Snow looks weightless as it falls, but it stops looking weightless the moment it sits on a roof. A single foot of wet spring snow can weigh as much as a foot of water spread across your entire roof — tens of thousands of pounds pressing on rafters, trusses and walls. Roof collapses are rare, but they are almost never sudden mysteries: the load builds for days, the warning signs are visible, and the arithmetic is simple. The roof snow load calculator does that arithmetic. Enter the depth of snow on your roof and the kind of snow it is, and you get the load in pounds per square foot plus the total weight on your roof — the two numbers that tell you whether to relax, watch, or call for help.
How to use this calculator
Snow load in pounds per square foot comes from two measurements: depth in inches and density in pounds per cubic foot. Multiply them and divide by 12 to convert inches to feet, and you have the pressure on the roof surface. Fresh powder is light, around 8 pounds per cubic foot; settled average snow is about 15; packed old snow reaches 22; and wet spring snow can hit 30 or more. Multiply the per-square-foot load by your roof area and you get the total weight the structure is carrying. The calculator shows both figures, because a per-square-foot number tells you about design limits while the total tells you the scale of the problem.
- Measure snow depth on the roof itself, not on the ground — wind drifts make roof depth very different from yard depth.
- Take several readings, including near drifts and valleys, and use the deepest representative depth for a cautious estimate.
- Judge the snow type: light and fluffy is fresh, dense and hard to shovel is packed, heavy and slushy is wet.
- Enter your roof area — the footprint of the roof, in square feet — and press Calculate.
- Compare your pounds-per-square-foot result with your local design snow load, found in building codes or from a local engineer.
Understanding your results
Pounds per square foot. The pressure the snow exerts. Most homes are designed for 20 to 40 pounds per square foot, with snowy regions designed higher. Approaching that figure is your warning zone.
Total roof load. Per-square-foot load times roof area. A 1,000-square-foot roof at 30 pounds per square foot carries 30,000 pounds — the weight of several cars.
Why density dominates. Two feet of fresh powder weighs less than one foot of wet snow. Depth alone tells you almost nothing; depth plus density tells you everything.
Zero results. Zero depth means zero load. If snow is on the roof and you see zero, check that the depth field is filled in.
Practical tips
- Measure after every major storm, not just once a winter — loads accumulate and a rain-on-snow event can double weight overnight.
- Watch for warning signs: sagging ceilings, sticking doors and windows, new cracks, and creaking sounds.
- Clear snow in balanced strips from the ridge outward, never all from one side, to avoid unbalanced loading.
- Keep gutters and roof edges clear so meltwater drains instead of refreezing into heavy ice dams.
- Hire professionals for steep, high or icy roofs — a fall from a roof is a far greater risk than the snow itself.
- After a heavy winter, have the roof structure inspected for deflection before the next one.
Common mistakes to avoid
- Judging danger by depth alone and ignoring wet, heavy snow.
- Measuring snow in the yard instead of on the roof, where drifts pile much deeper.
- Assuming a new roof can carry anything — design loads are limits, not guarantees, and older roofs may have weakened.
- Clearing snow from only one side of the roof, which creates uneven stress on the structure.
- Climbing onto an icy, snow-covered roof alone without fall protection.
- Waiting for a collapse warning sign that never comes — some failures give little visible warning.
Who benefits most
This calculator is for homeowners in snowy climates, landlords with properties in winter regions, facility managers responsible for warehouses and schools, and anyone who has looked at a sagging carport and wondered whether to worry. It is also useful for insurers and inspectors who need a quick, documented load estimate after a storm. If your winters regularly leave a foot or more of snow on the roof, this is a number worth knowing before the forecast turns serious.
Worked example
Take 12 inches of average settled snow at 15 pounds per cubic foot: the load is 12 × 15 ÷ 12 = 15 pounds per square foot, or 15,000 pounds on a 1,000-square-foot roof. Now take 24 inches of wet spring snow at 30 pounds per cubic foot: 24 × 30 ÷ 12 = 60 pounds per square foot — 60,000 pounds on the same roof, far beyond most residential design loads. And 12 inches of fresh powder at 8 pounds per cubic foot is only 8 pounds per square foot. Same depth, wildly different danger — which is exactly why density is part of the calculation.
Checking and re-running your result
Recheck your result after each storm and after any rain-on-snow event, since wet snow gains weight fast. If your result approaches your local design load, clear the roof or call a professional promptly, and save each calculation to history so you can show an inspector or insurer exactly how the load built up over the season. Re-measure whenever drifting changes the depth, because the deepest drift — not the average depth — is what stresses the structure. Ice dams and local codes deserve attention alongside the raw load number, because they decide how snow behaves on your particular roof. An ice dam forms when heat escaping through the roof melts snow from below, and the meltwater refreezes at the cold eaves, building a ridge that traps further meltwater behind it. That trapped water weighs heavily, seeps under shingles, and damages ceilings — and it is driven by attic insulation and ventilation as much as by snowfall. Keeping the attic cold, with even insulation and clear soffit-to-ridge airflow, is the long-term fix, while a roof rake used carefully from the ground can relieve the lower edge after storms. Building codes also matter: your local design snow load, published in the code for your region, is the number to compare against the calculator's result, and it varies enormously — from under 10 pounds per square foot in warm regions to over 100 in mountain towns. If your home predates modern codes or has had structural changes, an engineer's opinion is worth more than any online figure. Document each storm's depth and your calculated load with dated photos; insurers and inspectors value that record after a collapse or leak claim. And remember the human risk: roof falls kill more people than roof collapses, so the safest snow removal is often a professional crew with harnesses, or a ground-based rake, rather than a solo climb onto an icy slope. A quick reference for the loads you are most likely to meet: six inches of fresh powder is about 4 pounds per square foot; a foot of average snow is about 15; a foot of packed snow is about 22; and a foot of wet spring snow is about 30. Two feet of wet snow reaches 60 pounds per square foot, which exceeds most residential design loads outright. Memorise the wet-snow figures, because they are the ones that turn an ordinary winter into a structural emergency.