Roof Snow Load Calculator

The US method (ASCE 7 / IBC 1608) starts from the flat-roof load: pf = 0.7 × Ce × Ct × Is × pg, where pg is your ground snow load and Ce, Ct, Is are the exposure, thermal, and importance factors.

For a sloped roof, multiply by a slope factor: ps = Cs × pf. Low-slope roofs also have a minimum load that can govern.

Example: pg 30, heated home (all factors 1.0) → pf = 21 psf. This is a balanced-load estimate only — an engineer must confirm any structural design.

Ground snow load (pg) is the weight of snow on the ground for your location, set by ASCE 7 maps and your building department. Roof snow load is what the roof must carry, usually less.

The 0.7 factor reflects that roofs hold less than open ground — wind, melting, and geometry all reduce it. The Ce, Ct, and Is factors then adjust from there.

You always start from the ground value and convert; you never design to the raw ground number without the code adjustments.

It varies enormously — from 0 psf across the warm South to 100+ psf in high mountains. Much of the northern US falls in the 20–50 psf range.

You can’t reliably guess it from experience. Get the official figure from the ASCE 7 Hazard Tool (site-specific by address) or your local building department, which may set a jurisdiction minimum.

That number is the biggest driver of the result, so start there before trusting any roof-load estimate.

Steeper roofs shed snow, so the code reduces the load via the slope factor Cs. Up to a threshold slope Cs is 1.0 (no reduction); above it, Cs drops to zero as the roof approaches very steep angles.

For a warm, non-slippery roof the reduction starts around 30° (~7:12) and reaches zero near 70°. Cold roofs hold snow longer, so theirs starts around 45°.

A steep roof can carry noticeably less design snow than a low-slope one in the same location — though low-slope roofs have a minimum that prevents under-design.

The three adjustment factors in the flat-roof formula.

Ce (exposure) reflects wind: exposed roof 0.9, sheltered 1.2, partially exposed 1.0 (default). Ct (thermal) reflects heat loss: heated 1.0, unheated/cold 1.2. Is (importance) reflects risk category: 0.8 low-hazard, 1.0 typical homes, up to 1.2 essential facilities.

Each shifts the load up or down, so choosing them correctly matters — your building department or engineer confirms the right values.

For low-slope roofs, ASCE 7 sets a minimum so the roof isn’t under-designed just because the slope reduces the value. Where pg ≤ 20 psf the minimum is Is × pg; where pg > 20, it’s 20 × Is.

It applies to low-slope roofs (roughly under 15°) and can govern when the sloped load comes out lower. On steeper roofs it generally doesn’t apply.

The calculator flags when the minimum controls, so you can see whether the design load comes from the slope calc or the code floor.

No. This tool computes the balanced snow load only — a uniform layer. It does not cover drift, sliding, unbalanced, rain-on-snow, or partial loading, any of which can produce far higher local loads and often govern the design.

Snow drifts against walls, parapets, and roof steps; snow sliding off an upper roof piles on a lower one; rain soaking into snow adds surcharge. These conditions cause roof failures and need the full ASCE 7 analysis.

Use the balanced number as a starting reference — a licensed structural engineer must evaluate drift and the other cases.

It depends heavily on water content. Fresh light snow weighs only a few pounds per cubic foot, but settled or wet snow is far heavier, and ice heavier still.

Roughly, 10–12 inches of fresh snow — or as little as 3–5 inches of packed wet snow — can weigh about 5 psf, so a couple of feet of dense snow can approach or exceed a typical design load. That’s why loads are in psf, not inches.

If you’re worried about snow on a roof now, weight matters more than depth, and rain on snow raises it fast. When in doubt, use safe roof-rake practices from the ground or call a professional.