Managing wind - how to build a roof that takes in local area considerations

When designing a roof there are a number of factors to take into consideration before even consulting the Building Regulations.

The physical and environmental context of a building will often establish a range of pre-conditions for the design of a pitched roof and its covering.

The specifier should consider the following factors including the general climate, exposure and the microclimate.

The design of a pitched roof is dictated initially by the shape of the building, the span between the structural elements and the use to be made of the space enclosed by the roof.

After structure and function have been taken into account, the design of the roof may be modified by other considerations, such as weathering and aesthetics. When designing the roof there needs to be consideration of adequacy of support; wind loads and attachments.

Exposure

The first priority is to establish the exposure zone of the site. You can do this by referring to the map in BS 5534 or from BRE Report 262 and BS 8104. The ‘Code of practice for assessing exposure of walls to wind-driven rain’ and ‘Thermal insulation: avoiding risks’. The map applies to buildings of up to 12 metres in height at the ridge.

Calculate the wind suction loading either in accordance with BS EN 1991: Part 1 - 4 or use the design calculations on wind loads in BS 5534.

Where the roof slope exceeds six metres in length and / or the site is rated to be in a severe exposure category, guidance on the suitability of the slate should be confirmed by contacting Cedral’s Technical Advisory Team.

Ridges and hips

All ridge and hip tiles must be mechanically fixed with at least one mechanical fixing, (this includes mortar bedded units) as required in BS 5534. Mechanical fixing is best achieved with a dry fixing method which can also provide ventilation at the ridge. Should a mortar bedded system be adopted then a mechanical anchor is required either through the unit or in conjunction with a clip through the mortar joint.

Aircraft vortices

Roofs near airports can experience high local wind load forces due to air vortices created by certain aircraft when taking off and landing, which may be greater than the calculated wind loads to BS 5534.

Designers should seek advice from the Airport Authority Planning Department when designing roof fixings in these locations, and refer to the guidance contained in BRE Digest 467 ‘Slate and tile roofs: avoiding damage from aircraft wake vortices’.

Calculating the fixing specification

The procedures for calculating the wind loads and determining the fixing specification for tiles and slates in accordance with BS EN 1991-1-4 and BS 5534 are complex to undertake.

Designers are advised to obtain a full roofing fixing specification from the Technical Advisory Team.

Technical Support

Specifiers and installers require prompt, knowledgeable and detailed responses to a vast range of enquiries covering everything from the embodied energy of a typical roof slate, to the different ventilation options available, as well as installation advice and guidance.

Our Technical Advisory Team is staffed by a qualified team with specialist knowledge not only of all Cedral roofing products, but also crucially, of how those systems integrate with other roofing components and comply with Building Regulations, Health and Safety, environmental and other critical roofing criteria.