How to measure a roof in Ontario from satellite imagery: a step-by-step for contractors
A six-step method for Ontario roofers: pick a snow-free capture, trace the drip edge, set the pitch, label the edges and turn the roof into squares and square metres.
To measure a roof in Ontario from satellite imagery, pick a capture with no snow on the roof, trace the outline at the drip edge, set a pitch for each plane, label the edges, then let the pitch factor turn the footprint into roof area in square feet and square metres. The six steps below take a few minutes per house and work the same in Windsor, Thunder Bay and Ottawa.
What Ontario adds is weather and housing stock: a snow belt east of Lake Huron and Georgian Bay, a long winter in the north, Victorian semis with shared roofs in Toronto, and sprawling hip-and-valley roofs in the 905 subdivisions. Each one changes what you check before you draw a line.
Step 1: Find the house, not the pin
Check that the pin landed on the right roof before you trace. In a new subdivision in Milton, Barrie or Kanata, the address database can lag the houses by a year and drop the pin on a neighbour or an empty lot. On a concession road it may sit at the driveway entrance, a few hundred metres from the house. Confirm with the lot shape, the driveway and the garage position, or ask the customer for a photo of the front. A perfect measurement of the wrong roof is worth nothing.
Step 2: Choose a capture without snow
Ontario roofs carry snow from November to April, longer in Muskoka, Sudbury and north of North Bay. Snow hides the drip edge, rounds off hips and ridges and fills valleys, so a snowed-in roof traces fat. Leaves matter too: in the older parts of Toronto, Hamilton, London and Kingston, the canopy covers the eaves in July. The best Ontario capture is often April or May, after the snow and before the leaves, or October after the leaves drop. If every capture is snowed in or under a canopy, use your own drone or phone photo from the site visit instead of guessing the eave line.
Step 3: Trace the outline at the drip edge
Draw the outline on the drip edge, not the wall line: the overhang is roof you shingle and eave you flash. On a 1960s Ontario bungalow with wide eaves, the difference can be a square or more on a modest roof. Trace the main house first, then each attached piece: garage, porch, the bump-out over the bay window. On a Toronto semi or a row of townhouses, stop at the party wall; you are measuring one unit of a shared roof.
In the RoofStruct builder, Draw Roof is the trace tool, on satellite imagery or on your own drone or phone photo; the measure feature page shows the flow. A shape drawn over another facet is counted twice, which is what you want for a dormer on a slope, and a shape left without a pitch is counted flat.
Step 4: Set a pitch for every plane
Satellite imagery looks straight down, so it shows the footprint, never the slope. The pitch comes from the plans, a gauge on the rake at the site visit, or the free pitch finder from the address. Ontario subdivisions built since the 1990s often run a steep front over a lower-pitched rear or garage roof, so set each plane; a bungalow is usually one pitch all round. The pitch factor is the multiplier from footprint to roof area:
| Pitch | Factor | 1,000 ft² footprint becomes |
|---|---|---|
| 4/12 | 1.054 | 1,054 ft² (97.9 m²) |
| 6/12 | 1.118 | 1,118 ft² (103.9 m²) |
| 8/12 | 1.202 | 1,202 ft² (111.7 m²) |
| 10/12 | 1.302 | 1,302 ft² (121.0 m²) |
Step 5: Label every edge
Each line is an eave, a rake, a ridge, a hip, a valley, a step or a wall. The labels give you the lineal lengths for drip edge, starter, ridge cap, valley metal and the eave protection strip. The builder asks "What is this edge?" for each one and shows "Edges labeled X of Y" so none is skipped.
Ontario has its own provincial building code, and it asks for eave protection at the eaves of most sloped roofs, measured up the slope from the edge to a line past the inside face of the exterior wall. The width you order depends on the overhang and the pitch, not on the eave length alone, so measure the overhang from the outline or on site. Our guide to eave protection in Canada walks through the membrane width and the roofs the code exempts.
Step 6: Turn it into squares and metres
A square is 100 ft² (9.29 m²) of roof surface, and Ontario suppliers still sell shingles by the bundle with coverage printed per square, even when the quote to the homeowner is in square metres. The order of operations is footprint, then pitch factor, then waste, then bundles.
Worked example: a two-storey in Mississauga
| Measurement | Imperial | Metric |
|---|---|---|
| Footprint at the drip edge | 1,500 ft² | 139.4 m² |
| Pitch, all planes | 6/12 (factor 1.118) | 26.6° |
| Roof area | 1,677 ft² | 155.8 m² |
| Squares before waste | 16.8 squares | |
| Waste at 12% (hips and two valleys) | 18.8 squares, order 19 | |
| Eaves | 152 ft | 46.3 m |
| Ridge | 36 ft | 11.0 m |
| Hips, four | 84 ft | 25.6 m |
| Valleys, two | 26 ft | 7.9 m |
| Rakes on the front gable | 24 ft | 7.3 m |
The 1,500 ft² footprint is what the satellite shows; the 1,677 ft² (155.8 m²) is what the crew shingles. The 152 ft (46.3 m) of eave sets the drip edge, starter and the eave protection run; the hips and valleys set the cap and valley metal. Our post on bundles per square in Canada takes the 19 squares the rest of the way to a count. Ontario's HST applies to the whole quote, materials and labour alike.
Where you trip in Ontario
- Tracing a snowed-in capture. The outline runs fat by the depth of the snow at the eaves and the valleys vanish. Switch to a bare-roof capture or a site photo.
- Stopping at the wall line on wide eaves. A bungalow in Scarborough or Oshawa with 24-inch (0.6 m) overhangs loses a square of shingles and the whole eave protection run.
- One pitch for the whole house. A 10/12 front over a 4/12 garage roof differs by a quarter in factor. Set each plane.
- The shaded north eave in the snow belt or the Ottawa Valley. That is where the ice forms; label it as an eave even when the canopy hides part of it.
FAQ
Can I measure a roof in Ontario from satellite imagery in winter?
Yes, from a capture taken when the roof was bare. Satellite imagery is not live, so pick a snow-free date from the captures available, or use your own drone or phone photo.
Do Ontario roofers measure in square feet or square metres?
Both. The crew and the shingle supplier work in squares and feet; permit forms, insurers and some homeowners expect square metres. Record the footprint and the roof area in both so no one converts by hand later.
Does eave protection change what I measure?
Yes. Beyond the eave length you need the overhang and the pitch of each eave to work out the membrane width the Ontario code asks for. Our Canada measuring guide covers the general method; the eave protection post covers the width.