Snow Load Calculator — Roofs That Carry Winter, Not Just Weight
Estimates the design snow load on a steel roof from sk = μr · s0 (GB 50009-2012) using the roof shape factor for your pitch geometry, with the EN 1991-1-3 shape-coefficient method shown alongside for export verification. Snow rarely falls evenly — the code's drift, slide and accumulation cases are where small roofs fail, so the calculator includes the shape logic and the commentary explains where the real load concentrates.
Design Snow Load on the Roof
Enter the site's 50-year ground snow pressure and the roof geometry. The shape factor follows GB 50009-2012 Table 7.2.1 for the selected roof family.
Inputs — Site Climate and Roof Geometry
Beijing 0.40 • Harbin 0.45 • Shenyang 0.50 • Urumqi 0.60 kN/m² (GB 50009 App. E, typical values)
0 for flat; affects μr once α > 25°
At selected density the snow depth equivalent: sk / (ρ·g) ≈ 0.41 m of snow cover.
How to read the results
sk is the characteristic snow load on the projected (plan) roof area — codes load the horizontal projection, not the sloped surface, and the shape factor adjusts for how snow sits on your particular roof. For a flat or low-pitch roof (α ≤ 25°) the factor is 1.0: the full ground value acts on the roof. Above 25° the factor falls (snow slides), and at α = 50° a gable roof reaches μr = 0.0 under GB 50009 — but the sliding snow then accumulates on the lower roof or at eaves, which the two-level drift case captures.
sk = μr · s0 (GB 50009-2012 §7.1.1)
EN 1991-1-3 cross-check: the equivalent European load is s = μi · Ce · Ct · sk, where Ce (exposure, typically 1.0) and Ct (thermal, 1.0 for insulated roofs) bracket the same physics as the Chinese factor, and μi starts at 0.8 for low-pitch roofs (versus GB's 1.0) — a real difference, not a tolerance. When we design an export roof to EN, the European shape coefficients (with their 0.8 < μ < 0.8–1.6 drift range) govern, and the GB value is used as a check. Always tell your fabricator which code governs: the same roof about 20–25° pitch can be ~20% lighter or heavier depending on the national annex.
Partial snow and rain-on-snow: codes require the check of drifted (asymmetric) snow on duo-pitch roofs — one slope at μr, the other at 0.5 μr — because a purlin or truss sized only for uniform load derates when the unloaded slope relieves its neighbour. Our design reports always include the asymmetric case for gable roofs and the drift case for stepped buildings; this is where a "standard" steel package from a supplier who skips the drift case quietly under-designs the splice.
Roof Shape Factor μr — GB 50009-2012 Table 7.2.1
| Roof Geometry | Shape Factor μr | Design Note |
|---|---|---|
| Flat / single-pitch, α ≤ 25° | 1.0 | Full ground load on plan area; governing for most industrial roofs |
| Duo-pitch, α between 25° and 50° | 1.0 → 0.6 linear | Interpolate μr = 1.0 − 0.4×(α−25)/25; plus asymmetric case |
| α ≥ 50° | 0.0 (gable) / 0.2 minimum | Snow slides; check eaves accumulation and valley drift instead |
| Two-level / stepped roof | 1.0 + drift wedge (up to ~2.0) | Drift forms at the step when wind sweeps the upper roof; triangular wedge over the lower roof |
| Arched / curved roof | 0.5 – 0.8 by f/l ratio | Snow redistributes from crown to haunches; check both distributions |
Worked example: 24 m × 60 m gable warehouse in Shenyang
Shenyang ground snow s0 = 0.50 kN/m². Roof pitch 8° → μr = 1.0, so sk = 0.50 kN/m². At a frame spacing of 6 m, each portal frame picks up a roof band of 6 m × the 12 m half-width = 72 m² → sk × 72 ≈ 36 kN of snow per frame, distributed as a uniform vertical load on each rafter — roughly three times the dead load of the frame itself. Adding the asymmetric case (one slope 0.5 μr) and the wind-uplift combination, the rafter and eave connection are designed for the worst admissible envelope. This is exactly the arithmetic behind every "snow region" price premium in steel quotations — now you can run it yourself before you ask us to.
Why the drift case decides the splice
The classic failure: a steel warehouse with a tall monitor or a higher adjacent hall — the upper roof's snow slides and drifts into the step at the lower roof, piling up in a wedge that can exceed 2× the ground value over several metres of lower roof. The lower-roof purlins and the column at the step see a load the flat-roof assumption never included. Whenever possible we detail a transition detail (aerated parapet, snow fence, or a steepened lower slope) precisely so the code's drift case does not govern; when it does govern, the members are sized for it — both are engineering decisions, and we document which one we made and why.
Combining Snow With Wind — the Envelope, Not the Peak
- Gravity governs at the mid-span: for a flat roof the governing combination is usually 1.2×Dead + 1.4×Snow (GB) or 1.35×G + 1.5×S (EN) — the rafter, purlin and splice all size up for it.
- Uplift governs at the eaves and edge zones: wind suction on the roof field (see the wind load calculator) can exceed the snow + dead combination locally, particularly in the corner zones. The bracing and anchorage must hold the roof down against the suction case where uplift dominates.
- Rain-on-snow surcharge: for low-pitch roofs (< 5°) certain codes add a rain-on-snow surcharge during thaw. We include the case where the code requires it, and we check the drainage design so ponding cannot accumulate a secondary load on an already loaded span.
- Long-span sensitivity: on 100 m+ grids and trusses, the snow pattern (uniform, drifted, partial) is checked member by member: a drift over the crown can produce local hogging moments the uniform case never creates. This is why large-span snow loads in our reports are always accompanied by the load-pattern diagrams, not a single number.
Design for snow — checklist your steel supplier should answer
1) Which national code governs (GB 50009, EN 1991-1-3, ASCE 7-22) and what is the design sk for your city? 2) Which shape factors were applied, and were the asymmetric and drift cases run? 3) How were wind + snow combined per the governing code's partial factors? 4) What is the snow load at the eave, the ridge and the step (if any)? 5) Is the roof drainage designed so ponding cannot add load on top of snow? If a quotation answers all five with numbers — and not with "standard design" — you are talking to an engineer. That is the standard Permanent Steel holds on every project, and the same numbers you can reproduce with the calculator on this page.
Send us the city, roof geometry and code — we return the snow case (uniform + drift + asymmetric) with the frame budget in 24 hours.
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Wind + Snow Combined
Run the snow load first, then combine with wind uplift — the two load cases work as a pair.
Full Design Check for Snow Regions
Snow-country projects sized with drifted and asymmetric cases — request a load report today.
Roofs Engineered for the Snow It Actually Gets
Send us the city, roof geometry and governing code — we return the uniform, drifted and asymmetric snow cases with member implications, plus the frame budget that goes with them.
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