BS EN 1991-1-4 Wind Actions Explained

1) Who does BS EN 1991-1-4 apply to?

• Buildings and civil engineering structures where wind creates structural pressures, suction or forces.
• Whole structures and individual elements, including roofs, walls, cladding panels, parapets, canopies and attached components.
• Roof coverings and their fixings where wind uplift must be calculated and resisted.

What it’s not: BS EN 1991-1-4 does not provide the complete design of every roof covering or structural material. It determines the wind actions that the relevant roof, cladding, timber, steel, masonry or fixing design must then be capable of resisting.

2) The big idea in plain English

Wind does not act evenly across a building. Its effects change according to location, exposure, height, shape and the part of the building being considered. BS EN 1991-1-4’s core message is:

• Wind pressure is site-specific. A sheltered urban property is different from a coastal, hilltop or open-country building.
• Wind increases with height and exposure. Taller and more exposed buildings generally experience greater wind actions.
• Edges and corners are critical. Local suction can be considerably higher around roof perimeters, corners, ridges and verges.
• Internal pressure matters. Openings and air permeability can increase or reduce the net load acting on a roof or wall.

If you remember one thing: the design wind load is not one figure for the whole roof—different roof zones can require different fixing strengths.

3) Wind speed & pressure

BS EN 1991-1-4 begins with a basic wind velocity and then applies a series of factors to determine the wind conditions affecting the building.

• Basic wind velocity: derived from national wind data and adjusted where appropriate for direction and season.
• Mean wind velocity: takes account of terrain roughness, height above ground and topography.
• Peak velocity pressure: represents the pressure associated with the mean wind together with shorter-term gust effects.

Client takeaway: wind load is calculated from more than the nearest weather forecast. It is based on standardised design wind data and the particular characteristics of the site and building.

4) Terrain & exposure: location matters

BS EN 1991-1-4 classifies terrain according to its roughness and the obstacles located upwind of the building.

• Coastal and open terrain: generally allows wind to reach the building with less obstruction.
• Open country: includes low vegetation and widely spaced trees or buildings.
• Suburban and wooded terrain: includes more regular buildings, vegetation and obstacles.
• Dense urban terrain: contains a significant concentration of taller buildings.

Hills, ridges, cliffs and escarpments can further accelerate the wind and may require an additional orography factor.

Client takeaway: two identical roofs in different locations may require different fixing specifications because their wind exposure is not the same.

5) Roof pressure zones: the heart of wind design

Wind passing around a building creates both pressure and suction. On roofs, suction or uplift is often most severe around perimeter zones and corners.

What this means in practice

• Different roof zones: central, edge and corner areas can have different pressure coefficients.
• Roof shape matters: flat, monopitch, duopitch, hipped, multispan and curved roofs are assessed differently.
• Roof pitch matters: changing the pitch changes how wind flows over the roof and the resulting pressure coefficients.
• Loaded area matters: small components and individual fixings can experience higher local coefficients than large roof areas.

Client takeaway: a uniform fixing pattern may not be adequate. Roof edges and corners commonly require enhanced restraint.

6) External pressure, internal pressure & net load

Wind acts directly on the external surfaces of a building, but it can also enter through openings and act on the internal surfaces.

• External pressure: the pressure or suction created as wind passes over walls and roofs.
• Internal pressure: pressure acting inside the building due to openings, leakage and permeability.
• Net pressure: the combined effect of the external and internal pressures acting across the building element.

A large opening, damaged door, missing rooflight or partially completed elevation can significantly alter internal pressure.

Client takeaway: the roof must resist the pressure difference across it, not simply the wind acting on its external surface.

7) Edges, corners, parapets & fixings

Local wind actions are commonly greatest at the exposed edges of buildings. BS EN 1991-1-4 gives particular consideration to:

• Roof corners and perimeter zones: areas where local suction can be significantly higher.
• Ridges, hips and verges: details exposed to rapidly changing airflow and uplift.
• Parapets and free-standing walls: elements subjected to pressure on opposing faces and local end effects.
• Canopies and overhangs: constructions exposed to wind pressure above and below the surface.

Client takeaway: failures often begin at the perimeter because that is where wind suction and local fixing demand are commonly highest.

8) Alterations, openings & temporary works

BS EN 1991-1-4 requires the relevant wind actions to be considered for different design situations, including changes made while a building is under construction or being altered.

• Temporary exposure: a partially completed roof or elevation can behave differently from the finished building.
• Open doors and windows: openings assumed closed during storms may need to be considered as an accidental design situation.
• Changes to the building: extensions, roof conversions, new openings, parapets and rooftop equipment can alter wind flow and pressure.

Client takeaway: a structure can be more vulnerable during roofing works than after completion. Temporary restraint, sequencing and weather planning therefore matter.