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Roof Wind Load Calculator — BS EN 1991-1-4
Estimate wind pressure on a duopitch roof using BS EN 1991-1-4 and the UK National Annex. Includes basic wind speed by location, an indicative exposure adjustment, and UK NA Table NA.7a pressure coefficients by pitch and zone.
Duopitch zone coefficients (UK NA Table NA.7a) vary with pitch
Adjust for site altitude
1.0 for worst-case all directions
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How We Calculate This
This calculator applies a simplified version of the BS EN 1991-1-4 (Eurocode 1, wind actions) method, with the UK National Annex, for estimating wind loads on a duopitch roof.
The formula
q = 0.613 × v² (dynamic pressure, kN/m²)
The constant 0.613 = ½ × ρ, with the UK NA air density ρ = 1.226 kg/m³ (NA.2.18). v is in m/s, giving q in kN/m².
w = q × (cpe − cpi) (net wind pressure per zone)
External coefficients cpe come from the UK National Annex Table NA.7a (duopitch, θ=0°), which replaces the CEN Table 7.4a/7.4b: zone F is the windward corner/edge (highest suction), zones H/I cover the interior. Internal pressure cpi is taken as the UK NA default +0.2 for the worst-case net uplift.
Site wind speed v = vb × exposure adjustment × altitude factor × direction factor. The exposure adjustment here is indicative; the full method reads the exposure factor ce(z) from the UK NA charts.
The headline figure is the peak zone F corner suction (a local pressure for fixing design). The whole-roof uplift uses the lower mean interior coefficient over the full slope area, because zone F applies only to small corner patches — using the corner peak over the whole roof would roughly double the true figure.
Working figures for this are collected in our BS 5534 Wind Zones reference table.
Frequently Asked Questions
The fundamental basic wind velocity vb,map is read from the BS EN 1991-1-4 UK National Annex map (Figure NA.1). It ranges from roughly 21.5 m/s in south-west England to about 27 m/s in northern Scotland and Shetland before altitude and direction corrections. Sheltered inland areas sit at the lower end; exposed coastal and elevated sites at the higher end.
Pressure coefficients describe how wind pressure varies across the roof surface. Positive values indicate pushing force (on windward slopes), negative values indicate suction (pulling force). Steeper roofs have more positive pressure on the windward side.
The highest suction occurs in the windward corner/edge patches (zone F in BS EN 1991-1-4 Figure 7.8). For a duopitch roof at wind direction θ=0°, the UK National Annex Table NA.7a (which replaces the CEN Table 7.4a) gives the zone F overall coefficient cpe,10 as roughly −1.8 at a 5° pitch, −1.1 at 15°, −0.5 at 30° and near zero by 45° — so suction is highest on low-pitch roofs. Local cladding coefficients (cpe,1) over very small areas are more negative still. This is why roof tiles and slates at eaves, verges and ridges need extra mechanical fixings per BS 5534.
In the full Eurocode method, terrain enters through the exposure factor ce(z) (with the ce,T town correction), which depends on the height above ground, the terrain category and the distance upwind to the sea or town edge — it is read from the UK National Annex exposure charts (SCI P394), not a single fixed multiplier. This calculator applies a simplified indicative speed adjustment only (sheltered town below open country, coastal above). For design values, read ce(z)/ce,T from the UK NA charts.
This calculator gives indicative wind pressures based on simplified BS EN 1991-1-4 methods. For structural design, consult a chartered structural engineer who will apply the full Eurocode method with all relevant factors and load combinations. You may also want to check snow loading with our Roof Snow Load Calculator.
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Last updated: June 2026
Verified against UK standards · estimates only, confirm with your supplier.