One Thursday in July 2023 I drove out to Rangiora to look at a roof that had, according to the owner, started leaking in three places overnight. It had not rained. It had snowed, about 180 mm of the wet, heavy kind that Canterbury gets when a southerly meets moist air off the sea. The house was a 1970s brick place on Ivory Street with long-run corrugate on the main roof and a low-pitch lean-to over the back porch. The three leaks were all along the wall where the lean-to met the main roof, and there was nothing wrong with the roof at all. The snow was doing what snow does when it melts on a roof not designed to hold water.
What 180 mm of wet snow weighs
Fresh dry snow weighs about 100 kg per cubic metre. The wet snow that falls on the Canterbury plains is closer to 300 to 400 kg per cubic metre, so 180 mm is 55 to 70 kg on every square metre of roof. NZS 3604 puts low-lying Canterbury in a snow zone that is modest but not zero, and roof framing built to it is designed for a ground snow load in the order of 1 kPa. The Building Code requirement is set out on building.govt.nz, and the point is that a compliant house carries an ordinary Canterbury snowfall without structural worry.
Lean-tos, carports and pergolas added over the years with light framing are the structures I see sagging. So are old spouting brackets. Spouting full of wet snow and ice weighs 8 to 12 kg per metre, and I have seen 20 metres peel off a fascia in a single afternoon.
Drifts, slides and the roof below
Snow does not sit evenly. Wind piles it against chimneys, parapets, second-storey walls and skylight upstands. A 180 mm fall can become a 500 mm drift on the leeward side of a chimney. Snow on a steel roof also slides: once the sun gets on a painted long-run roof at 25 degrees the whole slab lets go at once and lands on whatever is below. I have measured 400 mm of compacted slide snow on a lean-to that received 150 mm of actual snowfall. That is what happened at Rangiora. The main roof shed onto the lean-to, which was pitched at about 5 degrees, and it held a bank of snow 300 mm deep against the wall.
Where the water goes when it melts
A snow bank melts from the bottom, where the roof steel is warmed by the house beneath. Meltwater runs downhill under the snow into a spouting full of snow with the downpipe plugged by ice. So the water rises. On corrugate at 5 degrees, water backs up only 20 mm before it is above the corrugation height and running sideways to any lap it finds. Corrugate has a minimum pitch of 8 degrees for exactly this reason; a lean-to at 5 degrees is fine in rain, not with 300 mm of snow melting over three days.
The other place water goes is behind the fascia. When the spouting is frozen and meltwater reaches it, water runs over the back edge, down the fascia and into the soffit, producing a leak 600 mm inside the exterior wall that convinces owners their roof has failed. What happens after the water reaches the downpipe outlet is covered in roof runoff and where the downpipe ends.
What is worth doing before the next fall
- Check spouting brackets. Screwed brackets at 600 mm centres hold snow load; the 1970s clip-on brackets at 900 mm often do not. Refitting runs NZ$12 to NZ$25 per bracket.
- Look at the lap over any low-pitch lean-to. A 200 mm end lap with a bead of neutral-cure silicone or butyl lap tape resists melt back-up far better than a dry 150 mm lap.
- Clear leaves out of the spouting in autumn. A spouting half full of leaves freezes solid an hour faster and thaws a day slower.
- Do not get up and shovel. A snow-covered steel roof is the most dangerous surface I know, and the roof is not going to fail under the load. Wait for the melt.
The Rangiora owner spent about NZ$600 having the lean-to laps taped and two brackets refitted. The next snow, in 2024, produced no leaks at all.
Frequently asked questions
How much snow can a Canterbury house roof hold? A house built to NZS 3604 on the plains is designed for a ground snow load in the order of 1 kPa, which is roughly 250 mm of wet snow or 1 metre of dry powder. Add-on lean-tos and carports built with light framing are the structures that sag, not the main roof.
Why does my roof leak when snow melts but not when it rains? Meltwater runs under the snow bank into a spouting that is frozen or full, then backs up the sheet until it climbs over a lap or the back of the spouting. Rain never sits long enough to back up, so laps that are fine in rain fail under melting snow.
Should I clear snow off my roof? No. A snow-covered steel roof is far too dangerous to walk on, and a code-compliant roof is not at risk from an ordinary fall. Clear the spouting outlets from the ground with a hose if you can reach them, and let the rest melt.