How Much Does Snow Weigh? Roof Load by Depth and Type

The short answer: it depends on the water in it

FEMA's Snow Load Safety Guide gives the range for fresh snow: 1 foot of new snow weighs 3 lb per square foot when it is light and dry, and up to 21 lb per square foot when it is wet and heavy. That is a factor of seven between two snowfalls of the same depth. In metric terms, the same range is about 48 to 336 kg per cubic meter of snow.

FEMA adds that snow in the western United States is typically lighter and less dense than the wetter, denser snow of the East Coast, with local exceptions.

Ice sits far above that range. FEMA puts 1 inch of ice at a little less than 5 lb per square foot and 1 foot of ice at about 57 lb per square foot. Inch for inch, ice is heavier than even wet, heavy snow.

Snow density and snow water equivalent

Two terms from the National Weather Service and the USDA Natural Resources Conservation Service (NRCS) do all the work in this guide:

• Snow water equivalent (SWE) is the water you get by melting the snow. NRCS describes it as the depth of water that would result if the whole snowpack melted at once. NRCS measures it at many remote SNOTEL sites and uses the data for streamflow forecasts.
• Snow density is the snow water equivalent divided by the snow depth. NRCS states it as a share of the weight of the same volume of water, usually as a percentage. Snow at 10% density holds one inch of water in every ten inches of depth.

Water itself weighs about 62.4 lb per cubic foot at temperatures just above freezing, from NIST's property data for water (about 1,000 kg per cubic meter). Spread one inch of water over one square foot and it weighs 62.4 ÷ 12 = 5.2 lb. In metric, one millimeter of water over one square meter weighs about 1 kg.

NRCS works through an example: 36 inches of new, powdery snow at 10% density holds 36 × 0.10 = 3.6 inches of water. By the formula above, that snow weighs 3.6 × 5.2 = 18.7 lb per square foot. NRCS also gives the reverse: SWE ÷ density = snow depth.

Settling changes the depth, not the water. NRCS explains that snow gets denser after it falls through gravitational settling, wind packing, melting and recrystallization. Snow that sinks from 12 to 8 inches still holds the same water unless meltwater drained away, so it weighs the same.

The 10:1 ratio is only a rough guide

You will often read that 10 inches of snow equal 1 inch of rain. The NWS office in La Crosse, Wisconsin, calls this an old rule of thumb that is far from the norm, and says recent studies point to about 12:1 as more representative on average for the Upper Midwest. The ratio can even change within a single storm. Snow near freezing has a lower ratio (denser snow), deep cold tends to give a higher ratio, and wind can break flakes apart, which lowers the ratio.

NOAA's National Severe Storms Laboratory gives the US average as 13 inches of snow to 1 inch of rain, and a range from about 2 inches for sleet to nearly 50 inches for very dry, powdery snow.

For observers who cannot melt or weigh a sample, NOAA publishes an estimating table that ties the ratio to the air temperature, meant only as a guide for newly fallen snow. The weight columns are computed from its ratios.

The table notes that it cannot be used for old snow, because packing and melting and refreezing change the density of a snowpack substantially. Roof snow that has sat for days is exactly that kind of snow.

Weight per cubic foot by snow type

NRCS gives these density ranges. New snow runs from about 5% at an air temperature of 14 °F to about 20% at 32 °F. East of the Cascades, typical snowpack densities are 10 to 20% in winter and 20 to 40% in spring; in the Cascades of Washington and Oregon they are higher, about 20 to 30% in winter and 30 to 50% in spring. The rows below use round values from those ranges, plus FEMA's figure for ice. Your snow can sit anywhere in between.

FEMA's range of 3 to 21 lb per square foot for a foot of fresh snow works out to densities of about 4.8% to 33.7%, which fits the rows above. If you need these numbers in other units, the weight converter handles pounds and kilograms, and the length converter turns inches of snow into centimeters.

Weight per square foot by depth

Each cell is depth × density × the weight of water, for snow on a level surface. These are snow weights, not a roof's design load.

The same table in metric units, with depth in centimeters and the load in kilograms per square meter:

Two feet of new snow at 10:1 and six inches of spring snowpack at 40% weigh the same, about 12.5 lb per square foot. That is why forecasts and snow surveys report the water equivalent next to the depth.

Measure the water in your own snow

The NWS snow measurement guidelines for its volunteer observers describe how to find the water content of snow on the ground with a core sample. The steps, adapted for a homeowner:

1. Pick a representative spot where blowing and drifting are minimal. Where wind cannot be avoided, the NWS asks for several measurements (typically three or more).
2. Measure the depth first. Snow depth is the total depth of snow on the ground, including any ice layers.
3. Take a core at the same spot. Observers push the inverted outer cylinder of a rain gauge straight down through the snow and slide something thin and sturdy, such as a spatula, under it to lift the core out.
4. Melt and measure. Let the snow melt indoors and measure the water. In a straight-sided container, the depth of the meltwater is the snow water equivalent of the core.

Example with made-up numbers: a core 8 inches deep melts to 1.2 inches of water. The density is 1.2 ÷ 8 = 15%, and the snow weighs 1.2 × 5.2 = 6.2 lb per square foot.

This tells you about the snow on the ground, not on the roof. FEMA is explicit that 1 foot of snow on the ground does not necessarily equal 1 foot of snow on a roof, and that measuring ground snow and assuming the roof carries the same load is incorrect. Do not climb onto a snow-covered roof to take a sample; see the safety notes further down.

Example: the total weight on a roof

The total weight is the load per square foot times the area it covers. This example uses a flat roof so the geometry stays simple. All figures are example values to show the arithmetic; they say nothing about whether any real roof can carry them.

A detached garage has a flat roof of 24 × 24 ft = 576 square feet (about 53.5 m²), covered evenly with 12 inches of snow.

The depth never changed in the first two rows, yet the load tripled. In the third row, rain soaked into the settled snow: every inch of rain the snow holds adds 5.2 lb per square foot. Real roof snow is rarely this even.

To get your own area, the square footage calculator adds up rectangles and other shapes, and the guide to measuring a room covers the tape-measure side. For a sloped roof, the roof pitch calculator handles pitch, rafter length and roof area. The area converter switches between square feet and square meters.

Roof snow is not ground snow: drifts, slides and ice dams

FEMA calls it unbalanced snow load when snow lies at different depths in different places on a roof, and says it poses a greater risk to the roof structure than an even load. Three effects cause it.

Drifting snow

Wind carries snow from one part of a roof to another. FEMA says drifts often form on a lower roof in the wind shadow of a higher part of the building, and against obstructions. Places it lists:

• the leeward side of the ridge on gable and single-slope roofs
• parapets on flat and low-slope roofs, and steps where one roof meets a higher one
• valleys and the intersections of sloping roofs
• rooftop equipment such as heating and cooling units and solar panels
• on houses: chimneys, dormer windows, porch roofs and skylights

Sliding snow

Unless a sloped roof has snow guards, snow can slide from a higher roof onto a lower one and pile up there. FEMA names porch and sunroom roofs and entrance canopies as common cases; the impact itself can overload the lower roof. It describes the angle at which snow tends to slide as about a 30 degree slope, often referred to as 6:12 or 7:12, while noting that snow on shallower roofs can slide too and that roofs as low as 10 degrees have been seen to shed snow. Slippery surfaces such as metal panels shed snow more readily than asphalt shingles. The roof pitch calculator converts a pitch like 6:12 into degrees.

Ice dams

When temperatures swing above and below freezing, snow melts and refreezes. FEMA explains that on sloped roofs, heat from the building can melt the snow from below; the meltwater runs down and freezes at the colder eave. The dam grows and holds back more water. FEMA lists two problems: the ice and snow at the eave create an unbalanced load and keep snow from sliding off, and water can get into the building. A blocked or poorly maintained drainage system makes this worse. With ice at about 4.8 lb per square foot per inch, a dam adds up quickly. Example values: ice 3 inches thick in a band 2 ft wide along a 40 ft eave covers 80 square feet at 14.3 lb per square foot, about 1,140 lb concentrated at the roof edge.

Rain on snow and melt between storms

FEMA notes that building code provisions for design snow loads include light rain on snow, but not heavy rainfall. Continuous rain can wash snow away, but a short period of rain can melt the snow and saturate it, which significantly increases the load. FEMA also warns that snow left in the sun can soften, become denser and then harden when it freezes again.

Melt between storms helps if the roof drains properly. If drains are blocked or poorly designed, FEMA says meltwater can collect in low areas of flat or low-slope roofs. This is called ponding, and it concentrates the load in one spot. A snow event, in FEMA's words, is not necessarily one storm: a series of storms can keep adding to the load on a building.

Design snow loads come from the code, not from this table

FEMA's guide states that it is not a manual for computing snow loads, and neither is this page. The weights above describe snow, not what a roof can carry. FEMA explains where that number comes from:

• Building codes, today usually the International Building Code (IBC), give a ground snow load for the location: the weight of snow on the ground. The values are based on National Weather Service data. The ground snow load maps in the IBC and in the ASCE 7 standard show a load with a 2 percent chance of being equaled or exceeded in any given year.
• The designer starts from that number to calculate the roof snow load, the weight of snow on the roof surface used to design the structure. It depends on the building's importance and use, wind exposure, roof slope and shape, obstructions on the roof and the building's thermal condition.
• The detailed design rules are in ASCE 7, Minimum Design Loads for Buildings and Other Structures, and states or local jurisdictions may amend the code.

Older buildings may follow older rules: FEMA notes that drift loads first entered one of the regional model codes in 1975, and that a building constructed 40 years ago may not have been designed for snow loads as they are understood today. Reroofing over an old roof and added rooftop equipment can also reduce the capacity left for snow.

To find the number for your own building, FEMA points to the construction drawings, which state the design snow load. If there are no drawings, it advises retaining a licensed design professional before the snow season to determine the structural capacity. If you know the age of the building, your local building department should be able to tell you which code edition was in force when it was built, and it knows the design snow load required where you live.

Warning signs of an overloaded roof

FEMA's guide says most buildings are not at risk of snow-induced failure. It also says overstressed roofs typically show warning signs, and lists these as common in wood, metal and steel buildings:

• sagging ceiling tiles or boards, ceiling boards falling out of the ceiling grid, sagging sprinkler lines and sprinkler heads
• sprinkler heads deflecting below suspended ceilings
• popping, cracking and creaking noises
• sagging roof members, including metal decking or plywood sheathing
• bowing truss bottom chords or web members
• doors or windows that can no longer be opened or closed
• cracked or split wood members
• cracks in walls or masonry
• severe roof leaks
• excessive water collecting at non-drainage locations on low-slope roofs

FEMA's instruction if you see any of these: evacuate the building promptly and contact your local building authority or a qualified design professional, such as a licensed professional engineer, for a detailed structural inspection. FEMA also notes that secondary structures such as porches, carports, detached garages and sheds typically do not perform as well as the main building during snow events.

If snow has to come off the roof

FEMA's guide opens with a caution: more often than not, attempting to remove snow from a roof is more hazardous than beneficial, for the people doing it and for the roof. When removal is needed, FEMA recommends a licensed, insured professional roofing contractor with experience in removing snow from roofs, and says removal should only be done by qualified individuals following safety protocols. Its guidance for that work includes:

• Follow OSHA protocols for work on rooftops and use fall arrest harnesses where applicable.
• Keep someone on the ground to keep people away from falling snow, ice and icicles.
• Mark buried skylights and other rooftop hazards beforehand.
• Leave at least a couple of inches of snow on the roof, because removing it completely can damage the roof covering.
• Do not use mechanical snow removal equipment or sharp tools such as picks; use plastic rather than metal shovels.
• Remove drifted snow first, then work from the center of the roof, in the direction of the main structural members, so the load stays balanced. Do not pile snow on the roof.
• A long-handled non-metallic snow rake can sometimes be used from the ground. Metal rakes can damage roofing and pose an electrocution risk, and roof snow can slide at any moment, so stay clear of the eave.

FEMA's warning applies throughout: if there is any concern that the snow may cause the roof to collapse, stop all removal and evacuate the building.

On the ground, the National Weather Service advises avoiding overexertion, such as shoveling heavy snow, if you are not in good health, because the strain from the cold and the hard labor may cause a heart attack.

Frequently asked questions

Sources

• FEMA: Snow Load Safety Guide, FEMA P-957, January 2013, copy hosted by the Whole Building Design Guide (snow weight per foot, ice weight, drifting and sliding snow, ice dams, rain on snow, ground and roof snow load definitions, warning signs, snow removal methods)
• FEMA: Snow Load Safety Guidance flyer, FEMA P-957 (warning signs of overstress, evacuate and contact a local building authority or design professional, safety measures and methods for snow removal)
• National Weather Service: Snow Measurement Guidelines for National Weather Service Surface Observing Programs, September 2013 (snow depth, water content of snowfall and of snow on the ground, core sample method)
• NOAA National Centers for Environmental Information: Estimating the Water Equivalent of Snow, New Snowfall to Estimated Meltwater Conversion Table (ratios from 10:1 at 28 to 34 °F to 100:1 at −21 to −40 °F; new snow only)
• National Weather Service, La Crosse, WI: What Are Snow Ratios? (10:1 as an old rule of thumb, 12:1 more representative in the Upper Midwest, what raises and lowers the ratio)
• NOAA National Severe Storms Laboratory: Severe Weather 101, Winter Weather FAQ (on average 13 inches of snow equals 1 inch of rain in the US; about 2 to nearly 50 inches)
• National Weather Service glossary: Snow Density and Snow Water Equivalent (definitions)
• USDA Natural Resources Conservation Service: Field Office Technical Guide Supplement, Climate Data (snow water equivalent, new snow density about 5% at 14 °F to about 20% at 32 °F, snowpack densities in winter and spring, SNOTEL)
• USDA Natural Resources Conservation Service: Interactive Map Glossary (snow density as snow water equivalent divided by snow depth)
• NIST Chemistry WebBook: thermophysical properties of water at 14.696 psia (density 62.42 lbm/ft³ at 33 to 40 °F)
• NIST Special Publication 811, Appendix B.8: Factors for units listed alphabetically (lb/ft² to kg/m², lb/ft³ to kg/m³, pound to kilogram)
• National Weather Service: What To Do If You’re Caught in a Winter Storm (avoid overexertion such as shoveling heavy snow if you are not in good health)

Written by Tobias. Updated October 22, 2026.