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Most residential roofs in the United States are designed to carry somewhere between 20 and 42 pounds per square foot of snow, and the depth that represents depends completely on what kind of snow it is. As a rough conversion for a 20 psf roof: about 4 feet of fresh powder, about 2 feet of settled packed snow, or about 4 inches of solid ice will all reach the design load. Double those figures for a roof rated to 40 psf. Depth is the number people quote and density is the number that matters — a wet snowfall can weigh seven times what the same depth of dry powder weighs.
How much does snow actually weigh?
Snow density varies more than almost any other common material you will encounter. University of Minnesota Extension, in its guidance on preventing roof collapse, gives the working range as 3 pounds per cubic foot for light fluffy snow up to 21 pounds per cubic foot for wet heavy snow, with ice at around 57 pounds per cubic foot. That is a sevenfold spread in snow alone, and nearly twentyfold once ice is involved.
| Type | Weight per cubic foot | Load from 1 ft depth | Depth to reach 20 psf | Depth to reach 40 psf |
|---|---|---|---|---|
| Fresh light powder | ~3 lb | ~3 psf | ~6 ft | ~12 ft |
| Typical fresh snow | ~5 lb | ~5 psf | ~4 ft | ~8 ft |
| Settled snow | ~12 lb | ~12 psf | ~20 in. | ~3 ft 4 in. |
| Wet heavy snow | ~21 lb | ~21 psf | ~11 in. | ~23 in. |
| Ice | ~57 lb | ~57 psf | ~4 in. | ~8 in. |
The row that catches people out is settled snow. A roof carrying 20 inches of old, compacted January snowpack is at the same load as one carrying 4 feet of the fresh stuff that fell overnight — and it looks like far less snow.
How to estimate your own load without guessing
You do not need a snow-load calculator. You need a coffee can and a kitchen scale.
- Push a straight-sided container — a coffee can, a large tin, a section of pipe — down through the snow to the ground in an open area of your yard, and lift out a full-depth core.
- Weigh the contents in pounds. A kitchen scale in ounces divided by 16 is fine.
- Divide by the container's opening area in square feet. A 6-inch-diameter can has an opening of about 0.196 square feet.
- The result is pounds per square foot on the ground. Roof load is generally somewhat less on a steep pitch and roughly equal on a low-slope roof, so treat the ground figure as a conservative estimate.
A quicker version: melt the core and measure the water depth. Every inch of liquid water equals about 5.2 pounds per square foot. Two inches of water content across your snowpack is roughly 10 psf regardless of how deep the snow looks.
What is your roof actually rated for?
There is no single national number. Roof snow load is derived from the local ground snow load in the building code your house was permitted under, then adjusted for exposure, roof slope, thermal condition and importance. In practice:
- Warm regions and older light-frame structures: often around 20 psf.
- Snow country: substantially higher. Minnesota state statute sets residential roof snow load at 42 psf in northern Minnesota and 35 psf in the south, and mountain jurisdictions in Colorado, Utah and northern New England go well beyond that.
- Agricultural and light commercial buildings: frequently designed to 20 psf, which UMN Extension notes corresponds to about six feet of dry fluffy snow or one foot of wet heavy snow. This is why barns, machine sheds, carports and flat-roofed additions collapse in storms that leave houses untouched.
- Flat and low-slope roofs anywhere: the highest-risk category, because nothing slides off and drifting piles snow unevenly. Drift loads against a parapet or a taller adjoining wall can be several times the uniform load on the same roof.
Your local building department can tell you the ground snow load your permit was issued against, and the US Department of Energy's Building America guidance on managing snow loads on roofs and decks is a useful primer on how those figures are derived and where drifting changes the answer.
The scenario that causes most collapses
It is rarely a single record snowfall. The common pattern is a sequence: snow falls, partially melts, refreezes into a dense layer, more snow falls on top, and then rain arrives. Rain onto an existing snowpack is the worst case in this whole subject, because the snow absorbs the water rather than shedding it and the roof gains weight with no visible change in depth. Add a blocked or frozen drain on a flat roof and you get ponding on top of the snow load.
The second common pattern is drifting. Wind strips snow off one side of a roof and piles it against a parapet, a dormer, a chimney or a step up to a taller section — concentrating several times the average load in one strip. If your roof has a lower section abutting a higher wall, that is the spot to watch.
Warning signs, in order of urgency
- Creaking, popping or cracking noises — leave the building. Wood framing announces overload audibly before it fails.
- Doors or windows that suddenly stick or will not latch — the frame is being squeezed by deflection above it. This is the classic early sign and it is easy to dismiss.
- New cracks in drywall or plaster, especially radiating from door and window corners.
- A visibly sagging or bowed ceiling, or sprinkler heads that have dropped below the ceiling plane.
- Visible deflection in rafters or trusses when you look in the attic, or gaps opening at truss connections.
- Doors that push open on their own, or interior doors binding at the top.
Any of these means evacuate and call a structural engineer or the fire department. Do not go into the attic to investigate a roof that is making noise, and do not go onto it.
If you do need to reduce the load
Clear from the ground with a roof rake, working the lower three or four feet of the roof edge, and stand out of the fall line — released snow comes down as a slab. Our roof rake picks cover the tools; look for wheels or roller guides that keep the head off the shingles, because cold asphalt sheds granules easily.
Three things not to do. Do not climb onto a snow-covered roof — it is a pitched, low-friction surface over a hard landing, and this is how homeowners are seriously injured every winter. Do not chip ice with an axe, hatchet, hammer or ice pick; below about 40 °F asphalt shingles are brittle and the tool that breaks the ice removes the roofing with it. Do not clear one side or one strip only — uneven removal can create an unbalanced load that stresses the structure more than the original uniform load did.
If the roof genuinely needs full clearing, that is a contractor's job, done with fall protection and usually with low-pressure steam for the ice. And if your recurring problem is ice at the eaves rather than snow across the roof, you are looking at a different problem with a different fix — see how to prevent ice dams, because that is a heat-loss issue inside the house, not a snow-removal issue.
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