and the corner. You can often project the edge from the visible parts of the rake and eave on either side, but a fully hidden back slope is a guess. Flag it, and confirm on site before you order.
Flat and low-slope roofs. From above, a flat roof gives you no pitch cue at all: no shadow line, no ridge, nothing to read. That's fine for area (a truly flat roof has a pitch factor of 1.0), but you can't tell a 2/12 shed roof from a flat one off the image. Parapets, curbs and roof heights don't show either.
Lower roofs under upper eaves. A porch roof tucked under the main eave, or a lean-to on the back wall, can be partly hidden by the roof above it. Count the planes you saw in step 1 against the facets you traced.
Old imagery. A recent addition or new garage may not be in the picture yet. If the trace doesn't match what the homeowner describes, believe the homeowner and go look.
When the imagery isn't good enough, upload your own drone or phone photo of the roof, set the scale from a length you know, and trace on that instead.
Where you trip
- Tracing the walls instead of the drip edge. It undercounts every square and every foot of drip edge. The overhang is roof.
- One outline, one pitch. Wrong on anything with a porch, a dormer or a second pitch.
- Unlabeled edges. The area looks right and the material list comes up short. Watch the counter.
- Double-labeling shared edges. The line between two facets is one valley, not two. Doubled valleys mean doubled valley metal.
- Guessing the pitch. A one-step miss moves the area more than most tracing mistakes. Measure it or read it from a photo.
- Ordering with zero waste. The measurement is the roof. The order is the roof plus cuts, starter and cap.
FAQ
Can you measure a roof accurately from satellite imagery?
Yes, for the footprint and line lengths, when the roof is clearly visible. The area is only as good as the pitch you apply. Shadows, trees and old imagery are the usual problems.
How do you get roof pitch from a satellite image?
You don't, directly. An overhead image shows plan area, not rise. Get the pitch on site, from the job file, or from a street-level photo of a rake. A free pitch finder handles that last method. Then apply it per facet.
Is satellite roof measurement free?
In RoofStruct, measuring roofs yourself on satellite imagery is free and unlimited, with the full PDF and no card.
Should I trace to the wall or the drip edge?
The drip edge. The overhang is part of the roof you shingle, and your drip edge, starter and gutter run along it. A 1 ft overhang on a 40 × 30 ft house adds 144 sq ft of plan area.
What do I do when trees cover part of the roof?
Trace what you can see and project the hidden edges from the visible ones. If a whole slope is hidden, upload your own drone or phone photo or confirm on site before you commit to a number.