Roof Snow Load Calculator - Snow Weight on Your Roof (psf)

How the Roof Snow Load Calculator Works

Snow load is weight per square foot, and snow varies more than people think. FEMA's Snow Load Safety Guide puts a foot of fresh snow anywhere from 3 psf (light, dry) to 21 psf (wet, heavy), and it notes that a foot of snow in Utah does not necessarily weigh the same as a foot in Vermont. Old snow is heavier again, because it settles and soaks up melt and rain. That is why the calculator asks for the snow in layers: the older, packed snow on the bottom and the new snow on top, plus any ice.

For each layer it multiplies the depth (in feet) by the weight of a foot of that snow, adds the layers together, and gives you a low-to-high range in psf. It then multiplies by the roof footprint for total pounds and compares the load to the design snow load you enter. If the roof still has room, it shows about how many more inches of average new snow it would take to reach the design load.

I've lived and worked in Alaska, Colorado and South Dakota, and in snow country a roof is a structure that has to carry whatever winter piles on it. The number that matters is not the depth on the news. It is the weight on your roof today, against what that roof was built to hold.

Inputs You'll Need

• Roof length and width: the footprint, measured on the ground including overhangs. See how to measure a roof from the ground.
• Snow depth by layer: inches of older, settled snow on the bottom and new snow on top, measured straight down.
• Snow type for each layer: average new snow, light powder, wet and heavy, or old and packed.
• Ice thickness: inches of ice in or under the snow, if any.
• Roof design snow load: from your plans or building department. If you do not know it, use 20 psf.

How Much Does a Foot of Snow Weigh?

These are the weights the calculator uses. Pounds per square foot per foot of depth is the same number as density in pounds per cubic foot.

Depth to reach 20 psf = 20 / weight per foot x 12, rounded. FEMA notes that snow in the western US is typically lighter than the wetter, denser snow on the East Coast, with exceptions like the Seattle area. When you are not sure, pick the heavier type. These are typical figures, not ceilings: IBHS (via DOE) says 2 feet of old snow plus 2 feet of new snow could weigh as much as 60 psf, more than this calculator's top values give for that mix (52 psf).

Worked Examples

1. A house after two storms (the calculator's defaults)

A 40 x 30 ft footprint with 12 inches of old, packed snow and 12 inches of new snow on top. Design load 20 psf.

• Footprint: 40 x 30 = 1,200 sq ft
• Old snow: 1 ft x 12-20 psf = 12-20 psf
• New snow: 1 ft x 5-6 psf = 5-6 psf
• Total: 17-26 psf, which is 85%-130% of 20 psf
• Weight: 1,200 x 17 = 20,400 lb to 1,200 x 26 = 31,200 lb

The heavy end is past the design load. Two feet of snow does not sound like much, but the bottom foot is doing most of the work. This is the case FEMA describes: close to capacity, time to get some off.

2. A small shed roof with one fresh snowfall

A 12 x 10 ft shed with 18 inches of average new snow. Design load 30 psf.

• Footprint: 12 x 10 = 120 sq ft
• New snow: 1.5 ft x 5-6 psf = 7.5-9 psf, which is 25%-30% of 30 psf
• Weight: 120 x 7.5 = 900 lb to 120 x 9 = 1,080 lb
• Room left: (30 - 9) x 12 / 6 = 42 in to (30 - 7.5) x 12 / 5 = 54 in more of average new snow

Plenty of room, as long as the snow stays light. Rain on that snow would change the picture quickly.

3. A large roof in a hard winter

A 60 x 40 ft roof with 24 inches of old, packed snow, 12 inches of wet, heavy snow on top, and 2 inches of ice. Design load 40 psf.

• Footprint: 60 x 40 = 2,400 sq ft
• Old snow: 2 ft x 12-20 psf = 24-40 psf
• Wet snow: 1 ft x 21 psf = 21 psf
• Ice: 2/12 ft x 57 psf = 9.5 psf
• Total: 54.5-70.5 psf, which is 136%-176% of 40 psf
• Weight: 2,400 x 54.5 = 130,800 lb to 2,400 x 70.5 = 169,200 lb

Even the light end is well over the design load. Have a structural engineer look at it before anyone goes up, and if you see any warning sign, get people out. FEMA says that when there is concern the snow could bring the roof down, removal should stop and the building should be evacuated.

Where Do You Find Your Roof's Design Snow Load?

The design snow load is what the roof was built to carry. Here is how to find it, from best source to rough fallback:

• Your plans. If you have the permit plans, check them for the roof design loads.
• Your building department. Under the IRC, each jurisdiction fills in its ground snow load in Table R301.2. Section R301.6 then says a house roof is designed for the roof live load in Table R301.6 (12-20 psf, depending on slope and the area each member carries) or that ground snow load, whichever is greater.
• The 20 psf rule of thumb. The DOE Building America Solution Center, citing IBHS, says most residential roofs should be able to support 20 psf of snow regardless of location. Treat it as a rough fallback, not a rating for your roof.

Engineers turn a ground snow load into a roof design load with ASCE 7. In the 2022 edition the flat-roof equation is p_f = 0.7 x C_e x C_t x p_g; earlier editions also multiplied by an importance factor I_s, which ASCE 7-22 dropped in favor of separate ground snow maps for each risk category. Where the ground snow load is over 70 psf, the IRC requires an engineered design. Your jurisdiction may amend these sections, so check your local code. And FEMA warns that a building constructed 40 years ago may not have been designed for snow loads as they are understood today: drift loads first entered the BOCA code in 1975, and unbalanced roof snow loads did not enter the UBC until 1988.

Pro Tips From the Field

• Measure the deep spots. FEMA notes that snow drifts on lower roofs, in valleys where roof slopes meet, and around chimneys, dormers, porch roofs and skylights, and that this uneven load poses a greater risk to the roof structure than an even one. Sliding snow from a higher roof piles onto porch roofs too. Run the calculator on the drift depth, not just the average.
• Know what the framing has been through. The design load assumes the roof framing is still the way it was built. A rafter or truss cut for a skylight or attic stairs, or rot at the bearing points, means the roof may not carry what its design number says.
• Rain on snow is the sneaky one. FEMA notes that design loads account for light rain on snow but not heavy rain. A short rain can soak the snow and add weight without melting it off.
• Keep the drains and gutters clear. On a low-slope roof, melt that cannot drain refreezes into ice, and ice weighs about 57 psf per foot.
• Leave a couple of inches. FEMA says to leave at least 2 inches of snow on the roof when you clear it, so the shovel never touches the roofing.

Common Mistakes to Avoid

• Treating every foot of snow the same. A foot of old snow can weigh 2 to 4 times a foot of average new snow. Two feet of mixed snow can be past a 20 psf roof's design load.
• Using the ground snow load as the snow on your roof today. The ground snow load is a code design value, not a measurement. Measure what is actually on the roof.
• Measuring along the slope. Use the footprint, and measure the depth plumb. Measuring the roof along the slope overstates the area, and on a steep roof a depth reading taken square to the roof surface understates the load per square foot of footprint.
• Climbing up to check. Measure from the ground, or from a ladder at a gable end, not at the eave. FEMA warns that roof snow can slide at any moment and says to stay out of its sliding range, and buried skylights are a serious fall hazard.
• Using metal tools or a snow blower on the roof. FEMA says to use plastic rather than metal shovels, no picks or other sharp tools, and no mechanical removal equipment.

Warning Signs to Get Out and Call an Engineer

FEMA's Snow Load Safety Guide lists these signs of an overstressed roof. If you see any of them, FEMA says to evacuate promptly and have a qualified design professional inspect the building:

• Sagging ceilings, ceiling boards falling, or sagging roof sheathing or framing
• Popping, cracking, or creaking noises
• Bowed truss bottom chords or webs
• Doors or windows that can no longer open or close
• Cracked or split wood members, or cracks in walls or masonry
• Severe roof leaks, or water pooling where it does not normally collect on a low-slope roof