Eurocode 1 national annex differences — why the same EN 1991 gives different qb / sk per 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 gets calculated for three completely different load envelopes. The reason is the *national annex* (NA): every CEN member state customises 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 catch architects out on 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. There are 31 national annexes in active use across the EU, EEA + the UK as of 2024, plus an EU-mean default (Annex C, "informative") that some smaller markets adopt directly.

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 designed for DE NW lowland (sk 0.65) and shipped to a CH plateau site (sk 1.20) is under-designed by 85 % for snow. Cross-border projects need re-validation against the destination NA, not the origin.

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 we see in PONARC dealer projects:

- DACH dealers shipping into Italy. DIN sk values are conservative for South Tyrol; the IT NA gives a more generous treatment of altitude. Re-running the calc on the IT NA can drop the structural class by one tier. - UK architect specifying in the South of France. The GB NA pushes qb harder than the FR NA at most sites; the structure is over-designed if the GB calc is shipped without re-validation. - NO project being installed in the AT Alps. Snow falls in the UK / lowland EU is mostly accumulated; the NO NA assumes a saturated snowpack early — pergola design from NO will already cover AT sk 2.5 without modification.

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: re-validation against destination NA before final fabrication release." 2. Altitude clause: "Sites above the destination NA's trigger altitude (DE: 600 m, AT/CH: 1000 m) require a project-specific structural calculation. Standard-product pricing and lead time apply only to mapped-zone sites."

Linking back to our products + tools

Every PONARC group structural calculation is performed against the destination country's NA — Luxa Sereno pergolas, the Aperio retractable louvre roof, the VisioMod fence + glass railing range. 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 structural calculation re-validated against the destination NA for a cross-border project? Contact our engineering team — re-validation typically lands within five working days.*