Wind load and terrain category

The two effects of wind

Pressure

On the windward side it presses the façade inwards. An intuitive effect.

Suction

On the leeward side, on the side walls and above all on the roof it pulls outwards or upwards. Not an intuitive effect, and often larger than the pressure.

On a flat or low-pitched roof suction is the dominant effect — the roof “wants to fly off”.

What its magnitude depends on

• wind zone: from the map in the national annex,
• height: the higher the surface, the greater the wind speed,
• terrain category: greater in open country, lower in dense urban surroundings,
• the position on the surface: local suction at the edges, corners and roof perimeter is substantially higher than in the middle of the surface,
• the shape of the building, its openings and permeability.

Why it governs in light gauge steel

Because the upward force would be balanced by self-weight — and there is little of it.

It follows that wind typically sizes three things:

1. the fixing of the roof covering and the build-up — closer fixings are needed along the edges,
2. the roof-to-wall connection, which works in tension too,
3. the bracing and the anchorage.

Hold-down anchorage · Resisting uplift in light structures.

Local effects

Along the edges and corners of the building suction is far greater than in the middle of the surface. So:

• fixings have to be closer at the edges,
• the detailing of the edge trims is a structural question,
• the roof perimeter is one of the most common places for storm damage.

What matters on site

Closer fixings are the detail easiest to leave out, and the consequence only emerges in a storm. So mark the closer-fixing zones on the drawings and check them during the work.

The point

On a light building wind tests the fixings rather than the stability. Damage almost always starts at the edges and the perimeters.