Snow Load Calculator
Estimate roof snow load in pounds per square foot, from the ground snow load, roof pitch, exposure, and how warm the roof runs, using simplified ASCE 7 factors.
Reviewed by the ToolNestr Editorial Team — July 2026
How the snow load calculator works
ASCE 7, the Minimum Design Loads standard, starts from a location-specific "ground snow load" — how much snow, by weight, typically accumulates on the ground in your area — and adjusts it down through a series of factors to estimate the load a real roof carries. Roofs generally see less load than the open ground because wind scours snow off elevated surfaces and warmth from the building below can melt some of it.
This tool applies a simplified version of that method: the flat-roof snow load is 0.7 times an exposure factor, times a thermal factor, times your ground snow load. The exposure factor accounts for wind — exposed, windy sites lose more snow to drifting, while sheltered, wooded sites hold more. The thermal factor accounts for roof warmth — heated buildings melt snow faster than unheated structures or open-air roofs.
A slope reduction factor is then applied on top, since steep roofs shed snow more readily than flat ones. Roofs under about 30 degrees see little to no reduction; the reduction grows through the 30 to 70 degree range and roofs steeper than about 70 degrees are assumed to shed essentially all their snow load. This mirrors ASCE 7's slope-factor concept in a simplified form and omits several real-code factors like importance category and drift loading — it is for rough estimation and education only.
Based on the simplified structure of ASCE 7 (Minimum Design Loads and Associated Criteria for Buildings and Other Structures), Chapter 7, Snow Loads — not a substitute for the full standard or a licensed engineer's calculation.
Find your ground snow load
Get the official figure from your local building department or ASCE 7 maps.
Set roof pitch and site
Enter your pitch, exposure, and whether the roof is heated.
Treat as a rough estimate
Use this to understand the factors, then get a licensed engineer for any real design.
The formula explained
Flat-roof load
= 0.7 × Ce × Ct × Pg. 30 psf ground, typical exposure, heated: 0.7 × 1.0 × 1.0 × 30 = 21 psf.
Slope factor
A 6 in 12 roof (26.6°) is under the 30° threshold where slope reduction begins, so Cs = 1.0 — no reduction yet.
Roof snow load
= flat load × Cs = 21 × 1.0 = 21 psf, unchanged from the flat-roof value at this pitch.
Worked example
A cold-climate 50 psf ground load, steep 12 in 12 roof, exposed windy site, heated.
Real-world context
A sheltered, wooded lot in a moderate-snow region with a 40 psf ground load, a shallow 4 in 12 roof, and a heated house computes a flat-roof load of 0.7 × 1.2 × 1.0 × 40 = 33.6 psf. At 18.4°, the roof is well under the 30° slope-reduction threshold, so the roof load stays at the full 33.6 psf — a reminder that a sheltered site can outweigh a shallow pitch when it comes to actual accumulated load.
A mountain cabin with a 60 psf ground load, a steeper 9 in 12 roof, typical exposure, and an unheated structure gets a flat load of 0.7 × 1.0 × 1.1 × 60 = 46.2 psf. At 36.9°, the roof is just past the 30° threshold, giving a slope factor around 0.863 and a roof load near 39.9 psf — unheated roofs get a smaller slope-factor benefit than heated ones because a cold roof surface holds snow more readily even as it sheds.
A windy coastal ridge with a lighter 25 psf ground load, an 8 in 12 roof, and a heated home shows how exposure alone can matter as much as ground load: 0.7 × 0.9 × 1.0 × 25 = 15.75 psf flat load, reduced further by the 33.7° pitch's slope factor of about 0.908 to roughly 14.3 psf on the actual roof — well under half of what a sheltered site with the same ground load would carry.
Common misconceptions
"Ground snow load and roof snow load are basically the same number." They are usually quite different. Ground snow load measures accumulation on an open, unobstructed surface; roof snow load is reduced by wind exposure, roof warmth, and pitch, and can easily run 30-60% lower than the raw ground figure for an exposed, heated, sloped roof.
"A steep roof means I don't need to worry about snow load at all." Steep roofs shift the risk, they don't eliminate it. Snow that slides off a steep roof still has to land somewhere — often piling into deep drifts against lower roof sections, decks, or adjacent structures, which is exactly the kind of localized drift loading this simplified calculator does not account for.
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Frequently asked questions
How is roof snow load calculated?
ASCE 7 starts with the flat-roof snow load: 0.7 times an exposure factor, times a thermal factor, times an importance factor, times the ground snow load for your area. A slope reduction factor then reduces the load further on steeper roofs, since snow sheds more easily off a steep pitch.
Where do I find my ground snow load?
Ground snow load is a location-specific value published in ASCE 7 snow load maps or provided by your local building department. It varies enormously by region and even by elevation within the same county, so use the figure that applies to your exact site, not a national average.
Why does roof pitch reduce the snow load?
Snow slides off a steep roof more easily than a flat one, so codes apply a slope reduction: roofs under about 30 degrees see little to no reduction, while roofs over about 70 degrees are assumed to shed essentially all snow. A warm, slippery roof surface sheds snow more readily than a cold, rough one.
Is this a substitute for an engineered snow load calculation?
No. This tool applies simplified ASCE 7 factors for a rough planning estimate only. Actual structural design must use the full ASCE 7 Minimum Design Loads standard, your jurisdiction's adopted ground snow load, and a licensed structural engineer's review — never use this estimate alone to size or evaluate a roof structure.
Sources & references
This tool uses standard formulas and reference values from:
- • American Concrete Institute — ACI 318, Building Code Requirements for Structural Concrete. concrete.org
- • ICC — International Residential Code (IRC), span, footing and framing tables. codes.iccsafe.org
- • APA – The Engineered Wood Association, allowable span and load guidance.
Estimates for planning only. Span, load and code values vary by jurisdiction — verify against your local adopted code and a licensed engineer before building.