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Roof snow load calculator

The building code takes the mapped ground snow load for your site and adjusts it for how exposed the roof is, how warm it is underneath, how important the building is, and how steeply it sheds. This runs that arithmetic. It is a screening figure — a structural engineer signs the design.

Two-colour print of snow lying on a low-slope roof
An even blanket on a low-slope roof — the load case that governs.
Snow load, psf

Ground snow load comes from your building department or the ASCE hazard tool — it is a mapped value, not something you can estimate. Everything else is a table lookup.

30.8 psf

sloped-roof snow load at 26.6° · Cs 1.00

StepValue
Ground snow load pg40 psf
0.7 × Ce × Ct × Is0.770
Flat roof snow load pf30.8 psf
Slope factor Cs at 26.6°1.00
Sloped roof snow load ps30.8 psf

How the estimate is built

The flat-roof snow load is pf = 0.7 · Ce · Ct · Is · pg — the method the code adopts from ASCE 7 section 7.3. The sloped-roof load is then ps = Cs · pf, where the slope factor falls from 1 as the roof gets steep enough to shed. A warm, unobstructed slippery roof begins shedding at a much lower angle than a cold shingle roof, which is why the surface checkbox moves the answer as much as the pitch does.

Ground snow load is looked up, never guessed

pg is a mapped, site-specific value. Your building department has it; in mountainous regions it is set by a case study rather than a map, and neighbouring addresses can differ. Never carry a number over from another town.

Tear-off / structural

This calculator screens; it does not design. Drift loads against a wall or a higher roof, sliding snow onto a lower roof, unbalanced load on a gable and rain-on-snow surcharge all sit outside it, and any of them can govern. Adding a heavy covering — see the tile comparison — needs an engineer, not a web page.

Why the thermal factor matters twice

A well-ventilated cold roof carries a higher design load (Ct above 1) precisely because it does not melt its snow — and that same coldness is what prevents ice dams. A leaky warm attic lowers the design load and creates dams instead. The structural answer and the roofing answer point the same way: keep the deck cold, and check the ventilation on the ventilation calculator.

Related

Sources

  1. Building code §1608.3 (flat roof snow load; adopts ASCE 7 §7.3) — Ce and Ct tables — read 2026-09-04