Snow Load Calculator
Estimate the weight of snow on a roof from depth, area and snow density, in pounds per square foot and total tons, and compare it against your design snow load.
Snow Load Calculator: with the default inputs, total weight on the roof is 23,410 lb.
The flat (plan) area under the roof, not the sloped surface.
Measure away from drifts. Then measure the drift separately — that's where the load concentrates.
From your local building code or the structural drawings. 20–30 lb/ft² is common in the northern US; ask your building department.
- Load per square foot
- 15.6Pounds per square foot — the number that matters structurally.
- Load per square metre
- 76.2Kilograms per square metre.
- Water equivalent
- 3 inDepth of water you'd get if it all melted.
- Share of design load
- 52%
- Load still available
- 14.4Pounds per square foot of design load not yet used.
- Where this sits
- About 52% of the 30 lb/ft² design load — a normal winter load for a roof designed to that number.
Assumptions
- Densities: fresh 83 kg/m³ (5.2 lb/ft³), settled 250 kg/m³ (15.6 lb/ft³), wind-packed/wet 400 kg/m³ (25 lb/ft³), ice 917 kg/m³ (57 lb/ft³).
- Load is computed on the roof's plan (flat) area; slope, drift, sliding and unbalanced loads are not modelled.
- The comparison uses the design snow load you enter. This is not a structural analysis and does not consider framing, spans or condition.
| Condition | lb/ft³ | lb/ft² at 12 in | lb/ft² at 12 in |
|---|---|---|---|
| Fresh, light snow (just fell, powdery) | 5.2 | 5.2 | 5.2 |
| Settled snow (a few days old) | 15.6 | 15.6 | 15.6 |
| Wind-packed or wet, heavy snow | 25 | 25 | 25 |
| Ice layer | 57.2 | 57.2 | 57.2 |
A foot of snow at X lb/ft³ weighs X lb/ft², which is why the two middle columns match. Ice at 57 lb/ft² per foot is why a single inch of it matters.
How this is worked out
The formula
Load per area = depth × density Total weight = roof plan area × depth × density Water equivalent = load per area ÷ density of water (1,000 kg/m³ = 62.4 lb/ft³) Share of design load = load per ft² ÷ design snow load Fast check: 1 in of water on a roof = 5.2 lb/ft²
Open How it’s calculated above to see this worked through with your own numbers.
What you enter
- Roof area
- The flat (plan) area under the roof, not the sloped surface.in ft², m², yd² · 0 or more · defaults to 1500
- Snow depth
- Measure away from drifts. Then measure the drift separately — that's where the load concentrates.in in, ft, cm, m · 0 or more · defaults to 12
- Snow condition
- Choose one of 5 options.Fresh, light snow (just fell, powdery) — ≈ 5.2 lb/ft³ — 10–12 in of it equals 1 in of water · Settled snow (a few days old) — ≈ 15.6 lb/ft³ — 3–5 in of it equals 1 in of water · Wind-packed or wet, heavy snow — ≈ 25 lb/ft³ — dense, crusted or rain-soaked · Ice layer — ≈ 57 lb/ft³ — the density of solid ice · Custom density (More options)
- Design snow load
- From your local building code or the structural drawings. 20–30 lb/ft² is common in the northern US; ask your building department.from 0 to 300 · defaults to 30
- Custom density(under More options)
- Used only when the condition above is set to Custom. 83 kg/m³ is fresh powder, 917 kg/m³ is solid ice.from 10 to 1000 · defaults to 250
- Ice layer under the snow(under More options)
- A crust or ice dam adds weight fast — an inch of ice weighs as much as a foot of fresh snow.in in, cm, mm · 0 or more · defaults to 0
What you get back
- Total weight on the roofmain answer
- Load per square foot
- Pounds per square foot — the number that matters structurally.
- Load per square metre
- Kilograms per square metre.
- Water equivalent
- Depth of water you'd get if it all melted.
- Share of design load
- Load still available
- Pounds per square foot of design load not yet used.
- Where this sits
What this assumes
- Densities: fresh 83 kg/m³ (5.2 lb/ft³), settled 250 kg/m³ (15.6 lb/ft³), wind-packed/wet 400 kg/m³ (25 lb/ft³), ice 917 kg/m³ (57 lb/ft³).
- Load is computed on the roof's plan (flat) area; slope, drift, sliding and unbalanced loads are not modelled.
- The comparison uses the design snow load you enter. This is not a structural analysis and does not consider framing, spans or condition.
About this calculator
Snow does not weigh what people expect, because "a foot of snow" can mean anything from 5 to 25 pounds per square foot depending on how old and how wet it is. This calculator turns a depth measurement and an honest guess at the condition into pounds per square foot — the number that structural design actually uses — and compares it with the design load for your building.
Say this first
This is an estimate, not a structural assessment. It gives you the weight of the snow. It knows nothing about your trusses, your spans, your connections, whether a previous owner cut a web member for a chimney, or how the load drifts and concentrates. If the number worries you, the next call is to a structural engineer or your building department, not to another calculator.
How to measure
Push a ruler through to the roof surface somewhere in the open field of the roof, away from valleys and walls. Then measure the deepest drift separately and run it as a second calculation — drifts against a wall or in a valley routinely hold two to three times the field depth, and roof failures start there.
Then judge the condition:
- Fresh powder (fell in the last day, still light) is roughly 5 lb/ft³. Ten to twelve inches of it equals one inch of water.
- Settled snow (a few days old, footprints hold their shape) is about 15 lb/ft³. Three to five inches equals an inch of water.
- Wind-packed or rain-soaked snow reaches 25 lb/ft³ and is the condition that collapses roofs. Rain falling on an existing snowpack is the classic sequence.
- Ice is 57 lb/ft³. One inch of ice weighs as much as a foot of fresh powder, and ice dams put it at the eaves where reserve capacity is lowest.
Design load
Ask your building department for the ground snow load in your jurisdiction; the roof design load is derived from it in ASCE 7 and is typically 20–40 lb/ft² across the northern US, far higher in mountain towns. If you don't know it, leave the field and treat the pounds-per-square-foot figure as the useful output.
Reading the results
Load per square foot is the number to compare against anything a professional tells you. Water equivalent is what the snowpack would be as rain — useful because it's how weather services report snow water equivalent, and how you sanity-check a depth measurement against the season's snowfall.
Frequently asked questions
▸How much does snow weigh per square foot?
Roughly 0.4 lb/ft² per inch of fresh powder, 1.3 lb/ft² per inch of settled snow, 2.1 lb/ft² per inch of wet or wind-packed snow, and 4.8 lb/ft² per inch of ice. A foot of settled snow on a 1,500 ft² roof is about 23,000 lb.
▸When should I rake snow off my roof?
There is no universal depth. The usual triggers are: the load is approaching your design snow load, you see drifts far deeper than the field, doors or interior doors start sticking, or you hear creaking. Rake from the ground with a roof rake — going on the roof is how people get hurt.
▸Is ground snow load the same as roof snow load?
No. ASCE 7 derives the roof load from the ground load with factors for exposure, thermal condition, slope and importance, and it is usually lower than the ground load on a sloped, heated roof — but drift and sliding loads can be much higher locally.
▸Why is wet snow so much more dangerous?
Density. Wet or rain-soaked snow can be five times heavier than the powder that fell a week earlier, so a snowpack that was safe on Monday can be over the design load on Friday without a single new inch falling.
▸Does the roof slope change the weight?
The total weight is set by the plan area, which is what this calculator uses. Slope matters because steeper roofs shed snow, and ASCE 7 applies a slope factor to the design load — but until it slides, the snow that's up there weighs what it weighs.
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