Snow Load Calculator
Ground snow into roof snow — the 0.7 of the ground load, trimmed by exposure and heat, floored at the code minimum.
Snow Load Calculator
Results recalculate instantly on every keystroke. Nothing you type is transmitted.
What this result does not account for
- Balanced flat-roof load only — no drift or unbalanced
- Importance factor pinned at 1.0
In short: Forty psf of ground snow in a partial exposure, a heated building: the flat roof carries 0.7 of it, 28 psf — 56,000 lb across a 2,000 ft² roof, twenty-eight tons for the rafters. The 20 psf code minimum sits eight psf below and does not bite. Roofs carry less than the ground because wind scours and heat melts; the code just refuses to let the arithmetic promise less than the floor.
Formula
pf = 0.7 · Ce · Ct · Iₛ · pₘ ≥ the code minimum
Ground snow is measured where nobody builds; roofs get the 0.7 that wind erosion and building heat earn. Exposure trims or adds (sheltered sites hold their drifts), thermal factor punishes cold roofs, and the importance factor sits at 1.0 for ordinary structures. When the arithmetic dips under the minimum, the minimum governs — the code floor is not a suggestion. ASCE 7 Chapter 7 is the source; drift and unbalanced cases live beyond this page.
Worked Example
- Enter the ground snow load from your map.
- Set exposure and thermal factors for the site.
- Read psf, the tonnage, and whether the floor bit.
Defaults: 40 psf ground, partial exposure, heated, 2,000 ft² — 28 psf flat-roof, 56,000 lb, the 20 psf floor eight psf clear. Shelter that same roof and the exposure factor walks the load to 33.6 psf.
Strengths & Limits Of This Model
Where this engine is strong
- The code minimum floor is enforced, not forgotten
- Tonnage line ready for the rafter conversation
Where it stops
- Drift wedges are the real killer and live outside
Practical Use Cases
Shed and shop roofs
psf before the rafter span
Solar racking
the ballast under the panels
Permit prep
the number the plan checker asks for
Methodology & Editorial Standards
Computation runs in IEEE-754 double precision at full internal precision; rounding to two decimal places occurs strictly at the display layer, so no cumulative drift enters the result. All monetary outputs use accounting presentation — grouped thousands, two decimals, negatives in parentheses — so figures can be transcribed directly into a model or working paper. Division-by-zero and out-of-domain inputs return an em-dash rather than a misleading number.
This engine was reconciled against an independent reference implementation and hand-verified for the worked example above before release. Our full five-stage review process is published on the About Us page.
Disclaimer. This calculator is provided for informational and modelling purposes only and does not constitute financial, tax, legal, medical, or engineering advice. Verify all figures with a qualified professional before acting on them.
Snow Load Calculator — 8 Expert FAQs
8 analyst-written answers to the questions practitioners actually ask — optimised for voice and answer-engine retrieval.
Why does the roof only carry 70% of the ground?
Wind scours open roofs and building heat melts from below; decades of roof measurements averaged about seven tenths of the ground load. Sheltered or cold roofs lose those credits — their factors climb above one.
When does the 20 psf minimum govern?
Wherever the factors multiply to less than the floor: a 20 psf ground load in the open with a warm roof computes exactly at the line, and a light 15 psf map zone computes under it. The code then hands you the floor instead.
What ground snow load is my town?
From the snow-load map in the building code or your state's adopted map — the plains run light twenties, the Great Lakes and mountains stack fifties and beyond. The map value, not the last big winter, is the honest input.
Are drifts included in this number?
No — and drifts are how flat roofs actually fail. A parapet or a taller next-door wall piles the scoured snow into a wedge the balanced load never sees. Chapter 7's drift cases sit beyond this screen by design.
Why does a cold roof carry more?
Because nothing melts from below. An unheated structure takes a thermal factor of 1.2, so the same 40 psf ground load reads 33.6 psf instead of 28 — the heat you do not pay for, the rafters do.
What about the roof's own slope?
Steep smooth roofs shed snow and earn a slope factor below one; the flat-roof number here is the conservative ceiling the slope case starts from. Over about seventy degrees the code stops asking entirely.
How heavy is wet snow versus powder?
The map load already encodes regional density. What it does not encode is a stacked storm — rain on old snow can double the psf overnight, which is the load case the minimum floor quietly defends against.
What do I do with the tonnage line?
Hand it to the structure: beams and rafters are chosen in pounds, not psf. The beam deflection page takes the load from here and prices the sag the span will pay.