Technical Reference
Snow Load Design for Solar Mounting:
EN 1991-1-3, ASCE 7-22 & AS/NZS 1170.3
In Norway, Austria, Switzerland, and US mountain states, snow load — not wind — is the governing structural design case for mounting rail section size. This guide explains the Eurocode EN 1991-1-3, ASCE 7-22 Chapter 7, and AS/NZS 1170.3 snow load methods for solar mounting systems: ground snow load zones, shape coefficient tables, worked calculation examples, when snow governs over wind, and how to correctly specify snow load requirements in a mounting system purchase order.
When Snow Load Governs Solar Mounting Design
Most online discussion of solar mounting structural design focuses on wind load — and for good reason: wind is the governing case for fastener uplift capacity and clamp specification in most coastal and temperate-zone markets. However, for the substantial and growing solar markets in central and northern Europe, Canada, Japan, and US mountain states, snow load is the governing case for mounting rail design and is frequently the input that drives module selection.
Snow load acts as a uniformly distributed downward force on the panel face. Unlike wind uplift, which creates tension in clamps and fasteners, snow load creates bending in the mounting rail, compression in the roof structure, and cumulative bearing loads at each attachment point. The governing structural check for a mounting rail under snow load is its section modulus (bending resistance) — a metric that determines how deep and thick the aluminium C-section or rail profile must be.
To illustrate the relative magnitudes: in Region A Australia (45 m/s), the downward wind pressure component on a 10° tilted panel is approximately 0.4–0.6 kN/m². In Lillehammer, Norway, the design snow load on the same 15°-tilted panel is 3.2 kN/m² — roughly 5–8× larger. The rail spanning between two supports at 1.5m carry 5–8× more bending moment under snow than wind. A rail adequate for Australian wind load will fail under Norwegian snow load.
This is not a niche problem. Germany has approximately 80 GW of installed solar capacity, with significant portions in snow load Zones 2 and 3. Norway is expanding its solar sector rapidly. Austria, Switzerland, and southern Japan (which uses AIJ/BSL snow provisions with values up to 10 kN/m² in Hokkaido) all require correct snow load treatment in mounting design.
European Ground Snow Load Zones (EN 1991-1-3 National Annexes)
The Eurocode snow load standard EN 1991-1-3 provides a framework; each country publishes a National Annex with specific ground snow load maps. The values below are characteristic (50-year return period) ground snow loads s_k for representative zones. Design roof snow load = μ₁ × s_k (assuming C_e = C_t = 1.0 and μ₁ = 0.8 for tilts ≤ 30°).
| Country / Zone | Ground Snow s_k | Design Load (μ₁=0.8) | Representative Cities |
|---|---|---|---|
| Germany — Zone 1 (north) | 0.65 kN/m² | 0.52 kN/m² | Hamburg, Schleswig-Holstein, most of north Germany |
| Germany — Zone 2 (central) | 1.05–1.7 kN/m² | 0.84–1.36 kN/m² | Frankfurt, Stuttgart, Nuremberg, Bavaria flatlands |
| Germany — Zone 3 (pre-Alpine) | 2.75 kN/m² | 2.20 kN/m² | Munich metro, Alpine foothills, Black Forest heights |
| Norway — Coastal south | 2.5 kN/m² | 2.0 kN/m² | Bergen, Stavanger, Trondheim coast |
| Norway — Inland central | 4.0–5.0 kN/m² | 3.2–4.0 kN/m² | Lillehammer, Hamar, Gjøvik |
| Austria — Eastern plains | 1.5 kN/m² | 1.2 kN/m² | Vienna, Graz, Linz lowlands |
| Austria — Alpine (Tyrol) | 4.5–6.0+ kN/m² | 3.6–4.8 kN/m² | Innsbruck, Salzburg rural, Vorarlberg |
| Switzerland — Plateau (Mittelland) | 1.5–2.0 kN/m² | 1.2–1.6 kN/m² | Zurich, Bern, Basel |
* Design load calculated as μ₁ × s_k with μ₁ = 0.8 (EN 1991-1-3, slope ≤ 30°), C_e = 1.0, C_t = 1.0. Altitude corrections apply above reference altitude (200m for Germany, site-specific for Norway). Always verify with the applicable National Annex for the project municipality.
US Ground Snow Load Regions (ASCE 7-22 Chapter 7)
ASCE 7-22 Figure 7.2-1 provides a mapped ground snow load p_g for the contiguous United States (in psf). The flat roof snow load p_f is calculated as: p_f = 0.7 × C_e × C_t × I_s × p_g, where C_e is the exposure factor (0.7–1.3), C_t is the thermal factor (1.0 for solar panels), and I_s is the importance factor (1.0 for Risk Category I, 1.1 for Risk Category II). For sloped roofs, a slope factor C_s further reduces p_f.
| US Region | Ground Snow p_g | Flat Roof p_f (approx.) | Representative Cities |
|---|---|---|---|
| Pacific Northwest Coast | ≤ 0.5 kPa (10 psf) | ≤ 0.35 kPa | Seattle, Portland, San Francisco |
| Northeast / New England | 1.0–2.4 kPa (20–50 psf) | 0.7–1.7 kPa | Boston, Providence, Vermont, NH |
| Midwest | 0.5–1.0 kPa (10–20 psf) | 0.35–0.7 kPa | Minneapolis, Chicago, Cleveland |
| Colorado / Rocky Mountains | 1.44–3.84 kPa (30–80 psf) | 1.0–2.7 kPa | Denver, Colorado Springs, Aspen |
| Mountain West (Utah, Montana) | 1.44–4.80 kPa (30–100 psf) | 1.0–3.4 kPa | Salt Lake City, Bozeman, Jackson |
* Flat roof snow load p_f estimated at 0.7 × p_g (C_e = 1.0, C_t = 1.0, I_s = 1.0). Actual values require site-specific calculation. Mountain and CS zones use site-specific values not shown on the mapped figure — use local jurisdiction ground snow load data.
Step-by-Step Snow Load Calculation (EN 1991-1-3)
The following six steps outline the EN 1991-1-3 procedure for determining the design snow load on a solar mounting system in Europe. This is an overview for procurement purposes — actual engineering reports must be prepared by a qualified structural engineer under the applicable national requirements.
Determine the Ground Snow Load s_k
Look up the site location in the National Annex snow load map for the project country. For Germany, use DIN EN 1991-1-3/NA Figure NA.1 (snow load zones 1–3 plus altitude correction above 200m). For Norway, use NS-EN 1991-1-3 NA Table NA.1. For Switzerland, use SIA 261 Annex A. The ground snow load s_k represents the characteristic (50-year return period) value for a flat open site at the site altitude.
Determine the Shape Coefficient μ₁
The shape coefficient converts ground snow load to roof snow load and accounts for the reduced accumulation on sloped surfaces. Under EN 1991-1-3 Clause 5.3: for α ≤ 30°, μ₁ = 0.8. For 30° < α ≤ 60°, μ₁ = 0.8 × (60 − α)/30. For α > 60°, μ₁ = 0 (no snow accumulation assumed). Solar panels at typical fixed tilts of 10–30° use μ₁ = 0.8 — the same as a flat roof. Only panels at 35°+ begin to benefit from shape coefficient reduction.
Apply Exposure Coefficient C_e
The exposure coefficient C_e accounts for redistribution by wind. For normal topography with some upwind shelter: C_e = 1.0. For exposed sites (hilltops, open plains with strong prevailing wind): C_e = 0.8 (conservative — assumes wind blows snow off). For sheltered sites surrounded by higher features: C_e = 1.2 (conservative — snow drifts accumulate). Most solar farm sites use C_e = 1.0. Confirm with the national annex; the UK NA for example specifies C_e = 0.8 for unobstructed exposure.
Apply Thermal Coefficient C_t
C_t accounts for heat loss through the roof that melts snow from below. For solar panels: C_t = 1.0 (no heat transmission — panels are non-insulating, but snow melt from solar gain is not counted as it is unreliable in design). Heated greenhouses or warm roofs may use C_t < 1.0 where justified.
Calculate Design Snow Load s
Design snow load on the roof: s = μ₁ × C_e × C_t × s_k. For a Norwegian site with s_k = 4.0 kN/m², μ₁ = 0.8, C_e = 1.0, C_t = 1.0: s = 0.8 × 1.0 × 1.0 × 4.0 = 3.2 kN/m². This is the uniformly distributed load applied to the top face of each panel and transmitted through the mounting rail to the roof structure.
Check Load Combinations
Under EN 1990 (Eurocodes basis), snow and wind are variable actions combined using combination factors ψ. The governing ULS combination for downward loading is typically 1.35G + 1.5S (snow dominant) or 1.35G + 1.5W_s + ψ₀×S (wind dominant). For roof mounting rails, check both: the 1.5S combination for maximum bending, and the 0.9G + 1.5W_uplift combination for maximum uplift. The governing combination determines rail section, clamp type, and fastener specification.
Worked Example: Norwegian Rooftop at 4.0 kN/m²
Site: Commercial rooftop, Lillehammer, Norway. 15° panel tilt (fixed). Array spans 20 panels (10 rows × 2 columns). Panel size 2.1m × 1.05m. Rail span between supports: 1.4m.
Step 1 — Ground Snow Load
Lillehammer municipality → NS-EN 1991-1-3 NA
→ s_k = 4.0 kN/m²
Step 2 — Shape Coefficient
Panel tilt α = 15° (≤ 30°) → EN 1991-1-3 Table 5.2
→ μ₁ = 0.8
Step 3 — Exposure & Thermal
Normal exposure (commercial roof, no shelter obstruction); standard thermal for solar
→ C_e = 1.0, C_t = 1.0
Step 4 — Design Snow Load
s = μ₁ × C_e × C_t × s_k = 0.8 × 1.0 × 1.0 × 4.0
→ s = 3.2 kN/m²
Step 5 — Load per Rail (1 rail per 1.05m panel width, each rail carries 0.525m width)
w_rail = 3.2 × 0.525 = 1.68 kN/m
→ w_rail = 1.68 kN/m
Step 6 — Maximum Rail Bending Moment
M = w × L² / 8 = 1.68 × 1.4² / 8
→ M = 0.41 kNm
Result: Each mounting rail must resist 0.41 kNm of bending moment under the snow design load. Compare this to the wind-only design in Sydney (Region A TC3): w_rail ≈ 0.35 kN/m → M ≈ 0.09 kNm — roughly 4.6× less. For this Norwegian site, OmniSol would specify a 40mm × 60mm × 2mm extruded aluminium rail (section modulus ~3.0 cm³) versus the 30mm × 30mm × 1.5mm rail that would be adequate for an Australian coastal installation. The wind uplift check (0.9G + 1.5W) is performed separately and does not govern rail bending in this climate — but does govern clamp capacity.
Note: Simplified illustration. Actual design must include unbalanced snow load cases (EN 1991-1-3 Annex B for multi-span roofs), load combinations from EN 1990, and material partial factors per EN 1999 (aluminium structures). Engineering reports must be verified by a qualified structural engineer.
Use the OmniSol Wind & Snow Load Calculator to size your system
Input your snow zone, wind region, panel size and array configuration — the calculator checks both snow and wind, identifies the governing case, and generates a BOS BOM with correct rail section and clamp specification.
Snow vs Wind: Which Governs and What It Means for Specification
| Design Check | Governing Load (typical) | Implication for Mounting Specification |
|---|---|---|
| Rail section modulus (bending) | Snow (in snow regions) | Larger rail cross-section required; specify rail depth and wall thickness in RFQ |
| Rail span between supports | Snow (in snow regions) | Closer support point spacing required; affects number of rail brackets per row |
| Clamp uplift capacity | Wind uplift (virtually all locations) | Clamp type, clamp count per panel, and torque spec all driven by wind speed |
| Roof fastener pull-out strength | Wind uplift or snow (check both) | Hook bolt or ballast design must resist whichever combination is greater |
| Panel glass mechanical rating | Snow (in regions s_k > 4.0 kN/m²) | IEC 61215 test at 5,400 Pa — confirm 8,100 Pa rated modules for Alpine sites |
| Roof purlin / rafter capacity | Snow + dead load (in snow regions) | Building structural review required before adding solar load in snow regions |
Ground Mount Design in Snow Regions
Ground-mounted systems in snow regions require different design approaches than rooftop arrays:
- Tilt angle 30–45°: At slopes above 30°, μ₁ starts to reduce in the Eurocode framework. At 45°, μ₁ = 0.8 × (60 − 45)/30 = 0.4 — halving the design snow load. Many northern European ground-mount projects use 35–40° tilt to balance year-round yield (high winter sun angle) with reduced snow load.
- Panel ground clearance: In regions with 1m+ snowfall depth, arrays must be elevated to avoid snow burial. Standard OmniSol tripod ground-mount systems can be configured with extended legs for higher clearance; specify the design snow depth in your RFQ.
- Unbalanced snow load: EN 1991-1-3 Annex B requires checking redistributed (unbalanced) snow on multi-span structures. One side of a bi-facial or east-west ground array may accumulate more snow than the other due to prevailing wind direction. This can create torsional loads in the racking structure.
- Foundation depth: Frost heave in snow climates requires pile foundations to extend below the frost line (0.8–2.0m depending on latitude). Standard 300mm shallow auger piles used in Australian or Mediterranean ground mounts are not adequate for Norway or Finnish climates.
Common Mistakes in Snow Load Specification
Using the national average snow load instead of the site-specific zone
Germany spans Zone 1 (0.65 kN/m²) to Zone 3 (2.75 kN/m²) — a 4× difference. "Germany project" cannot be used as input. Provide the municipality name or GPS coordinates.
Ignoring altitude correction
EN 1991-1-3 National Annexes for Germany, Austria, and Switzerland include altitude corrections above a reference altitude (200–400m depending on country). A site at 600m in Bavaria may have an adjusted s_k of 1.5–2.0 kN/m² even if the base zone value is 1.05 kN/m².
Applying wind-only rail selection to a snow-region project
A mounting system datasheet rated for "wind speed 60 m/s" does not inherently certify the rail for 3.2 kN/m² snow load. Request the rail's allowable bending moment in kNm — this is the specification parameter for snow load design.
Not checking unbalanced snow accumulation for ground-mount arrays
EN 1991-1-3 Annex B and local national annexes specify redistribution coefficients for multi-span structures. A symmetric-looking ground array may develop asymmetric snow loads that create unexpected torsional forces in the structure.
Using a flat-roof module IEC test certificate without altitude/zone verification
IEC 61215 tests at 5,400 Pa front face. This covers most European snow zones (s_k up to 6.75 kN/m²). However, for Alpine sites with s_k > 5.0 kN/m² and μ₁ = 0.8: design load = 4.0 kN/m² — within 5,400 Pa. But if combined snow + live load or unbalanced cases push beyond 5.4 kN/m², a higher-rated module is required.
What to Include in Your RFQ for Snow Load Projects
To enable OmniSol to produce a correctly specified mounting system for a snow-region project, include the following in your RFQ:
Full site address or GPS coordinates with altitude (for snow zone and altitude correction)
Applicable snow standard and National Annex (EN 1991-1-3 + country NA, or ASCE 7-22)
Ground snow load s_k or p_g if known from local authority
Panel dimensions (length × width) and fixed tilt angle
Array type: rooftop (with roof structure details) or ground-mount
Design wind speed and standard (snow and wind must both be checked)
Desired support point spacing or span constraint
Any local building authority requirements for snow load calculation sign-off
Frequently Asked Questions
When does snow load govern over wind load for solar mounting design?
Snow load governs for downward structural actions — specifically the bending of mounting rails and the bearing capacity of roof purlins. In central and northern Europe (Norway, Austria, Switzerland, mountain Germany), Alpine Australia, and US mountain states (Colorado, Utah, Montana), ground snow loads of 1.5–5.0 kN/m² produce roof snow loads of 1.2–4.0 kN/m² that far exceed the downward wind pressure component (typically 0.3–0.8 kN/m²). For uplift (wind pulling panels off the roof), wind governs in virtually all locations. In practice, both snow and wind must be checked; the governing case for rail section size is usually snow, while the governing case for clamp specification is wind uplift.
What is the snow shape coefficient for solar panels at 15°?
Under EN 1991-1-3, μ₁ = 0.8 for slopes from 0° to 30°. At 15°, μ₁ = 0.8. The shape coefficient only reduces for slopes above 30°. Under ASCE 7-22, Cs = 1.0 for warm roofs up to 30°. For optimal snow shedding, mountain-region arrays are often designed at 30–45°.
What ground snow loads apply in Germany, Norway, and Austria?
Germany: s_k = 0.65 kN/m² (Zone 1, north) to 2.75 kN/m² (Zone 3, pre-Alpine). Norway: s_k = 2.5 kN/m² (southern coast) to over 9.0 kN/m² (inland highlands). Austria: s_k = 1.5 kN/m² (eastern plains) to over 6.0 kN/m² (Tyrol/Alpine). Always confirm with the National Annex map for the specific site.
Does snow load affect the mounting rail section size?
Yes — snow load frequently governs rail section selection. A 3.2 kN/m² snow load over a 2m × 1m panel produces 1.6 kN/m per rail at 1.5m span, requiring M = 0.45 kNm rail capacity. This typically requires a 40×40mm or larger C-section where a wind-only climate would be adequate with a smaller section.
What solar panel glass snow load rating is required for heavy snow regions?
IEC 61215 requires a 5,400 Pa front-face load test. For s_k above 6.0 kN/m² (Norwegian/Swiss Alpine sites), confirm the module manufacturer tests to the enhanced 8,100 Pa level. OmniSol supplies modules with test reports for the applicable load level on request.
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