European Snow Load — EN 1991-1-3 Snow Actions on Steel Structures
Complete reference for snow load determination on structural steel buildings and roofs per EN 1991-1-3 (Eurocode 1: Actions on structures — Part 1-3: Snow loads). Characteristic ground snow load sk mapping across Europe, snow load shape coefficients μi for mono-pitch, duo-pitch (pitched), multi-span, and barrel-vaulted roofs, drifted snow arrangements including the critical unbalanced load case, exceptional snow load provisions for Northern Europe, and a fully worked example for a multi-span steel portal frame.
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EN 1991-1-3 Snow Load Framework
EN 1991-1-3 provides the methodology for determining snow loads for structural design. The fundamental snow load on a roof is expressed as:
s = μi × Ce × Ct × sk
Where:
- s = snow load on the roof (kN/m²), projected on the plan area (horizontal projection)
- μi = snow load shape coefficient (accounts for roof geometry, drift, and sliding)
- Ce = exposure coefficient (accounts for site wind exposure affecting snow accumulation)
- Ct = thermal coefficient (accounts for heat loss through the roof reducing snow)
- sk = characteristic ground snow load (kN/m²) at the site
The load is applied to the horizontal projection of the roof — the same snow mass is spread over a longer sloped length, resulting in a lower load per unit area of sloping roof, which is captured through the shape coefficient μi.
Characteristic Ground Snow Load sk
sk is the characteristic value of snow load on the ground at the site, with an annual probability of exceedance of 0.02 (50-year return period). It is provided by the National Annex based on altitude and geographic zone.
UK Ground Snow Load Map (BS EN 1991-1-3 UK NA)
sk varies with site altitude H (metres above mean sea level):
| Zone | UK Region | sk at H = 0 m | sk at H = 100 m | sk at H = 200 m | sk at H = 400 m |
|---|---|---|---|---|---|
| 1 | South-east England (London, Kent, East Anglia) | 0.25 | 0.29 | 0.35 | 0.50 |
| 2 | Central and southern England | 0.25 | 0.33 | 0.45 | 0.72 |
| 3 | South-west England, Wales lower | 0.25 | 0.31 | 0.41 | 0.63 |
| 4 | Northern England, Midlands, Wales upper | 0.25 | 0.35 | 0.49 | 0.80 |
| 5 | Scotland lowlands, Northern Ireland | 0.30 | 0.40 | 0.55 | 0.88 |
| 6 | Scottish Highlands, Pennines (highest) | 0.35 | 0.47 | 0.64 | 1.05 |
The general formula for sk in the UK is: sk = (0.15 + 0.1 × Z + A/525) where Z is the zone number (1-6) and A is site altitude in metres, with a minimum of 0.25 kN/m² for Zones 1-4 and 0.30 kN/m² for Zone 5, 0.35 for Zone 6.
European Ground Snow Load Representative Values
Representative sk values for major cities (flat, low altitude):
| City | sk (kN/m²) | Notes |
|---|---|---|
| London | 0.25-0.30 | Zone 1, H ≈ 25 m |
| Paris | 0.45 | Zone A1 (NF EN 1991-1-3) |
| Berlin | 0.55-0.85 | Zone 1-2 (DIN EN 1991-1-3) |
| Warsaw | 0.90 | Zone 2 (PN-EN 1991-1-3) |
| Stockholm | 1.50-2.50 | Zone-dependent |
| Oslo | 1.50-3.50 | Extreme northern values |
| Vienna | 0.90-1.50 | Zone dependent |
| Madrid | 0.40-0.60 | Zone 1-2 |
| Rome | 0.50-0.60 | Zone I (Italian NA) |
| Helsinki | 1.50-2.75 | Zone-dependent — Baltic |
Exposure Coefficient Ce
EN 1991-1-3 Table 5.1 defines Ce based on site topography and wind exposure:
| Terrain | Ce | Description |
|---|---|---|
| Windswept | 0.8 | Flat unobstructed areas exposed on all sides — snow blows off, minimal accumulation |
| Normal | 1.0 | Areas with no significant snow removal by wind — standard suburban and rural |
| Sheltered | 1.2 | Areas significantly sheltered by terrain or higher structures — snow accumulates and lingers |
For most buildings, Ce = 1.0 is the default. Use Ce = 0.8 for exposed steel structures on open hilltops or coastal sites. Use Ce = 1.2 for buildings surrounded by taller structures or in deeply incised valleys.
Thermal Coefficient Ct
Ct accounts for reduced snow load due to heat transmission through a roof of high thermal transmittance (U-value):
| Roof Type | Ct | Condition |
|---|---|---|
| Normal unheated roof or well-insulated roof (U ≤ 1.0 W/m²K) | 1.0 | Standard case — no thermal reduction |
| Poorly insulated roof (U > 1.0 W/m²K) | ≤ 0.85 | Snow melts from below — reduced accumulation |
For most modern steel roof systems (insulated composite panels, built-up systems with mineral wool insulation), U-value is well below 1.0 W/m²K and Ct = 1.0 applies. The exception is uninsulated steel roofs (open canopies, agricultural sheds) where Ct may be reduced — but consult EN 1991-1-3 Cl. 5.2(8) and the National Annex.
Snow Load Shape Coefficients μi
Mono-Pitch Roofs (EN 1991-1-3 Cl. 5.3.2)
For a single-pitch roof (angle α), two load cases apply:
Case (i) — Uniform snow (undrifted):
μ1(α) = 0.8 for 0° ≤ α ≤ 30°
μ1(α) = 0.8 − 0.8 × (α − 30)/30 for 30° < α ≤ 60°
μ1(α) = 0.0 for α > 60°
| Pitch α | 0° | 10° | 15° | 20° | 25° | 30° | 35° | 40° | 45° | 50° | 55° | 60° |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| μ1 | 0.80 | 0.80 | 0.80 | 0.80 | 0.80 | 0.80 | 0.67 | 0.53 | 0.40 | 0.27 | 0.13 | 0.00 |
The unbalanced case is not required for mono-pitch roofs unless exceptional drift conditions apply.
Duo-Pitch (Pitched) Roofs (EN 1991-1-3 Cl. 5.3.3)
For duo-pitch roofs (two slopes with pitch angles α1 and α2), three load cases apply:
Case (i) — Uniform snow on both slopes: μ1(α1) and μ1(α2), each using the same function as mono-pitch.
Case (ii) — Drifted snow (unbalanced): 0.5 × μ1(α1) on one slope, μ1(α1) on the other. This accounts for snow blown off one slope and accumulated on the opposite slope.
Case (iii) — Drifted snow (unbalanced, reversed): same as Case (ii) but with the drift pattern reversed.
For symmetrical duo-pitch roofs (α1 = α2 = α):
- Case (i): uniform μ1(α) on both slopes
- Case (ii): 0.5 × μ1(α) on left slope, μ1(α) on right slope
- Case (iii): μ1(α) on left slope, 0.5 × μ1(α) on right slope
Design note: The unbalanced case (ii or iii) typically governs the design of portal frame rafters and haunch connections because of the asymmetric bending moment distribution.
Multi-Span Roofs (EN 1991-1-3 Cl. 5.3.4 and Annex B)
Multi-span roofs (saw-tooth, northlight, continuous pitched) require special drift consideration in valleys between ridges. EN 1991-1-3 Annex B provides the drift snow load distribution:
For valley regions, the shape coefficient increases above μ1 to account for snow accumulation:
μw = μ1 + (γ × h / sk) (limited to maximum values per Annex B)
Where:
- μw = shape coefficient in valley (drifted)
- γ = snow weight density (2 kN/m³, recommended)
- h = change in roof height across the valley
For a typical continuous multi-bay portal frame (equal spans, equal eaves height), the drift snow accumulation in the valleys between bays can be 1.5× to 2.0× the uniform snow load, significantly affecting purlin, rafter, and valley gutter design.
Cylindrical / Barrel-Vaulted Roofs (EN 1991-1-3 Cl. 5.3.5)
For arched or barrel-vaulted steel roofs:
μ3 = 0.2 + 10 × h/b (limited to 0 ≤ μ3 ≤ 2.0)
Where h = rise of the arch, b = chord width (span). For slender arches (h/b ≤ 0.05), μ3 = 0.8 (flat roof equivalent). For steep arches (h/b ≈ 0.3-0.5), μ3 can reach 1.5-2.0 at the crown, reflecting drift accumulation.
Drifted Snow — Exceptional Cases (Northern Europe)
For regions with significant snowfall (sk ≥ 1.0 kN/m²), Annex B of EN 1991-1-3 requires consideration of the drifted load case in addition to the uniform case. The drifted snow arrangement represents the accumulation of snow in geometric discontinuities:
- At roof level changes: Snow drifts against parapets, behind taller building portions
- At roof obstructions: Snow accumulates around rooftop plant, chimneys, vents
- In valleys and gutters: Deep snow between roof ridges
For steel portal frames with internal valleys, the drifted case can represent a 50-100% increase in snow load in the valley zone compared with uniform snow. Design the valley purlins and rafter sections for this enhanced load.
Worked Example — Multi-Span Steel Portal Frame
Building details:
- Roof: 3-bay duo-pitch, each bay 20 m span, pitch α = 15°
- Eaves height: 7 m
- Location: Sheffield, UK (Zone 4, H = 100 m)
- Terrain: Normal (Ce = 1.0)
- Roof: Insulated composite panel (Ct = 1.0)
Step 1 — Ground Snow Load
sk for Zone 4 at H = 100 m: sk = 0.15 + 0.1 × 4 + 100/525 = 0.15 + 0.4 + 0.19 = 0.74 kN/m². Minimum for Zone 4 = 0.25 → Use sk = 0.74 kN/m².
Step 2 — Shape Coefficients
For α = 15°, μ1 = 0.80 (constant for ≤ 30°).
Load Case (i) — Uniform: s = 0.80 × 1.0 × 1.0 × 0.74 = 0.59 kN/m² on the horizontal projection, uniform on all three bays.
Load Case (ii)/(iii) — Drifted (unbalanced): Bay 1: 0.5 × 0.80 × 0.74 = 0.30 kN/m² Bay 2: 0.80 × 0.74 = 0.59 kN/m² Bay 3: 0.5 × 0.80 × 0.74 = 0.30 kN/m²
But for multi-span roofs, Annex B valley drift must also be considered. In each valley between adjacent bays, snow accumulates at the geometric low point.
Valley Drift (Annex B): Assume valley depth from ridge to gutter h = 2.8 m (15° pitch over 10 m half-span). γ = 2 kN/m³.
Additional drift shape coefficient in valley: Δμ = γ × h / sk = 2 × 2.8 / 0.74 = 7.57 → capped at the Annex B maximum of 2.0.
Valley peak snow load: svalley = 2.0 × 0.74 = 1.48 kN/m² — this is 2.5× the uniform snow load, and applies over a length of approximately 5 m on either side of the valley.
Step 3 — Design Loads
| Case | Uniform (kN/m²) | Valley Drift (kN/m²) |
|---|---|---|
| Characteristic snow (SLS) | 0.59 | 1.48 |
| Design snow — ULS (γQ = 1.5) | 0.89 | 2.22 |
Step 4 — Portal Frame Member Design Implications
The valley drift case governs:
- Valley purlins: Designed for enhanced downward load of 2.22 kN/m² (ULS). Typically requires one purlin size up from uniform case.
- Valley gutter beam: Snow + ponded water + self-weight — check deflection and strength.
- Internal columns: The asymmetric snow load from drifted case produces a bending moment in the internal columns due to rafter thrust imbalance. Check internal columns for combined compression + bending per EN 1993-1-1 Cl. 6.3.3.
- Rafters in valley bay: Larger bending moment due to higher snow load. Verify cross-section resistance and lateral-torsional buckling.
Snow Load Quick-Reference by Roof Type
| Roof Type | Governing Case | μmax | Typical sk = 0.50 | Typical sk = 1.00 |
|---|---|---|---|---|
| Flat roof (0° ≤ α ≤ 30°) | Uniform | 0.80 | 0.40 kN/m² | 0.80 kN/m² |
| Mono-pitch (α = 15°) | Uniform | 0.80 | 0.40 kN/m² | 0.80 kN/m² |
| Duo-pitch (α = 30°) — uniform | Uniform | 0.80 | 0.40 kN/m² | 0.80 kN/m² |
| Duo-pitch (α = 30°) — unbalanced | Drifted (½ and full) | 0.80 | 0.40 kN/m² | 0.80 kN/m² |
| Barrel vault (h/b = 0.2) | Drifted | 2.0 (limit) | 1.00 kN/m² | 2.00 kN/m² |
| Multi-span valley | Valley drift | 2.0+ | 1.00+ kN/m² | 2.00+ kN/m² |
| Roof with parapet (hp = 1 m) | Behind parapet drift | 1.6 | 0.80 kN/m² | 1.60 kN/m² |
Steel Purlin Snow Load Design Considerations
For cold-formed or hot-rolled steel purlins spanning between portal frame rafters, the snow load arrangement can significantly affect the design:
- Uniform snow: Purlin designed for uniform UDL per metre length = s × purlin spacing
- Valley drift snow: Purlin load varies along the span (non-uniform). Design the purlin for maximum moment considering the actual load distribution
- Unbalanced snow on duo-pitch: Purlin bending about the minor axis may increase if the snow acts eccentrically on a sloping roof (roof sheeting diaphragm action reduces this)
- Snow overhanging at eaves: For gutters and eaves purlins, the overhanging snow load creates torsion in the purlin — typically addressed through bridging and sag rods
Frequently Asked Questions
How does the UK National Annex zone my site for snow load?
The UK NA (BS EN 1991-1-3) divides Great Britain and Northern Ireland into six snow zones (1-6). Zone 1 covers south-east England (lowest snow), Zone 6 covers the Scottish Highlands (highest snow). The zone number, combined with site altitude, determines sk using the formula sk = (0.15 + 0.1Z + A/525) kN/m², with minimum values of 0.25-0.35 kN/m² depending on zone. Use the Ordnance Survey or site survey to determine altitude, and consult the UK NA Figure NA.1 for exact zone boundaries. For sites near zone boundaries, use the more onerous zone.
When is the drifted snow case critical compared to uniform snow?
Drifted snow governs in three key situations: (1) multi-span roofs where valley drift can double the snow load at valleys, increasing rafter bending moments and governing valley purlin size; (2) duo-pitch roofs with pitch above 15° where unbalanced snow produces asymmetric rafter moments that can govern the haunch connection and column design; (3) roofs with parapets or abutting taller buildings where snow accumulates against the obstruction. The drifted case is mandatory for all roofs — it is not an optional check.
What snow load applies to a steel canopy or open-sided structure?
For open-sided steel canopies (petrol stations, bus shelters, walkway covers), EN 1991-1-3 still applies. The snow load shape coefficient for a canopy roof uses the same μ values. However, because the canopy has no thermal insulation from below, Ct may be reduced if the roof is uninsulated (Cl. 5.2(8)). Additionally, canopies in windswept locations (Ce = 0.8) benefit from reduced snow accumulation. The net design snow load on a steel canopy is often 0.5-0.7 × sk, which is significantly less than for an enclosed heated building.
How does exceptional snow load differ from the normal snow load provisions?
Exceptional snow loads (EN 1991-1-3 Cl. 4.3 and Annex A) apply when snow accumulation is governed by exceptional meteorological conditions that exceed the 50-year return period. These are design situations where the snow load is applied without the combination factor ψ0 — i.e., at full value alongside the permanent load only, and with modified partial factors (γA = 1.0). Exceptional snow load is particularly relevant for Northern Europe (Scandinavia, Alpine regions) and for roofs with geometries prone to drift in extreme snowfall events. In the UK, exceptional snow is generally not a governing case for building design (normal provisions control), but it must be checked for structures in Zone 6 at altitudes above 300 m.
What is the snow load on a roof with solar panels mounted on steel support rails?
Solar panels on a steel roof affect snow load in two ways: (1) the panels alter the roof surface — snow slides less readily from glass panels than from a profiled metal sheet, so μ1 should be taken as for a flat unobstructed surface (no sliding reduction assumed); (2) the panel support rail creates a local obstruction that accumulates drifted snow. Per EN 1991-1-3 Annex B, treat the bottom edge of each row of panels as a local obstruction. For rows with a height above roof surface ≥ 0.3 m, drift snow accumulation at the panel edges should be considered. The additional snow surcharge from panel drift can be significant (Δμ up to 1.0) and should be included in purlin and rail design.
Related Pages
- EN 1990 Load Combinations →
- European Wind Load per EN 1991-1-4 →
- EN 1993 Steel Design Overview →
- EN 1993 Column Buckling →
- EN 1998 Seismic Design →
- Snow Load Calculator Tool →
- ASCE 7 Snow Load (USA) →
- Australian Snow Load (AS/NZS 1170.3) →
Educational reference only. Verify ground snow loads against the current National Annex for the building jurisdiction and confirm site altitude from a qualified survey. Snow load calculations must be independently verified by a qualified structural engineer. Results are PRELIMINARY — NOT FOR CONSTRUCTION without professional structural engineering review.