Roof dimensions from satellite: perimeter, area, squares and facets explained
Perimeter, plan area, true area, squares, facets, line types and predominant pitch, defined one by one and worked through on a cross-gable house you can check yourself.
A satellite roof measurement gives you a handful of dimensions: perimeter, plan area, true area, squares, a facet count, line lengths by type and a predominant pitch. Everything level comes straight off the image; everything sloped depends on the pitch you apply.
Here is each term, then one cross-gable house worked through so you can check every number.
Perimeter: the roof edge, not the wall line
Perimeter is the length of the roof's outer edge: the eaves plus the rakes. It follows the drip edge, not the siding. A top-down image shows the roof edge, so the overhang is already in it. Tape measurements and property records are usually wall dimensions, short by the overhang on every side.
One catch: rakes slope. Traced from above, a rake reads at its horizontal run; along the slope it is longer by the pitch factor. Drip edge is ordered on the sloped length.
Plan area and true area
Plan area is the roof's footprint seen from straight above, the area you trace on the image. True area (slope-adjusted area) is the surface you actually shingle.
True area = plan area × pitch factor, where pitch factor = √(rise² + 12²) ÷ 12.
| Pitch | Pitch factor |
|---|---|
| 4/12 | 1.054 |
| 6/12 | 1.118 |
| 8/12 | 1.202 |
| 10/12 | 1.302 |
| 12/12 | 1.414 |
Satellite imagery cannot show rise, so pitch comes from a site visit, a street-level photo, a gauge or the free pitch finder. The RoofStruct builder shows both numbers side by side: "Plan Area (2D)" and "True Area (slope-adjusted)".
Squares
A square is 100 sq ft of roof surface, so squares = true area ÷ 100. Squares figured off the footprint come up short by the whole pitch factor.
Facets: one plane each
A facet is one flat plane of roof. Count planes, not visible patches.
- Gable: 2 facets.
- Hip (rectangular): 4 facets: two trapezoids, two triangles.
- Cross-gable: 4 facets. The wing adds two planes; the main slope it cuts into stays one plane, even with its eave split in two.
- Gable dormer: adds 2. Shed dormer: adds 1.
More facets means more valleys, more cuts and more waste.
Line types and what each feeds
Each edge gets a label, and the label decides which material it drives.
| Line | What it is | Feeds |
|---|---|---|
| Eave | Lower, level edge overhanging the wall | Starter, drip edge, ice and water |
| Rake | Sloped edge at a gable end | Drip edge |
| Ridge | Level peak where two slopes meet | Ridge cap, ridge vent |
| Hip | Outer sloped angle where two slopes meet | Hip cap |
| Valley | Inner angle where two slopes meet | Valley metal or ice and water |
RoofStruct has 12 edge types, asks "What is this edge?" when you pick one, and shows "Edges labeled X of Y" so an unlabeled eave doesn't drop out of the takeoff.
Predominant pitch
Predominant pitch is the pitch covering the most roof area, applied to any facet without its own value. RoofStruct's pitch card says it plainly: "Set the predominant roof pitch. Individual facets can override this when selected." A steep front gable on a shallow main roof needs its own pitch.
Worked example: a cross-gable house
The house: a main gable block 48 ft long and 26 ft deep at the roof edge, ridge running the long way. A centered front wing 20 ft wide projects 14 ft out from the main eave. Every facet is 6/12 (factor 1.118). All dimensions are at the drip edge.
Plan area. Main block 48 × 26 = 1,248 sq ft, plus wing 20 × 14 = 280 sq ft: 1,528 sq ft.
True area and squares. 1,528 × 1.118 = 1,708 sq ft, or 17.1 squares. The footprint alone says 15.3.
Facets. Four. In plan: back slope 48 × 13 = 624; front slope 624 minus the 100 sq ft triangle the wing covers between its valleys = 524; each wing slope is a 10 × 14 rectangle plus a 50 sq ft triangle = 190. 624 + 524 + 190 + 190 = 1,528.
Eaves. Back 48 ft, front 14 + 14 ft beside the wing, wing sides 14 + 14 ft: 104 ft, level.
Rakes. Four main rakes over a 13 ft run each = 52 ft, plus two wing rakes over 10 ft = 20 ft. 72 ft × 1.118 = 80.5 ft along the slope.
Perimeter. 104 + 72 = 176 ft from above; 104 + 80.5 = 184.5 ft along the edge.
Ridges. Main: 48 ft. The wing ridge sits 5 ft above its eave (10 ft run at 6/12) and dies into the main slope where that slope is 5 ft high, 10 ft in from the main eave. So it runs 14 + 10 = 24 ft. Total 72 ft.
Valleys. Each runs from a wing eave corner to the end of the wing ridge: 10 ft across and 10 ft in, √(10² + 10²) = 14.1 ft in plan, climbing 5 ft. True length √(14.1² + 5²) = √(200 + 25) = 15 ft; two valleys, 30 ft.
| Dimension | From above | Along the slope | Feeds |
|---|---|---|---|
| Area | 1,528 sq ft | 1,708 sq ft (17.1 sq) | Shingles, underlayment |
| Eaves | 104 ft | 104 ft | Starter, drip edge, ice and water |
| Rakes | 72 ft | 80.5 ft | Drip edge |
| Ridge | 72 ft | 72 ft | Ridge cap |
| Valleys | 28.3 ft | 30 ft | Valley metal |
Where you trip
- Ordering from plan area. On this house that leaves out 180 sq ft, almost two squares.
- Using the pitch factor on valleys and hips. They cross the slope diagonally and climb slower than a rake: 14.1 × 1.118 = 15.8 ft, not the true 15 ft.
- Counting the split front slope as two facets. One plane, one facet, one pitch.
- Leaving edges unlabeled. An unlabeled rake feeds nothing, and the drip edge comes up short.
Reading it in RoofStruct
RoofStruct gives you free, unlimited self-serve measuring and PDF reports, no card, on satellite imagery or your own drone or phone photo. The PDF includes "Length, Pitch & Area Diagrams", "Area by Pitch" and "Materials to Order". For the tracing, see how to measure a roof from satellite imagery and measure your first roof; for where these numbers slip, how accurate satellite measurements are.
FAQ
Is roof perimeter the same as the house perimeter?
No. Roof perimeter follows the drip edge, outside the walls by the overhang, and its rakes are longer along the slope than they look from above.
How many facets does a cross-gable roof have?
Four, before dormers or porch roofs: two on the main block, two on the wing.
Why is true area bigger than plan area?
A sloped roof covers more surface than its footprint, by the pitch factor: 1.118 at 6/12, 1.414 at 12/12.
What does predominant pitch mean on a measurement report?
The pitch covering the most roof area, used for any facet without a pitch of its own.
Confirm the pitch, label every edge, order from true area.