How much snow can a roof hold? Weigh the load, not the depth

How much snow a roof can hold depends on two numbers: the weight of the snow per square foot, and the design snow load the roof was built for. Depth alone tells you surprisingly little. A foot of fresh, fluffy snow weighs about 5 lb per square foot, while a foot of wet, heavy snow weighs about 25. Your roof's design load comes from the local building code and the building's plans, and your local building department can tell you what applies where you live.

I like snow load questions because they are among the few winter worries you can settle with a yardstick and some multiplication. Here is how to estimate the weight, where it piles up, and how to take some off without climbing up there.

Why does snow depth tell you so little?

Snow is mostly air. What weighs on the roof is the water locked in the crystals, and that fraction changes enormously from one snowfall to the next and as snow ages. The arithmetic starts from one fact: an inch of water spread over one square foot weighs about 5.2 lb. Multiply the snow depth by the fraction that is water, then by 5.2, and you have the load in pounds per square foot.

Here is a foot of snow at the four densities our gauge uses as presets, on a 1,200 square foot roof:

Real snow does not read the table. Cold, dry storms in the interior West or off the Great Lakes can come in lighter than the fresh preset, and a wet snow on a 33 °F morning in the Appalachians can top the wet one. Solid ice is close to the weight of water, nearly 5 lb per square foot for every inch, which is why a roof that has been through a few thaw and freeze cycles can carry far more than its depth suggests.

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How do you estimate the load on your own roof?

Our roof snow load calculator does the multiplication. Enter the snow depth on the roof in inches or centimeters, pick the kind of snow, and add the roof area if you want a total. It returns the load in pounds per square foot and kilograms per square meter, the depth of water held in the snow, and the weight on the whole roof. With its default inputs, 12 inches of settled snow on 1,200 square feet, it shows 3 inches of water (7.6 cm), 15.6 lb per square foot (76 kg/m²), and about 18,720 lb (8,491 kg) on the roof as a whole.

The gauge shades the result into three bands: under 20 lb per square foot, 20 to 40, and over 40. Those are attention markers, not a rating of your roof. A roof built for heavy mountain snow can shrug at 40, and an old carport may be in trouble well before 20.

A few habits make the estimate better:

• Measure depth on the ground in an open spot away from buildings and trees, then look at the roof. Wind often scours exposed roofs below ground depth and piles drifts deeper elsewhere.
• If the snow sits in layers, a dense old base under fresh fluff, run each layer separately and add the results.
• Add rain. Snow on a roof soaks up rain like a sponge, and every inch it holds adds about 5.2 lb per square foot. Check the forecast at weather.gov before deciding a big load can wait.
• If you want a better density figure than the presets, weigh a level container of snow and then the same container filled with water. The ratio is the water fraction.

Where does the extra weight pile up?

Averages hide the spots where roofs actually get into trouble. Wind carries snow over the ridge and drops it on the sheltered side, so one face of a gable roof can hold much more than the other. Valleys, where two roof planes meet, collect snow from both. A lower roof tucked against a taller wall, such as a porch, an addition or an attached garage, catches drifts and everything that slides off the roof above it. Engineers treat these drifts as a separate load for exactly that reason.

Flat and low-slope roofs have their own problem. When drains or scuppers freeze, meltwater ponds under the snow, and water is the heaviest form snow can take.

Then there are ice dams. Heat leaking from the house into the attic melts snow on the upper roof; the water runs down and refreezes over the cold eaves, building a ridge of ice that concentrates weight at the edge and backs water under the shingles. Attic air sealing and insulation are the lasting fix, and our green tips page covers the dull, effective version of that job.

Outbuildings deserve the first look after a big storm. Carports, porch roofs, sheds, older barns and light metal buildings are often built to a lower standard than the house, or to whatever code applied decades ago.

What is your roof's design snow load?

Building codes set the snow load a roof must be designed to carry, in pounds per square foot. The figure starts from a ground snow load for the location and is adjusted for the roof's slope, how exposed it is to wind, how much heat escapes through it, and what the building is used for. It varies enormously by region and even by elevation within a county: a mountain town in the Rockies, a lakeshore town in the Great Lakes snowbelt and a city in the Southeast work with very different numbers.

There is no honest "typical roof holds this much" figure, and you should be suspicious of anyone who quotes one. The number for your building comes from these places:

1. Your local building department, which can tell you the design snow load used in your area and often which code applied when the house was built.
2. The original plans or permit file, which may list the roof design load.
3. Truss paperwork, for newer houses with manufactured trusses.
4. A structural engineer, for older buildings, additions, barns, or any roof that has been altered or patched.

Age and condition matter as much as the original number. Rot, a cut rafter, a removed brace or an old leak can leave a roof weaker than its paperwork.

What are the warning signs of too much snow?

Walk through the house and the attic after a heavy storm, especially if rain is on the way:

• A sagging ridge line or a new dip in a ceiling.
• Fresh cracks in drywall or plaster, often above doors or where walls meet the ceiling.
• Interior doors that suddenly stick or will not latch.
• Creaking, popping or cracking sounds from the roof or attic.
• New leaks or water stains.
• Bowed or split rafters or trusses in the attic.

If you see these with a heavy load overhead, get everyone out of that part of the building and call a contractor or a structural engineer. If it looks like collapse is close, call emergency services. Do not go up on the roof or stand underneath it to look.

How do you clear a roof safely from the ground?

Never climb onto a snowy roof. A roof rake lets you pull snow off the lower part of the roof while your boots stay on the ground.

The rake on the shortlist was picked on paper, from the maker's published reach and blade details and from points owners raise again and again about how the pole sections lock and how the blade treats shingles. No one here has raked a roof with it for this article. Whatever rake you use, look for a pole long enough to reach several feet up the roof from a safe distance, sections that lock firmly, and a blade that rides on small bumpers or rollers instead of scraping the shingles.

1. Look up first. Find the power line that feeds the house and any others nearby, and keep the pole well away from them; a long metal or wet pole can conduct electricity.
2. Stand to the side of the strip you are clearing, never directly under the eave. Snow and icicles can come down all at once, and a slab off a metal roof can knock a person flat.
3. Start at the eave and pull snow down in strips, working a few feet higher each pass.
4. Leave an inch or two on the shingles. Scraping to the surface damages them.
5. Do not chip at ice dams with a hammer or an axe.
6. Keep pulled-down snow off walkways, the gas meter, and furnace or water heater vents on the side wall; a buried exhaust vent can push fumes back inside.
7. For large, high, steep or flat roofs, hire an insured contractor who does roof snow removal.

The same heavy, wet snow that loads a roof also loads tree limbs and power lines, so outages often arrive with it. If you run a fuel generator, it belongs outdoors and well away from doors and windows, never in a garage. EcoFlow's battery power stations make no exhaust and can sit indoors to keep lights, phones and a small appliance going.

Questions

How much does a foot of snow weigh on a roof?

Anywhere from about 5 lb per square foot for fresh, light snow to about 31 for packed snow with ice layers, using our gauge's presets, with settled snow around 15.6.

Should I rake the roof after every storm?

No. Most houses are designed for the snow their area normally gets. Rake when the estimated load is climbing toward your roof's design figure, when heavy rain is forecast onto deep snow, to keep ice dams from forming at the eaves, or on outbuildings you are unsure about.

Can rain make a snow load worse?

Yes, often sharply. The snowpack holds rain instead of shedding it, so an inch of rain absorbed adds about 5.2 lb per square foot without changing the depth much. Rain on deep snow over frozen ground also feeds winter floods, the kind of runoff our flash flood explainer describes.

Is it safe to let snow slide off a metal roof?

Letting it slide is fine for the roof and dangerous for anything below. Keep people, pets and cars out from under the eaves, and do not park beside a metal roof after a big storm.

When the next storm clears, stand in the yard with a yardstick and a cup of coffee and do the multiplication. If the answer is a comfortable fraction of your roof's design load, go back inside and enjoy the quiet. If not, the rake is in the garage.