Snow and wind load calculation BS EN 1991 for solar carport and PV structures in the UK
Method to identify your snow load and wind speed zone under BS EN 1991-1-3/1-4 and the UK National Annex, with worked examples for London, Manchester and Edinburgh. Engineering check included in every Sunrak quote.
A poorly sized photovoltaic mounting structure is not a minor technical detail — it is an insurance liability. The three most common failures on solar carports and PV canopies — collapse under snow load, lift-off in storms, and slow fatigue deformation — almost always trace back to a calculation that ignores BS EN 1991-1-3 (snow loads) and BS EN 1991-1-4 (wind loads). These standards, together with the UK National Annexes, have been mandatory in Britain since 2010, replacing the older BS 6399 series. Any load-bearing PV structure without a formal calculation to BS EN 1991 is not contractually defensible.
This article explains, without unnecessary jargon, how our engineering team identifies your real climate zone, reads the characteristic ground snow load sk and the fundamental basic wind velocity vb,map, and sizes a structure that withstands North Sea storms, Scottish snowfall and Atlantic gales. You will find here the 3-step method, three worked examples (London, Manchester, Edinburgh), and the common pitfalls of “cheap solar carport” kits that advertise a generic “snow resistance” without any calculation evidence.
Why BS EN 1991 is mandatory for any solar mounting structure
PV mounting structures are the most commonly under-engineered components on the UK market. Three reasons:
- Adverse weight-to-area ratio. A traditional pitched timber roof weighs 50-80 kg/m². An aluminium solar carport weighs 8-12 kg/m². Per unit area, the climatic loads (snow, wind, modules) dominate by an order of magnitude.
- Wind uplift on canopies is massive. A shallow-pitched PV canopy in exposed terrain experiences suction up to 1.7 kN/m² in edge zones — enough to rip off any structure with under-sized anchorage.
- Snow does not accumulate uniformly. EN 1991-1-3 prescribes different shape coefficients depending on roof geometry, edge drift, and shading behind PV modules. The 2025 revision introduces explicit rules for PV panel rows on flat roofs — older studies do not include these.
A stand-alone structural calculation by an independent engineer costs in the UK between £500 and £1,800. At Sunrak it is included in every quote, because it is what separates a 20-year installation from a stack of aluminium that fails on the first proper storm and voids your buildings insurance.
Snow loads under BS EN 1991-1-3 in the UK
The UK National Annex (NA to BS EN 1991-1-3) maps characteristic ground snow loads sk across England, Wales, Scotland and Northern Ireland. Unlike France or Germany with discrete numbered zones, the UK uses a continuous isopleth map with regional values.
Typical sk values (sea level)
| Region | Characteristic ground snow load sk |
|---|---|
| Southern England (Cornwall, Devon, Hampshire, Kent) | 0.30 - 0.40 kN/m² |
| Midlands and Wales | 0.40 - 0.55 kN/m² |
| Northern England (Yorkshire, Lake District) | 0.55 - 0.75 kN/m² |
| Lowland Scotland (Edinburgh, Glasgow) | 0.55 - 0.80 kN/m² |
| Highland Scotland | 0.80 - 1.20 kN/m² (varies steeply with altitude) |
| Northern Ireland | 0.40 - 0.65 kN/m² |
Altitude correction
The sk map values are referenced to sea level (A = 0 m). The UK National Annex provides a linear correction for altitude:
sk(A) = sk(0) + 0.10 × A / 100 (for A ≤ 100 m)
sk(A) = sk(0) + 0.10 × A / 100 + ... (continued formula above 100 m)A solar carport in the Cairngorms (Aviemore, A ≈ 300 m, base sk ≈ 1.00 kN/m²) faces a corrected sk ≈ 1.30-1.50 kN/m². A standard “Zone B kit” will collapse there in its first winter.
Roof snow load
The actual roof load is then computed:
s = μi × Ce × Ct × skwith:
μi— shape coefficient (0.8 for pitch ≤ 30°, linear decrease 30°-60°, zero above 60°)Ce— exposure coefficient (1.0 normal, 0.8 windswept, 1.2 sheltered)Ct— thermal coefficient (1.0 unless glazed heated roof, ≥ 1.0 W/m²·K transmittance)
For PV modules on flat or low-pitched roofs, EN 1991-1-3:2025 introduces extra drift coefficients in the shaded edge zone of each module row.
Wind loads under BS EN 1991-1-4 in the UK
The UK National Annex to BS EN 1991-1-4 defines the fundamental basic wind velocity vb,map at 10 m above ground over open country (Z₀ = 0.05 m), with a 50-year return period.
Typical vb,map values
| Region | vb,map (10 m, open country) |
|---|---|
| South-East England (London, Kent, Sussex) | 22 m/s |
| South-West and Midlands | 23-24 m/s |
| North-West England, Yorkshire | 24-25 m/s |
| Wales coast | 25-26 m/s |
| Western and Northern Scotland (Glasgow, Inverness) | 26-29 m/s |
| Shetland and Outer Hebrides | 30-32 m/s |
From basic velocity to peak velocity pressure
The peak velocity pressure on your structure is:
qp(z) = ce(z) × qbwhere qb = 0.613 × vb² (kN/m²) and ce(z) is the exposure factor depending on terrain category (Town vs Country in the UK NA), altitude A, and reference height z of the structure.
For a 2.8 m high solar carport in London Country category (suburban areas with houses and trees):
qb = 0.613 × 22² = 0.297 kN/m²
ce(2.8) ≈ 2.0 - 2.4 depending on town/country and altitude
qp(2.8) ≈ 0.60 - 0.71 kN/m²In Edinburgh exposed Country terrain (vb,map = 26 m/s):
qb = 0.613 × 26² = 0.414 kN/m²
qp(2.8) ≈ 0.85 - 1.10 kN/m²This pressure is then multiplied by external pressure coefficients cpe per roof zone — edge zones can experience up to 2.5× the central pressure.
3-step method to identify your zone
Step 1 — Postcode and altitude
The postcode identifies the locality. Ordnance Survey and Google Earth give the site altitude to the metre. Our quote form systematically asks for the postcode for this reason.
Step 2 — Read snow + wind values
From postcode + altitude we read:
- the
skground snow load (with altitude correction) - the
vb,mapbasic wind velocity (with altitude correction)
Step 3 — Action combinations to BS EN 1990
We apply the load combinations of BS EN 1990:
- Persistent ULS:
1.35·Gk + 1.5·Qk,s + 0.75·Qk,w(and permutations) - Accidental ULS: where exceptional snow load applies
- SLS characteristic and frequent for deflection (typically L/200 under characteristic combination)
This is what determines the section sizes, the post spacing, the foundation type and — often — the 8-15% premium over a “standard kit” that ultimately saves your structure from a winter or storm failure.
Three worked examples
Site 1 — London (W1 area)
| Parameter | Value |
|---|---|
| Town | Central London |
| Altitude | 25 m |
sk (with altitude correction) | 0.35 kN/m² |
vb,map | 22 m/s |
| Roof snow load on 30 m² structure | 8.4 kN |
| Estimated peak wind pressure | ≈ 0.65 kN/m² |
Site 2 — Manchester (M1 area)
| Parameter | Value |
|---|---|
| Town | Central Manchester |
| Altitude | 38 m |
sk (with altitude correction) | 0.55 kN/m² |
vb,map | 25 m/s |
| Roof snow load on 30 m² structure | 13.2 kN |
| Estimated peak wind pressure | ≈ 0.85 kN/m² |
Site 3 — Edinburgh (EH1 area)
| Parameter | Value |
|---|---|
| Town | Central Edinburgh |
| Altitude | 47 m |
sk (with altitude correction) | 0.70 kN/m² |
vb,map | 27 m/s |
| Roof snow load on 30 m² structure | 16.8 kN |
| Estimated peak wind pressure | ≈ 1.00 kN/m² |
Three cities, three radically different design loads. A single “national kit” sold across Britain is mathematically under-engineered for at least one of these cities — typically Edinburgh on both wind and snow simultaneously.
Common pitfalls
Pitfall 1 — Generic “snow resistance” claims. Many kits advertise “supports 60 kg/m² snow load” with no mention of ULS combination, pitch or accumulation coefficient. This is marketing copy, not a calculation report.
Pitfall 2 — Underestimating altitude correction. A site at 500 m in Cumbria sees
skrise from 0.55 to 1.20+ kN/m². A “Manchester-rated” carport installed there is mechanically under-engineered.
Pitfall 3 — Wind suction in edge zones ignored. Pressure coefficients
cpein roof zones G, H, J (corners and edges) are 2-3 times more severe than in the central F zone. The four corner anchorages are what limit the structure under storm loading.
Pitfall 4 — Ignoring Scottish topographic effects. Many Scottish sites have orographic effects (
c₀ > 1.0) on top of base velocities. A glen, ridge or windward slope can lift effectiveqpby 20-30%.
Pitfall 5 — Foundations forgotten. A perfectly sized structure on an undersized concrete pad is equally useless. Our quote always includes the foundation plan with minimum allowable ground bearing pressure.
Why have our engineering team handle this
Our in-house engineering team delivers for every quote:
- Automatic identification of your
skandvb,mapfrom postcode + altitude - Structural calculation report 10-25 pages detailing each coefficient, action combination and section verification
- Dimensioned plans assembly + foundation, suitable for planning submission (where required)
- Exhaustive parts list: each profile numbered, exact length, mass, quantity
- Installation manual in English with torque values and assembly order
The calculation is included in the quote price — no separate structural engineer to commission, no surprise fees. You receive within 48 business hours a complete technical dossier, signed, usable for permitted development declaration or full planning application.
Request your structural study →
FAQ — BS EN 1991 for solar mounting structures
Does BS EN 1991 apply to small carports under 30 m²?
Yes. BS EN 1991 applies to any load-bearing structure regardless of area. Permitted development rules may simplify the planning application, but the calculation requirement remains. Your buildings insurance may also require evidence of compliance in case of a claim.
Is CE marking on the profiles enough?
No. CE marking certifies conformity with the Construction Products Regulation. It does not substitute a project-specific BS EN 1991 calculation. A profile can be CE-marked and still be under-sized for your site.
What is the difference between BS EN 1991 and the old BS 6399?
BS 6399 was the UK pre-Eurocode standard and is fully superseded since March 2010. Modern PV mounting projects must use BS EN 1991 with the UK National Annex. Calculations to BS 6399 alone are no longer accepted by most building control bodies.
My site is at 400 m altitude — how does sk change?
The UK NA provides a linear correction. For example, a base sk of 0.55 kN/m² at sea level becomes ≈ 0.95 kN/m² at 400 m. Our engineering team applies this correction automatically from your postcode and altitude.
Do I need wind calculation for a ground-mount array?
Yes — and this is where wind matters most. A ground-mount is more exposed than a roof, and its anchorage (driven piles or ballasted footings) must resist uplift. Inadequate calculation leads to complete uplift failure in the first serious storm.
Does Sunrak include the structural calculation in the quote?
Yes, always. Every Sunrak quote includes a full BS EN 1991-1-3 + 1-4 calculation, dimensioned plans, parts list and installation manual in English. No additional structural engineer to commission, no surprise fees.
How long does the calculation take after my enquiry?
48 business hours from receipt of complete information (postcode, dimensions, photos for roof installations). This is our contractual commitment for any Sunrak quote.
Is this also useful for the Republic of Ireland?
Yes. Ireland uses the same Eurocodes with the Irish National Annex (I.S. EN 1991-1-3 and I.S. EN 1991-1-4). The values for `sk` and `vb,map` differ slightly from UK NA values but the calculation methodology is identical. We deliver calculations to either NA on request.
Further reading: