• | ------------------------------------------ | ------------------------------------------------ | | 1 | 0.75 | London, SE England, East Midlands | | 2 | 0.90 | Most of England, Wales lowland | | 3 | 1.10 | Northern England, most of Scotland, Wales upland | | 4 | 1.50 | Scottish Highlands, Pennines | | 5 | 1.80+ | Cairngorms, high altitude areas |

Altitude adjustment: sₖ(altitude) = sₖ + 0.001 × (altitude - 100) for sites above 100 m in Zones 1-3, and for all altitudes in Zones 4-5.

Altitude Corrections

Example: Sheffield (Zone 2, altitude 150 m): sk = 0.90 + 0.001 × (150 - 100) = 0.95 kN/m²

Roof Snow Load

[ s = \mu_i C_e C_t s_k ]

Where:

• μi = roof shape coefficient
• Ce = exposure coefficient (0.8 for windswept, 1.0 for normal, 1.2 for sheltered) — UK NA Table NA.2
• Ct = thermal coefficient (1.0 for normal buildings, heating < 1.0 for high-thermal-loss roofs)

UK NA Exposure Coefficient Ce

Roof Shape Coefficients (EN 1991-1-3 Table 5.2)

Monopitch roof:

Duopitch roof (symmetrical):

Multi-span roof drift coefficient: μ2 = μs × (h/sk⁰·⁵) for the valley between roofs, where:

• h = height difference between high and low roof
• s = snow drifting parameter
• μs = 2.0 (maximum drift coefficient)

Drift Loading at Roof Level Changes

For roofs with level changes (lower roof adjacent to higher wall):

μw is limited to 2.0 maximum per EN 1991-1-3 Clause 5.3.6.

Snow Load Tables for UK Roofs

Snow load on roofs — Zone 1 (London: sk = 0.75 kN/m²), Ce = 1.0, Ct = 1.0:

Drifted case for 30° pitch: μ₂ = 0.8 + 0.4 × (height/spacing) for multi-span roofs.

Worked Example — Snow Load on a 30° Pitched Roof in Manchester

Given:

• Building: Manchester (Zone 2, sk = 0.90 kN/m²)
• Altitude: 100 m (no altitude correction needed)
• Roof: 30° duopitch, normal exposure (Ce = 1.0)
• Building dimensions: 30 m span × 50 m length
• Frame spacing: 6 m

Step 1 — Ground snow load: sk = 0.90 kN/m² (Zone 2, no altitude adjustment needed)

Step 2 — Roof snow load: μ1 = 0.8 (for α = 30°) s = 0.8 × 1.0 × 1.0 × 0.90 = 0.72 kN/m²

Step 3 — Snow load on each rafter (6 m spacing): qSnow = 0.72 × 6.0 × cos(30°) = 0.72 × 5.20 = 3.74 kN/m (on slope length)

Step 4 — Snow load effects: Vertical load per rafter: 3.74 kN/m (on plan projection) Moment due to snow (span = 30 m): Msnow = 3.74 × 30² / 8 = 421 kNm

Step 5 — ULS combination: 1.35 × dead + 1.5 × snow (snow as leading variable action) Per EN 1990, when snow is the leading variable: ψ0 = 0.7 for imposed load combination

Step 6 — Drifted snow at valley (if multi-span): For adjacent span with same roof pitch (30°), valley coefficient: μ2 = 0.8 + 0.4 × (h / sk) where h = 30 × tan(30°) / 2 = 8.66 m μ2 limited to 2.0: μw = 1.2 × (h / s0)⁰·⁵ ≤ 2.0

Step 7 — Exceptional snow load: EN 1991-1-3 Clause 4.3 requires checking: SAd = 2.0 × sk = 1.80 kN/m² (exceptional drift) This is typically used only for roofs with unusual geometry.

Design Resources

• UK Wind Load — Wind loading companion
• UK Beam Design — Roof beam design
• UK Framing Systems — Portal frame design
• UK Steel Beam Sizes — Section data
• UK Steel Properties — Material data
• All UK References

Frequently Asked Questions

How is UK snow load calculated per EN 1991-1-3?

UK NA provides ground snow load sk values: Zone 1 (sk = 0.75 kN/m²), Zone 2 (sk = 0.90 kN/m²), Zone 3 (sk = 1.10 kN/m²). Roof snow load s = μi × Ce × Ct × sk. For a typical 15° pitched roof in London (Zone 1): s = 0.8 × 1.0 × 1.0 × 0.75 = 0.60 kN/m². For altitudes above 100 m, an additional increment of 0.001 kN/m² per metre applies.

What roof shape coefficients apply per UK NA?

UK NA uses μ1 = 0.8 for low-pitch roofs (0-30°), reducing linearly to 0 for 30-60°. For multi-span roofs, drift coefficients μ2 = 0.8 + 0.4 × (h/s0) for high/low roof interfaces, capped at 2.0. For curved roofs, the coefficient varies with the roof rise/span ratio. For cylindrical roofs, the coefficient is based on the roof angle at the eaves up to a maximum of 2.0 at the crown.

What is the difference between undrifted and drifted snow loading?

Undrifted (uniform) snow loading uses μ1 = 0.8 for low-pitch roofs. Drifted (non-uniform) snow occurs due to wind redistribution, creating triangular load patterns at roof level changes, valleys, and parapets. EN 1991-1-3 Clause 5.3 requires both undrifted and drifted cases to be checked: the undrifted case often governs simple pitched roofs, while drifted case governs multi-span buildings and roofs with level changes. For a 30° duopitch roof: undrifted s = 0.8 × sk, drifted has additional accumulation.

What combination factors apply for snow with other loads?

Per UK NA to EN 1990: when snow is the leading variable action, use ψ0 = 0.7 for wind (accompanying variable). When wind is the leading variable, use ψ0 = 0.7 for snow. For accidental combinations (exceptional snow): ψ1 = 0.5 for snow (frequent value), ψ2 = 0.2 (quasi-permanent). In UK practice, snow + wind rarely govern simultaneously because heavy snow typically occurs in calm conditions.

What is the exceptional snow load case?

EN 1991-1-3 Clause 4.3 requires consideration of exceptional snow loads for certain roof types at altitudes above 1000 m, or where local conditions cause abnormal accumulations. The exceptional load is defined as: SAd = 2.0 × Ce × Ct × sk. This is treated as an accidental design situation with γA = 1.0. For most UK buildings below 200 m altitude, the exceptional case does not govern, but it should be documented in the design basis.

Reference only. Verify all values against the current edition of EN 1991-1-3:2003 and UK NA. This information does not constitute professional engineering advice.