Wind forces

Winds create considerable forces on both the topside and the underside of roof cladding, and you must consider
these forces in the design and fixing of any roof. The forces are:
• inward forces tending to collapse the roof cladding inwards, caused by wind acting directly on the windward side; and
• outward forces tending to lift the roof cladding from its framing, and the entire roof structure from the rest of the
building.

Outward forces can be caused both by uplift from negative wind pressures, outside the building; and
by positive wind pressure inside the building. Generally the greatest wind forces imposed on roofs are
due to the outward forces. Because the dead weight of roofing materials is relatively small, the outward forces must
be resisted by the roof fasteners. It is very important that the battens and roof framing are
adequately fixed to the rafters and walls, and that claddings and flashings also be fixed to withstand these pressures.