Weather FIELD NOTES · 4 MIN READ
Snow Load Explained: Why a Foot of Powder and a Foot of Slush Differ by 5x
Depth times density is the whole story of roof snow loads — typical densities by snow type, water equivalent, and when to grab the roof rake.
Two feet of snow on a roof sounds alarming. Two feet of fresh powder weighs about the same as six inches of wet spring slush — and that comparison is the entire secret of snow loads. Depth is what you see; density is what breaks rafters. The arithmetic connecting them is one multiplication, and learning it changes how you look at every snowfall.
Load is depth times density
A roof snow load is simply weight spread over area: pounds per square foot (psf) or kilograms per square meter. Get it by multiplying the snow's density by its depth. Ten inches of fresh powder at about 4 lb/ft³ works out to (10/12) × 4 ≈ 3.3 psf — a cat nap worth of weight. Ten inches of dense, wet snow at about 20 lb/ft³ is (10/12) × 20 ≈ 16.7 psf. Same depth, five times the load.
In metric units the densities run from about 64 kg/m³ for fresh powder to 320 kg/m³ for rain-soaked snow, with settled snow near 144, wind-packed drifts near 240, and icy layered old snow up to 480 kg/m³. Multiply density by depth in meters and you have kilograms per square meter directly — which converts to psf by dividing by 4.88.
Why density varies so much
Fresh snow is mostly air: crystals settle into a loose lattice that is roughly 94% void. Over days, crystal tips sublimate and re-freeze, grains pack, and density drifts upward without any new snow falling. Wind does the same mechanically, grinding and packing drifts into slabs. Then liquid water enters the picture: rain or a mid-winter thaw fills air pockets, and because water is a thousand times denser than air, a modest soak moves density enormously. A snowpack that started the week at 64 kg/m³ can end it above 320 kg/m³ with its depth barely changed.
Water equivalent: the honest gauge
Hydrologists sidestep the taxonomy by reporting snow water equivalent (SWE) — the depth of liquid water the snowpack would produce if melted. It equals depth times the snow's density relative to water (about 6% for powder, 32% for wet snow). Ten inches of settled snow at 144 kg/m³ holds about 1.7 inches of water.
SWE is also the best early-warning number for roofs, because it measures the mass directly. The dangerous scenario is not a big snowfall but a modest old snowpack that gets rained on: depth stays flat while SWE jumps, and the load on your rafters doubles overnight without the yard looking any whiter.
From load to decisions
What should worry you? Residential roofs in snowy regions are commonly designed around ground snow loads of 30–70 psf depending on location, with the roof design load usually somewhat lower after slope and exposure adjustments. As field guidance: around 30 psf sustained — roughly a foot of wet snow or two feet of settled snow — is a sensible moment to clear, and any creaking, sagging at ridge lines, or popping sounds means act immediately. Flat roofs, decks and lower roofs that receive drifted snow from above reach their limits first.
Slope cuts both ways. Steep roofs shed by sliding, and design codes apply reduction factors for pitch — which is why the flat-surface number from a calculator is an upper bound for most pitched roofs. But drifting ignores averages: a 15 mph wind can strip the windward slope and stack three feet of wind-packed snow on the leeward side of a dormer, loading one section far beyond the uniform estimate.
Doing the math yourself
Measure depth in a few representative spots, judge the snow type honestly (dig a small pit and look at the layers), and multiply. If you want the total weight on a structure, multiply the load by roof area: 16.7 psf across a 2,000 ft² roof is about 33,000 pounds of wet snow — the mass of six elephants sitting quietly above your ceiling. Numbers like that make the case for the roof rake long before anything creaks.
Try it yourself
The Snow Load Calculator does the depth-times-density work for five snow types, reports the load in psf, kg/m² and kPa, converts it to water equivalent, and totals the weight over any roof area in ft² or m². When that snow melts and the same roof starts collecting rain instead, the Rainwater Harvesting Calculator picks up the water math; the Area Converter handles the roof-area unit conversions along the way.