Snow Curls on Roofs: Why Snow Slides Off a Cabin Roof and Hangs in Mid-Air

A thick sheet of snow slides off a cabin roof and stops in mid-air, curling over the edge like a rolled-up carpet. Pictures like this, first shared on Reddit by user Meikami of an uncle’s cabin, look impossible. They have a simple explanation: melting, sliding and refreezing, repeated over several days.

They are usually called snow curls or snow rolls, and they turn up on houses, porches and especially smooth metal roofs.

Why snow curls off a roof

Fargo meteorologist Jesse Ritka of StormTracker describes the recipe step by step:

1. Wet snow builds up. Heavy, sticky snow piles up into a thick, cohesive layer on the roof.
2. The base melts. When temperatures climb to around or just above freezing, and the roof gives off heat from the house below, the bottom of the layer melts. The meltwater cuts friction, and the whole pack begins to slide towards the eaves.
3. A quick freeze stops it. If the air turns cold again, the part that has slid over the edge refreezes while still bent downwards, so it hangs in place.
4. The cycle repeats. The roof keeps melting the base while the overhang stays frozen. New snow slides down behind it, and the frozen front curls under to make room.
5. It finally falls. When temperatures rise for good, the curl drops off the roof.

Metal roofs, eave cornices and snow guards

Metal roofs are the classic place to see curls, because snow slides easily on them. Snow-guard maker Alpine SnowGuards, testing roofs at its research facility in Morrisville, Vermont, reported snow and ice creeping over the eaves as cornices after heavy drifting followed by cold weather. Heat escaping from the building left a thin layer of ice near the roof surface, and the whole mass slowly slid outwards on it.

Snow guards are the standard fix. They hold sliding snow upslope of where they are mounted, so the pack melts and drains away as water. They cannot control snow below them, which is why their position and number matter more than the strength of any single bracket.

Keep clear of the eaves under a big curl: the hanging mass is heavy and drops without warning when it releases.

A mini glacier on the roof

Roofthings.com calls one of these curls a mini glacier on a metal roof, and the comparison helps. A real glacier also gains snow in one zone and loses it in another. The U.S. Geological Survey defines a glacier’s mass balance as the difference between accumulation, the snow added each year, and ablation, the snow and ice lost through melting, runoff, calving icebergs and evaporation. When accumulation wins, a glacier thickens and advances; when ablation wins for years, it thins and retreats.

The USGS has measured three benchmark glaciers since 1957: South Cascade Glacier in Washington, and Wolverine and Gulkana glaciers in Alaska. All three have lost mass since monitoring began, and the losses have sped up since 1989.

A roof curl runs the same budget in miniature over a few days: new snow at the top, melt at the base and a front that pushes slowly outwards.

More roof snow and winter shapes

Explore more: snow depth measured in a pit; wind-shaped snow around a doorway.

Related reading

• Unusual Ice Forms: Penitentes and Delicate Frost Ribbons
• A Roof-Snow Removal Tool: Understanding the Demonstration
• A Roof-Snow Release Beside a Ladder
• Roof Ice Falling at a Lodge in Grapevine, Texas

Sources