Number Snow Load Calculator
Two questions, one tool: what load is my roof designed for (the ASCE 7 number an engineer uses), and what is the snow actually sitting on it right now weighing (the number that decides whether you shovel)? Ground snow load in, both answers out.
Ground snow load (pg) is a code-map value, not a weather report: ASCE 7 maps and your local building department publish it (roughly 0 psf in the deep South, 20–50 across the northern US, 100+ in mountain snow belts). The slope factor here uses the standard warm-roof line for a roof kept at normal heat; cold-roof and drift cases (against taller walls, in valleys, behind parapets) carry additional loads an engineer handles — drifts, not uniform load, are what actually collapse roofs.
How it works
Design side (ASCE 7): flat-roof load pf = 0.7 × Ce × Ct × Is × pg. Sloped-roof load ps = Cs × pf, where the slope factor Cs = 1.0 up to the slide-off angle (30° for a non-slippery warm roof, 5° for slippery) and decreases linearly to 0 at 70°. Weight side: snow density γ = 0.13 × pg + 14 lb/ft³ (capped at 30) — the code’s own density line — so the current load of fresh snow at design depth implies a depth of ps ÷ γ, and the total weight on the roof is ps × plan area. A 28 psf design load on 1,500 sq ft is 21 tons of snow: roofs are quietly the strongest thing you own.
Formula & worked example
Formula: ASCE 7: flat-roof snow load pf = 0.7 × Ce × Ct × Is × pg. Slope factor Cs = 1.0 up to 30° (non-slippery) or 5° (slippery), then linear to 0 at 70°. Sloped load ps = Cs × pf. Snow density γ = 0.13 pg + 14 pcf (cap 30). Total weight = ps × area; equivalent depth = ps ÷ γ.
Worked example: pg 40, standard factors, 6/12 roof on 1,500 sq ft: pf = ps = 28.0 psf; total snow weight 21.0 tons; equivalent settled depth 17.5 in.
When to use this
- A big storm is sitting on your roof and you want weight, not inches.
- You are checking a design, shed or carport plan against code snow load.
- You are comparing a metal roof’s shedding behavior against shingles.
- An insurer or permit office asked for the design snow load.
How to use it, step by step
- Find pg for your address (building department or ASCE map) and enter it.
- Set exposure, thermal and importance factors — defaults are the standard house.
- Enter your roof pitch honestly; pick slippery only for true slide-off surfaces.
- Read the design load, the total tons, and the equivalent depth to watch for.
What the inputs mean
| Input | What it is | Typical values |
|---|---|---|
| Ground snow load pg | Code-map design value for your location, psf | 20–50 northern US; 100+ mountains |
| Slope factor inputs | Pitch + slippery surface drive Cs | 6/12 = 26.57°, Cs 1.0 non-slippery |
| Roof area | Plan (horizontal) area the snow sits on | 1,200–2,000 sq ft typical houses |
| Factors Ce/Ct/Is | Exposure, thermal and importance multipliers from the standard | Typical house: 1.0 / 1.0 / 1.0 |
Another worked example
A slippery 45° roof at pg 50 with Ce 0.9 and Ct 1.1: pf = 0.7 × 0.9 × 1.1 × 1.0 × 50 = 34.7 psf, but Cs falls to 0.38 at that pitch, so the design load is 13.3 psf and the 1,200 sq ft roof carries 8.0 tons at design — the sliding roof’s whole structural bargain, in one line.
Frequently asked questions
What snow load is a roof designed for at pg 40?
Flat-roof design load pf = 0.7 × 1.0 × 1.0 × 1.0 × 40 = 28.0 psf. At a 26.57° pitch (a 6/12 roof) the slope factor is still 1.00 — sliding relief has not started on a non-slippery warm roof — so the sloped design load is also 28.0 psf, which on 1,500 sq ft totals 42,000 lb: 21.0 tons.
How deep is 28 psf of snow?
Density at pg 40 by the code line is γ = 0.13 × 40 + 14 = 19.2 lb/ft³. Depth = 28 ÷ 19.2 = 1.46 ft ≈ 17.5 inches of settled snow. Fresh powder at half that density needs three feet to weigh the same — which is why "inches of snow" headlines mislead and weight is the honest unit.
Why does a steep metal roof carry almost nothing?
Slope factor. On a slippery surface the slide-off line starts at 5°, so at 45° with a ground load of 50 (and Ce 0.9, Ct 1.1 giving pf 34.7) the factor falls to 0.38 and the design load to 13.3 psf — the roof sheds before it accumulates. The snow does not vanish, though: it lands at the eaves, on the deck below, and on whoever is standing there. Sliding-snow loads are their own design case.
Where do I find my ground snow load?
Your local building department enforces it, and ASCE 7’s maps (and the free ATC Hazards tool) publish it by location. If you are between map lines, use the higher. Mountain properties: site-specific case studies govern in much of the high country — the map will say so, and a local engineer is not optional there.
Should I shovel my roof?
Watch the load, not the calendar. Warning signs: interior doors newly sticking, ceiling cracks or sag, icicle-heavy ice dams adding hidden weight, and snow depth approaching the design-depth figure this tool prints for your roof. Ground-level roof rakes from the eaves up a few feet are the safe DIY limit; beyond that, insured roof-snow services exist in every snow market.
What is unbalanced snow load?
Wind strips one slope and dumps it on the other; gable roofs are designed for a fraction of the load on the windward side and more than full on the leeward. Valleys, dormers and lower roofs below sliding upper roofs collect the same effect. This calculator gives the balanced uniform case — the drift and unbalanced cases are why rafters that "met code" sometimes still complain.
Does rain on snow matter?
Enormously. A saturated snowpack drinks rain and gains weight without gaining depth — the mechanism behind many late-season collapses of flat and low-slope roofs. If heavy rain is forecast on a loaded low-slope roof, the weight math here (already at settled density) becomes optimistic: clear what you safely can before the rain arrives.