How do I find the basic wind speed for my location?

UK basic wind speeds are given in the BS EN 1991-1-4 National Annex wind speed map. Sheltered inland areas are typically 21–23 m/s, while exposed coastal areas can be 27–30 m/s. Scotland generally has higher wind speeds than southern England.

What are pressure coefficients (Cp)?

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.

What is the maximum suction zone on a roof?

The highest suction typically occurs at the windward eaves and verge edges (Zone 1 corners). Here, Cp values can reach -1.3 or more. This is why roof tiles at edges need extra fixings per BS 5534.

How does terrain affect wind loading?

Buildings in towns benefit from shelter from surrounding buildings, reducing wind speed by approximately 25%. Open country provides no shelter (factor 1.0). Coastal and hilltop sites increase the effective wind speed by 15–20%.

Is this calculator suitable for structural design?

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.

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Roof Wind Load Calculator — BS EN 1991-1-4

Calculate wind pressure on roofs using BS EN 1991-1-4. Includes basic wind speed by location, terrain factors and pressure coefficients by pitch and zone.

Basic Wind Speed

From BS EN 1991-1-4 NA wind map

Terrain Category

Affects wind speed at site

Building Height

Height to ridge

Roof Pitch

Affects pressure coefficients

Roof Span

Roof Length

How We Calculate This

This calculator applies a simplified version of the BS EN 1991-1-4 method for determining wind loads on roofs.

The formula

q = 0.613 × v\u00b2 (dynamic pressure in Pa)

w = q × Cp (wind pressure per zone)

Where v = site wind speed = vb × terrain factor × altitude factor × direction factor

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.