Load safely.
Roof snow load in pounds per square foot. Compares your actual snow against design load capacity: flags when roof is over-stressed.
How we calculated this
Snow weight varies hugely by type. Fresh powder is only about 5 pounds per cubic foot. Settled snow after a few days is 15 lb/ft³. Wet, partially melted snow is 25 lb/ft³. Old compacted snow or ice-crusted snow approaches 35 lb/ft³. Solid ice is the heaviest at 57 lb/ft³.
Roof snow load in pounds per square foot (psf) is snow depth times density. A 2-foot accumulation of packed snow at 15 lb/ft³ = 30 psf. Same depth of wet snow = 50 psf. Same depth with a 1-inch ice glaze adds another 4-5 psf. Multiply by roof area to get total pounds of load the structure carries.
Compare actual load against your region's design snow load, the code-specified ground snow load your roof was engineered to handle. Southern US typically 20 psf. Mid-Atlantic and central US 30-40 psf. Northeast, Great Lakes, and Rockies 50-70 psf. Mountain resort areas and Upper Peninsula can require 80+ psf design loads.
When actual load approaches 80% of design, consider removing snow: heavy drifts, ice dams, or prolonged accumulation can exceed design capacity even in normal storms. When actual load exceeds design, immediate action: clear the roof (professional service in heavy loads), and consult a structural engineer to inspect for damage.
Important caveats: these calculations use uniform snow distribution. Drifting concentrates snow on leeward sides, behind dormers, and in roof valleys: local loads in these areas can be 2-3× the uniform load. Flat roofs retain more snow than pitched. Unheated structures accumulate more than heated (melt from below). In doubt, call a structural engineer, not this calculator.
Not captured: dynamic loads from wind+snow combinations, seismic considerations for heavy snow regions, non-uniform drift loading per ASCE 7 section 7.7, or rain-on-snow load increases. For engineering purposes, use a professional per ASCE 7-22 chapter 7 rather than this educational tool.
Depth is the wrong question. Weight is the question.
Two feet of fresh powder weighs about 10 pounds per square foot. Two inches of ice weighs roughly the same. A roof does not know or care how deep the snow looks; it responds to the weight, and weight depends almost entirely on density. That is why the neighbor with less snow can be in more trouble than you are, and why the dangerous moment in a winter is usually not the blizzard but the rain that lands on top of it three days later.
The calculator above converts what is actually on your roof right now into pounds per square foot and compares it against your design load. This page covers both halves of the subject: how much snow weighs by type, and how to find the design snow load your roof was built to, which is a different number from the ground snow load your county publishes.
How we calculated these numbers▾
Snow density figures follow FEMA snow load safety guidance and standard engineering values: roughly 5 lb/ft³ for fresh low-density snow, 15 for settled snow, 25 for wet snow, and 40 to 57 for ice. Design load conversion follows ASCE 7-22 Chapter 7, where flat roof snow load pf equals 0.7 times exposure, thermal, and importance factors times ground snow load pg. Ground snow loads are set by the authority having jurisdiction, referencing IRC Table R301.2(1). This page is a planning and awareness tool, not a structural analysis.
Snow weight by depth: the table to keep
Fresh powder | Settled snow | Wet snow | Ice | |
|---|---|---|---|---|
| 6 inches | 2 psf | 8 psf | 13 psf | 28 psf |
| 12 inches | 5 psf | 15 psf | 25 psf | 57 psf |
| 18 inches | 7 psf | 23 psf | 38 psf | 85 psf |
| 24 inches | 10 psf | 30 psf | 50 psf | 114 psf |
| 36 inches | 15 psf | 45 psf | 75 psf | 171 psf |
Load in pounds per square foot for a given depth and snow type. Find your depth, read across to your snow type, and compare against the design load in the section below.
Read that table next to a typical residential design load of 20 to 40 psf and the picture gets clear fast. Three feet of powder is a non-event on almost any roof in snow country. One foot of wet snow is already at the design limit for a mild-climate roof. And any significant thickness of ice is a serious problem anywhere, which is the practical reason ice dams matter structurally and not just as a leak source.
Ground snow load vs roof snow load
These are different numbers and confusing them is the most common error in this subject. Ground snow load, written pg, is a statistical value for your location: roughly the snow load with a 2 percent annual probability of being exceeded, measured on open ground. Roof snow load is what the structure is designed to carry, and ASCE 7 derives it from pg rather than using it directly.
The first step multiplies by 0.7, because wind and melting mean a roof rarely holds as much as open ground. Then exposure and thermal factors adjust it: a windswept roof holds less, a sheltered one in trees holds more, and an unheated structure like a detached garage holds more than a heated house because nothing melts from below. The second step applies a slope factor, which reduces the load on steep slippery roofs that shed and does essentially nothing for asphalt shingles at ordinary residential pitches. For a typical heated house in a 40 psf county, the flat roof design load works out near 28 psf.
What raises the load | What lowers it | |
|---|---|---|
| Exposure | Sheltered by trees or taller buildings | Windswept and fully exposed |
| Heating | Unheated garage, barn, or carport | Heated living space below |
| Roof slope | Low slope, or any rough surface | Steep and slippery, such as standing seam metal |
| Geometry | Valleys, dormers, and roof steps that collect drift | Simple gable with no obstructions |
| Occupancy | Essential buildings, higher importance factor | Ordinary houses and accessory structures |
The adjustment factors in plain terms. An unheated detached garage in an open field and a heated house in dense trees can end up with very different design loads from the same county number.
Finding your ground snow load
There is no single national map you can trust for a legal answer, and this is the part people find frustrating. The IRC publishes Table R301.2(1) as a blank form precisely because each jurisdiction fills in its own values. ASCE 7-22 replaced the old printed contour map with a location-based database, and the ASCE Hazard Tool returns a value for a specific latitude and longitude. Many states and counties publish their own adopted table, and where they do, that number governs over anything else.
Mountain regions add a further wrinkle. Large parts of the western US are designated case study areas, meaning snow load varies so sharply with elevation that no single published value applies and a local engineer has to establish the number. If you are building at altitude, expect to be told exactly that, and expect the answer to be considerably higher than the nearest town in the valley. The practical sequence is always the same: call the building department, ask for the adopted ground snow load, and use their number.
Carports, canopies, and temporary garages
Portable car shelters and fabric canopies are the structures most likely to fail under snow, and they usually carry a manufacturer rating rather than a code-derived design load. Many are rated somewhere between 10 and 20 psf, which the table above tells you is about eight inches of settled snow or four inches of wet snow. These are not roofs in the structural sense; they are meant to be cleared after every meaningful snowfall, and the instructions generally say so.
The same caution applies to attached patio covers, aluminum awnings, and older sunroom roofs. They are frequently the first thing to go in a heavy winter because they were designed to a lower standard than the house, they often have a low slope that sheds nothing, and they sit exactly where snow sliding off the main roof lands. If your house has one, it deserves clearing long before the main roof does.
A 1970s ranch with a 6:12 asphalt shingle roof, 1,800 square feet of footprint. The county publishes a 50 psf ground snow load, so the flat roof design load is roughly 50 x 0.7 = 35 psf, and the 6:12 asphalt slope contributes essentially no reduction.
In late January the roof carries 20 inches of settled snow: 20 x 1.25 = 25 psf, comfortably inside the 35 psf design load. Then a warm front brings an inch of rain, which the snowpack absorbs. The same depth now behaves as wet snow at roughly 2.1 psf per inch, or 42 psf, past the design load. Nothing on the roof looked different from the driveway. This sequence, snow followed by rain, is the mechanism behind most residential roof collapses in the Northeast, and it is why the calculator asks about snow type rather than just depth.
Composite illustration based on typical project dimensions, regional contractor pricing, and 2026 material costs. Not a specific real project.
Warning signs and what to do about them
A structure under distress usually says so before it fails. Doors and windows on interior walls that suddenly stick, new cracks in drywall near the ceiling, visible sagging in a ridge line, and creaking or popping sounds from the framing are the recognized signs. Sprinkler heads dropping below ceiling tiles is the commercial version. Any of these in a heavy snow winter is a reason to leave the building and call someone, not a reason to go up on the roof.
On clearing: a roof rake used from the ground is the safe method, and clearing the lower three or four feet of roof is usually enough to relieve both drift load and ice damming. Do not use a shovel or anything metal on a shingle roof, do not climb onto a snow-covered roof, and do not chip at ice. Falls and cardiac events during snow clearing injure far more people every winter than roof collapses do.
The rest of the winter roof
Snow load sits next to several other roof questions. Framing capacity starts with the joist span reference, the roofing calculator handles squares and material for a reroof, and ice damming is largely an attic problem the insulation calculator and attic ventilation calculator address together. Meltwater volume is a gutter calculator question.
Measure the depth, identify the snow type honestly, and run the numbers. Knowing you are at 25 psf against a 35 psf design is worth more than any amount of looking at the roof and guessing.
Frequently asked
How much does roof snow weigh?
Varies by type. One foot of fresh powder = 5 psf. One foot of packed snow = 15 psf. One foot of wet snow = 25 psf. One inch of ice = 5 psf. A 2,000 sq ft roof with 18" of packed snow carries 45,000 lbs, the weight of 15 cars. The calculator above handles any combination.
How do I know my design snow load?
Check your original building permit or plans (typically labeled 'roof snow load' in psf). If unavailable, contact your local building department: design snow loads are mapped by ZIP code per ASCE 7. Quick reference: southern US 20 psf, mid-US 30-40 psf, northern US 50+ psf, mountain areas 60-100+ psf.
When should I clear snow off my roof?
When actual load exceeds about 80% of design load. For a standard 30 psf design: clear if packed snow exceeds 22 inches or wet snow exceeds 14 inches. If ice dams form, address those first (usually a separate issue from structural load). Don't wait until you see sagging: by then, damage is already happening.
How do I measure roof snow depth safely?
From the ground with a long pole marked in inches (paint marks on a yardstick taped to a broom handle works). Measure at several points. Don't climb up: wet snow is slick, and structural failures are unpredictable. For professional assessment, roof rake companies provide measurement and removal services after major storms.
What's the difference between ground and roof snow load?
Ground snow load (pg) is measured on flat terrain. Roof snow load is typically 70% of ground snow load due to wind scour and thermal effects on the roof. The design values in this calculator are roof loads. The heavier ground load is the input to ASCE 7 calculations; code converts it to roof load automatically.
Why does wet snow weigh more?
Water is heavy (62.4 lb/ft³). Wet snow is a mix of ice crystals and liquid water, about 40% water by volume. Fresh powder is mostly air (95%+ air, 5% ice). As snow ages, melts, and refreezes, air escapes and density increases dramatically. A wet spring storm can triple the load of the equivalent depth of fresh powder.
Does roof pitch affect snow load?
Yes. Steeper roofs shed snow more readily. Roofs 15° or steeper (3/12+ pitch) can reduce snow load significantly: ASCE 7 allows reductions up to 40% for very steep unobstructed roofs. Low-slope roofs (under 3/12) retain snow longer. The calculator uses actual measured depth, so you don't need to calculate pitch reduction separately.
Can my roof collapse from snow?
Yes: it happens every winter in storm-affected regions. Warning signs: cracking sounds, ceiling sag, doors/windows jamming, drywall cracks, water leaking at unusual spots. If you see warning signs, evacuate and call a professional immediately. Do NOT continue using the structure: collapses progress from initial failure to total collapse quickly.
Sources
- ASCE 7-22, Chapter 7: Snow Loads - The flat roof conversion pf = 0.7 Ce Ct Is pg and the slope factor Cs
- ASCE 7 Hazard Tool: Snow Load by Location - Returns a ground snow load for a specific latitude and longitude under ASCE 7-22
- IRC 2021, Table R301.2(1): Climatic and Geographic Design Criteria - The table each jurisdiction fills in with its own adopted ground snow load
- FEMA: Snow Load Safety Guide - Snow density values, roof distress warning signs, and safe clearing practice
- NRCA: Roof Snow Removal Guidance - Industry practice for clearing snow without damaging roof coverings
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