Snow Load Calculator
Weigh the snow on your roof, or work out its design snow load. Compare depth and snow type with the design load, or apply the ASCE 7 method to the ground snow load.
How to Use
- Choose snow on the roof now to weigh what is up there, or design load to work out what a roof should be built for.
- For snow now: enter the depth and pick the type of snow, then the roof’s design load if you know it (often 20 to 40 psf on houses).
- For a design load: enter the ground snow load pg for your site from your building department or the ASCE hazard tool, then the exposure, heating and risk category.
- Enter the roof pitch and, for design loads, whether the surface is slippery (metal, slate, membranes).
- Enter the roof footprint to get the total weight on the roof.
Worked Example
Snow now: 24 in of settled snow on a 1,500 ft² roof designed for 30 psf. Settled snow weighs about 15 lb/ft³, so the load is 2 ft × 15 = 30 lb/ft². On the whole roof: 30 × 1,500 = 45,000 lb, 22½ tons. That is 100% of the design load. The same roof would reach it with 60 in of fresh snow, or 17 in of wet snow.
Design load: a heated house with a 6/12 shingle roof where pg = 30 psf, partially exposed, risk category II. pf = 0.7 × 1.0 × 1.0 × 1.0 × 30 = 21 psf. The roof is 26.6°, under the 30° knee of the slope curve for a warm, non-slippery roof, so Cs = 1.0 and ps = 21 psf. With slippery metal at 8/12 (33.7°) and pg = 50, Cs = 1 − (33.7 − 5) ÷ 65 = 0.56 and ps = 0.56 × 35 = 19.6 psf.
The common mistake: judging by depth alone. A foot of powder weighs 6 psf; a foot of March snow that has thawed and refrozen can weigh 20. Roofs fail most often after rain falls on deep snow, or in late winter when it has compacted, not during the storm that brought it.
Show Work
Formulas
Why Roofs Hold Less Snow Than the Ground
Ground snow loads come from decades of weather-station records of the snow’s water content, the weight that a 50-year (now reliability-targeted) winter puts on open ground. Roofs carry less, which is where the 0.7 comes from: wind scours them, and heat escaping from the building melts the snow from below. Colder and more sheltered roofs keep more, so Ct and Ce go up.
The danger is where snow piles up rather than where it falls evenly: in drifts behind a parapet or against a taller wall, in valleys, and on the leeward side of a ridge. Many roof collapses under snow involve drifts or rain on snow, which is why codes add those loads on top of the uniform load this calculator gives.
About This Calculator
This calculator weighs the snow on a roof from its depth and type and compares it with the design load, or works out the uniform design roof snow load from the ground snow load with the ASCE 7-16 factors for exposure, heat, importance and slope. It shows the load in lb/ft² or kPa and the total on the roof, and draws the snow against the limit.
Everything runs in your browser; nothing is sent anywhere. Snow densities vary widely; the values here are typical. Design loads need the official ground snow load for your site, and drifts, sliding and unbalanced loads need an engineer.
Related calculators: Joist & Rafter Span Calculator, Beam Load Calculator, and Roofing Calculator.
Frequently Asked Questions
How much does snow weigh on a roof?
It depends on how dense the snow is. Fresh powder weighs about 6 lb per cubic foot, so a foot of it is 6 lb/ft²; settled snow is about 15 lb/ft³, so 2 ft of it is 30 lb/ft²; wet, packed snow is over 20 lb/ft³ and ice 57. On a 1,500 ft² roof, 30 lb/ft² is 45,000 lb, 22½ tons.
How much snow can my roof hold?
As much as its design snow load, which in the US is set by local code from the ground snow load, often 20 to 40 psf for homes and 50 to 100 or more in mountains. A roof built for 30 psf holds about 5 ft of fresh snow, 2 ft of settled snow or 17 in of wet snow. Codes include a safety margin, but damaged, rotted or altered framing has less.
How is the design roof snow load calculated?
ASCE 7 starts from the ground snow load pg for the site. The flat-roof load is pf = 0.7 Ce Ct Is pg: 0.7 because roofs hold less than the ground, Ce for wind exposure, Ct for heat through the roof and Is for importance. Steep or slippery roofs shed snow, so ps = Cs pf. Low-slope roofs have a minimum. Drifts and unbalanced loads are added separately.
What are the signs of too much snow on a roof?
Sagging ridges or ceilings, new cracks in walls or ceiling drywall, doors and windows that start sticking, and creaking or popping sounds. If you see them, get people out and call a professional. Clear snow from the ground with a roof rake rather than climbing up, and never leave heavy drifts against a step or parapet.
Why did ASCE 7-22 change the formula?
ASCE 7-22 introduced reliability-targeted ground snow loads, mapped separately for each risk category, so the importance factor Is is no longer in the formula. This calculator uses the ASCE 7-16 method, still adopted by many codes; for a building permit, use the edition and ground snow load your jurisdiction specifies.
How do I use the Snow Load Calculator?
Just type your numbers. The answer shows up right away — there is no button to press. Change anything and it updates by itself.
Do I need to install or sign up for anything?
Not at all — it runs in the browser with nothing to install and no account. After it loads once, it even works without an internet connection.
Is my information private?
Yes. Everything happens in your browser. Nothing you type is sent to a server or saved anywhere.
Common Use Cases
After a storm
12 in of fresh snow on a 1,500 ft² roof: 6 psf, 4½ tons, a fifth of a 30 psf design load.
Late winter
24 in of settled snow: 30 psf, 22½ tons, right at a 30 psf design load.
Design load
pg 30 psf, heated house, 6/12 shingles: ps = 21 psf.
Metal roof
pg 50 psf, 8/12 slippery metal on a heated house: 35 psf flat-roof load, 19.6 psf on the slope.
Low slope
pg 20 psf, 1/12, fully exposed: 12.6 psf calculated, but the 20 psf minimum governs.
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