Eurocode 1 National Annex Guide: Wind and Snow Loads by Country.

EN 1991 is one document for the whole of Europe — and yet a 4-by-4-metre pergola in Hamburg, Marseille and the Alps can be checked against very different load envelopes. The reason is the *national annex* (NA): each CEN member state adapts the Eurocode for its own climate and geographic risk profile. This guide explains how the national annex layer works, why specifiers should care, and the deltas that can affect cross-border projects.

What a "national annex" actually is

EN 1991 is published by CEN as the harmonised baseline. Each CEN member state then publishes its own NA, which:

1. Sets the country-specific values for any "Nationally Determined Parameter" (NDP) — fundamental basic wind velocity vb,0, characteristic ground snow load sk per zone, and the partial safety factors that go on top. 2. Maps the country into zones — wind zones for EN 1991-1-4, snow zones for EN 1991-1-3. 3. Adds country-specific clauses for shielding effects, altitude correction formulas, and how to handle special topographies (Alpine valleys, North Sea coast funnels, sea-side exposure).

The base Eurocode is the same; the NA is what makes the calculation different in practice. Project teams should confirm the currently applicable national annex and any local authority requirement before using a country table as a final basis.

The wind side — EN 1991-1-4 NA

Wind starts with vb,0, the *fundamental basic wind velocity* (10-minute mean at 10 m height in terrain category II, 50-year return period). Each NA publishes a wind-zone map that gives vb,0 per zone, then the calculation cascades through:

- Roughness factor cr(z) — based on terrain category I–IV. - Topography factor co(z) — for hills, escarpments and Alpine valley corridors. - Exposure factor ce(z) = cr(z)² · co(z)². - Reference pressure qb = ½ · ρ · vb,0². - Peak velocity pressure qp(z) = ce(z) · qb.

The NDP that varies wildly across NAs is vb,0. In our 21-country specifier dataset:

| Country | vb,0 floor (zone 1) | vb,0 ceiling (top zone) | Comment | |---|---|---|---| | DE | 22.5 m/s (Zone 1, S/E inland) | 30.0 m/s (Zone 4, North Sea coast) | 4-zone map, hinterland coast included | | GB | 22.0 m/s (S England) | 30.0 m/s (Coastal Scotland / N. Isles) | Map blends Met Office 50-year basic wind speed | | FR | 22.0 m/s | 28.0 m/s | 4-zone map per Cerema, with overseas territory annex | | NO | 22.0 m/s | 32.0 m/s (Lofoten, exposed N. Norway) | Highest peak in EU/EEA mainland | | GR | 27.0 m/s (inland) | 33.0 m/s (Aegean) | Aegean island wind tunnel pushes the ceiling | | ES | 26.0 m/s | 29.0 m/s (Cantabrian Atlantic) | A/B/C zones, Mediterranean often Zone A |

Implication for a specifier: the same fence brought into a DE Zone 4 site sees qb = 0.56 kN/m²; in GR coastal it's 0.68 kN/m². A 21 % difference in design pressure for the *same physical fence* — the difference is purely in the NA mapping.

The snow side — EN 1991-1-3 NA

Snow uses sk, the characteristic ground snow load, in kN/m². NAs publish:

- A snow-zone map of sk per zone. - An altitude-correction formula sk(A) — typically a piece-wise polynomial or step function above a threshold altitude. - A site-specific note for high-altitude / Alpine sites where the formula above runs out and a project-specific calc is required.

Key NA deltas in our dataset:

| Country | Lowest sk | Highest mapped sk | Altitude trigger for site-specific | |---|---|---|---| | DE | 0.65 kN/m² (NW lowland) | 1.32 kN/m² (Bavarian foothills) | ≥ 600 m | | AT | 0.90 kN/m² (Burgenland) | 2.50 kN/m² (alpine, mapped) | ≥ 1000 m | | CH | 1.20 kN/m² (Plateau) | 4.00 kN/m² (Alps ≥1000 m) | always (alpine national annex) | | GB | 0.40 kN/m² (S England) | 1.20 kN/m² (Highlands) | ≥ 400 m | | NO | 1.50 kN/m² (Oslo lowland) | 4.00 kN/m² (coastal N. Norway) | ≥ 600 m | | PT | 0.20 kN/m² (Lisbon) | 0.60 kN/m² (Serra da Estrela) | low altitude, mostly lowland |

Implication: a pergola initially checked for DE NW lowland (sk 0.65) may need a higher snow-load review before it is specified for a CH plateau site (sk 1.20). Cross-border projects should be re-checked against the destination NA, not only the origin-market assumption.

Three NA traps that catch architects out

1. Altitude correction is non-linear. Above the trigger altitude (typically 600–1000 m), sk grows faster than linear with elevation. A site at 800 m in Bavaria isn't sk × 1.3 — the polynomial in DIN EN 1991-1-3 NA produces values 1.5–1.9× the lowland figure depending on local topography. Don't extrapolate; use the formula or commission a site-specific calc.

2. Coastal exposure modifiers stack on top of zone vb,0. GB's NA includes a "topography correction" co for sites within 2 km of the open coast that adds 5–10 % to qb. CH and NO have similar Alpine-corridor multipliers. The zone-table vb,0 is just the floor.

3. Shielding by adjacent buildings is country-specific. Some NAs (DE, AT) apply explicit shielding factors when a structure sits in the lee of another building of similar height; others (GB, FR) require this to be argued in a site-specific calculation. A pergola in a downtown German courtyard may be designed for 70 % of qb due to shielding; the same pergola in central Paris needs full qb in the absence of an explicit project-level argument.

What this means for cross-border projects

Three patterns that often deserve an early cross-border check:

- DACH dealers shipping into Italy. DIN sk values can be conservative for parts of South Tyrol; the IT NA may treat altitude differently. Re-running the calculation on the destination NA can affect the structural class. - UK architect specifying in the South of France. The GB NA can push qb harder than the FR NA at many sites; a GB-based calculation should not be treated as the final French project check without review. - NO project being installed in the AT Alps. Snow assumptions can differ sharply between lowland EU, Norwegian and Alpine sites; the destination NA should confirm whether the existing design envelope is sufficient.

The cleanest move on any cross-border project: re-run the calculation against the destination country's NA. Our wind & snow specifier calculator covers 21 EU/EEA countries and will surface the local zone table for the chosen country — useful as a first-cut sanity check before you commission a full site-specific structural calc.

What to write in the tender

Two lines that make the NA explicit:

1. NA reference: "Structural calculation per EN 1991-1-3 + EN 1991-1-4, destination country national annex (e.g. DIN EN 1991-1-3/NA). Cross-border supply: review against destination NA before final fabrication release." 2. Altitude clause: "Sites above the destination NA's trigger altitude should be reviewed by the project engineer. Standard-product pricing and lead time should be confirmed after the site load envelope is checked."

Linking back to our products + tools

PONARC group projects should be reviewed against the destination country's NA — Luxa Sereno pergolas, the Aperio retractable louvre roof, and the VisioMod fence + glass railing range all depend on site-specific wind, snow, substrate and use-category assumptions. The full set of structural references (EN 1991, EN 1090 execution class, alloy specs, surface treatment) lives in the structural section of the standards hub. For a first-cut load envelope on a candidate site, use the wind & snow specifier calculator.

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*Need a destination-NA review for a cross-border project? Contact our engineering team with the site, height, exposure and product family so the correct calculation route can be confirmed before fabrication.*