Roof snow load calculator: snow weight per square foot

After a big storm, the number that matters for a roof is weight, not depth. A foot of powder and a foot of slush look much the same from the driveway, yet one can weigh several times as much as the other. This gauge turns depth and snow type into pounds per square foot, and into a total for the whole roof if you give it an area.

Pick your units with the buttons at the top, Inches / ft² or cm / m². Switching converts whatever is already in the boxes, so 12 inches becomes 30.5 cm. Under Snow depth on the roof, enter the depth. You don't need to climb up for it: the depth on open, level ground nearby is a fair stand-in, although wind piles deeper drifts below ridges and against walls. Under Kind of snow, choose the closest of four presets. Roof area is optional; the simplest figure is the area the roof covers seen from above, the house's outside length times its width plus the overhangs.

How it works

Snow is mostly air. What weighs on a roof is the water locked in it, so the gauge first works out the snow's water content, called snow water equivalent: the depth times the fraction of the snow that is water. One inch of water spread over one square foot weighs about 5.2 pounds, because a cubic foot of water weighs about 62.4 pounds and an inch is a twelfth of a foot. Multiply the inches of water by 5.2 and you have the load in pounds per square foot. Multiply that by the roof area for the whole roof.

pounds per square foot = depth in inches × water fraction × 5.2

The presets give the water fraction: Fresh, light snow (about 8% water), Settled snow, a few days old (about 25%), Wet, heavy snow (about 40%), and Packed snow or old snow with ice layers (about 50%). Real snow varies, so treat them as starting points.

The readout, titled "Load on the roof", shows pounds per square foot as the big figure. A banded scale from 0 to 60 runs under it: under 20 lb per square foot, then 20 to 40 (worth knowing your roof's rating), then over 40 (check the design load before more snow arrives). The lines below give the water held in the snow, the load in kilograms per square meter, and the whole roof.

Worked example

The gauge opens on 12 inches of settled snow, about 25% water, on a 1,200 square foot roof. Twelve inches times 0.25 is 3 inches of water, and 3 × 5.2 is 15.6. The readout shows:

• Load on the roof: 15.6 lb per square foot, in the band under 20 lb per square foot
• Water held in the snow: 3 in (7.6 cm)
• In kilograms per square meter: 76
• Whole roof: about 18,720 lb (8,491 kg)

Change only the kind of snow and watch the same foot of depth. As fresh, light snow it comes to about 5 lb per square foot. As wet, heavy snow it reaches 25, and the marker moves into the 20 to 40 band, with the whole roof carrying about 29,952 lb. Same ruler reading, five times the weight between the fresh and wet presets.

What the gauge can't tell you

What your roof can carry. That number is the roof's design snow load, and it varies by region and by building: a newer house, an old barn and a carport may share a road and still differ widely. The local building department or the building's original plans give the figure. The bands on the scale are reference points for reading the result, not a rating of your roof.

Uneven snow. The gauge assumes an even layer. Drifts collect on the downwind side of ridges, in valleys between roof sections, and on lower roofs where snow slides off a higher one, and those spots can carry much more than the average.

Rain on snow. Snow soaks up rain like a sponge, so rain falling on a snowpack adds weight without adding depth. After a rain on snow event, switch the preset to wet, heavy snow and run the numbers again.

A better density. The presets are rough. For a real figure, push a straight-sided tube down through the snow on level ground, melt the core in a rain gauge, and read the water. Inches of melted water times 5.2 gives the load directly. Volunteer observers in the CoCoRaHS network take cores the same way, and measure depth on a white snowboard to the nearest tenth of an inch, averaging several spots. The oft-quoted "10 to 1" snow-to-water ratio is only an average, and real storms stray far from it.

Warning signs and safe clearing

Watch for sagging rooflines, new cracks in walls or ceilings, interior doors that start to stick, and creaking or popping overhead. Any of those means get people out and get the structure looked at by someone qualified before more snow or rain arrives.

Clear from the ground with a roof rake, working from the edge and pulling snow down in thin layers. Never climb onto a snowy roof. Keep clear of the drip line while snow comes down, and watch where it lands: snow and ice sliding off a metal roof can flatten what sits below, one reason our piece on heat pumps in cold mountain valleys keeps outdoor units out from under the eaves. For the forecast before the next storm, the National Weather Service issues winter storm watches and warnings, and our maps and data guide points to radar and other public weather tools.

Sources

• The density of water, about 62.4 pounds per cubic foot, which gives about 5.2 pounds per inch of water per square foot.
• Typical water content of fresh, settled, wet and packed snow from snow measurement guidance; real snow varies.
• Snow measuring methods from the National Weather Service and the CoCoRaHS volunteer network.
• Roof design snow loads: the local building department or the building's original plans.

Questions

How much snow can my roof hold?

There is no single answer, and any rule of thumb you hear may not fit your building. The roof's design snow load comes from the local building department or the original plans. Compare the gauge's pounds per square foot with that number, not with a neighbor's guess.

Why does wet snow weigh so much more than fresh snow?

Fresh snow is mostly air, about 8% water in the gauge's preset. Wet, heavy snow is closer to 40% water, so the same depth carries about five times the weight. Settling, melting and rain all push snow toward the heavy end.

Should I measure snow on the roof or on the ground?

On the ground, on an open, level spot away from buildings and trees. It is far safer, and on a simple roof it is a reasonable stand-in. Expect drifts on the roof itself to run deeper in places.

Does rain on top of snow make it worse?

Yes. The snowpack soaks up the rain and gets heavier without getting deeper. Rerun the gauge with the wet, heavy preset after rain on snow, and keep an eye out for the warning signs.