Measuring a roof under snow cover: what imagery shows and what it hides
A snowed-in capture traces fat at the eaves, loses the valleys and hides the drip edge. What you can still read from it, and when a site photo beats the satellite.
A roof under snow can be measured from satellite imagery, but only for the things snow cannot move: the footprint of the walls, the ridge line and the general shape. The drip edge, the valleys, the step flashings and the pitch are hidden or distorted, so a snowed-in trace runs fat at the eaves and short on the line items. If the only capture for the address is white, take the shape from it and the edges from a bare-roof source: the plans, a photo from the site visit, or a drone or phone photo of your own.
In most of Canada that is the normal case, not the exception. A roof in Winnipeg, Saskatoon, Edmonton, Québec City or St. John's carries snow for a third of the year or more, and satellite imagery is a stored capture, not a live feed; whether the capture on offer is bare depends on when it was taken.
What the snow shows and what it hides
| Roof feature | Under snow | Why |
|---|---|---|
| Footprint and shape | Visible, but fat | Snow overhangs the eaves and rounds the corners |
| Ridge | Usually visible | It melts and sheds first, and the two slopes meet in a line of shadow |
| Hips | Faint | A change of plane under a smooth blanket; low sun helps |
| Valleys | Hidden | Drifting fills them level; a valley is the first thing to vanish |
| Drip edge | Hidden | The eave cornice extends past it, sometimes by a hand's width or more |
| Pitch | Never | Imagery looks straight down in any season |
Snow keeps the big numbers roughly right and destroys the small ones. The footprint comes in a few percent high; the valley metal, the step flashing and the drip-edge count come in wrong or missing. On the quote, those small lines are where the margin lives.
Reading a snowed-in capture
Trace the walls, not the snow. On a house with ordinary overhangs the wall line shows as the shadow or the siding colour just inside the snow edge; draw the outline there, then add the overhang you measured on site or read from the plans. A 400 mm (16 in) overhang on a 12 m (40 ft) eave is 4.8 m² (52 ft²) of roof per side, which is not rounding error.
Look for melt lines. The ridge melts first, then the south-facing slope, then the slopes over heated rooms. Where two slopes meet at a valley there is often a faint seam once the melt starts; it tells you the valley is there, not how long it is.
Count what you can and flag the rest. In the RoofStruct builder you can trace the outline on the satellite imagery and leave a facet without a pitch; it is counted flat, and the drawing opens on any device for the site visit, as the measure feature page shows. Set the pitch per plane once you have it, from the attic, from a gauge on the rake or from the free pitch finder, which works from a street-level photo and is not affected by snow on the roof.
When there is no bare capture: two fallbacks
- A drone or phone photo of your own. Taken on a clear day and scaled off a known length such as the garage door. It shows the drip edge in winter as long as the snow has slid off the lower courses, which on a 6/12 or steeper roof it often has. Our guide to measuring from a drone photo covers the scaling.
- The plans or the permit drawing. The overhang, the valleys and the pitch are all on them. Trace the footprint from the capture for the as-built shape, then take the edge lengths from the drawing.
What does not work is tracing the snow edge and hoping the waste factor covers it. A fat outline errs high on the shingles and the hidden valleys err low on the metal, so no single percentage fixes both.
Worked example: a Saskatoon bungalow in February
A 4/12 bungalow with a gable over the main house and an attached garage that forms two valleys. The bare footprint at the drip edge is 12.2 m by 9.1 m (40 ft by 30 ft).
| Figure | Bare-roof trace | Snowed-in trace |
|---|---|---|
| Footprint | 111.5 m² (1,200 ft²) | 118.1 m² (1,271 ft²), about 150 mm (6 in) wide on each side |
| Roof area at 4/12 (factor 1.054) | 117.5 m² (1,264.8 ft²), 12.6 squares | 124.5 m² (1,339.6 ft²), 13.4 squares |
| Valleys | Two of 4.3 m (14 ft), 8.5 m (28 ft) of valley metal | None seen |
| Waste factor | The valleyed rate | The plain-gable rate |
A square is 100 ft² of roof surface, which is 9.29 m². The snowed-in trace adds about three quarters of a square, two bundles and change at three bundles per square, that the crew brings back to the yard. The same trace drops 8.5 m (28 ft) of valley metal, the ice and water strip the valleys need, and the extra waste a valleyed roof eats in cuts. The overcount on the field and the undercount on the valleys do not cancel: one is money sitting in the truck, the other is a Thursday supply run.
The fix was a photo from the driveway. The garage valleys were plain from the ground, the drip edge showed below the slid snow on the south side, and the footprint came from the capture with the outline pulled in to the wall line plus the 400 mm (16 in) overhang from the plans.
Where you trip
- Tracing the cornice. The snow at the eave hangs past the drip edge, so every eave length is long and the starter and drip-edge counts are padded while the valleys are missing.
- Treating a smooth white roof as a plain gable. The hips and valleys are under there. Check the shape against a photo or the plans before you pick the waste rate.
- Reading pitch from the melt. A slope that sheds early is probably steeper or warmer, not measurably either. Pitch comes from a gauge, the plans or a street-level photo.
- Forgetting the eave protection run. The provincial code sets a minimum width of eave protection up the slope on most sloped roofs, and the width depends on the overhang, which the snow hides. Our eave protection guide covers the width; measure the overhang on site.
- Quoting in square metres from a fat trace. The error is the same in either unit. Fix the outline first; our Ontario satellite guide walks the bare-roof trace step by step.
Take the shape from the capture and the edges from a bare source, and note on the quote which capture or photo each number came from. In April you will want to know.
FAQ
Can I measure a roof from satellite imagery when it is covered in snow?
Partly. The footprint and the shape are readable, with the outline pulled in to the wall line plus the overhang. The drip edge, the valleys and the pitch are not, so take those from a photo, the plans or a gauge.
How much does snow change the measured area?
It depends on how far the snow overhangs the eaves. A trace that runs 150 mm (6 in) wide on every side of a 12.2 m by 9.1 m (40 ft by 30 ft) footprint adds about 6% to the area, close to a square on a bungalow once the pitch factor is applied.
Which is better in winter, a drone photo or the satellite capture?
Whichever is bare. A drone or phone photo taken on the day shows the roof as it is, including the drip edge where the snow has slid off. The satellite capture may be older and may be bare, which is better still. If neither is, the photo plus the plans.