Roofs have to withstand tough environmental conditions, and now climate change is posing even more challenges. Here, Barbour Index and Scott Brownrigg weigh up the options
Climate change is subjecting Roofs to increasingly extreme external conditions. The UK Climates Impact Programme (www.ukcip.org.uk) shows that all weather elements are becoming increasingly powerful and have the potential to disrupt any roofing installation. And it’s going to get worse – wind speeds and rainfall levels are predicted to increase steadily. These effects are forcing the specifier to reconsider what has been thought of as good practice. Specifiers should weigh up the weather conditions that will be encountered even if the roof is only intended to last between 10 and 15 years. Without taking climate change into consideration specifiers may end up with a roof that performs badly except in the mildest conditions.
1. British Standards
The specifier should ensure that any roof is designed in accordance with current standards BS 6399 for wind and BS EN 12056 for rainwater disposal. The storms that regularly visit the UK with high wind speeds and driving rain result in effects that can exceed the values predicted in the standard. The storm return frequency in the standard is also forecast to change. Care and local advice should be a key driver when selecting the design parameters. Be wary of the rainwater design data in BS EN 12056, as it is more than 30 years old. New data has been collected but has not been published yet.
Gusting winds can cause significant damage to the perimeter of large and exposed roofs. These need to resist roof uplift caused by suction as well as direct wind pressure. For lightweight coverings such as single-ply roofing types ensure that additional fixings or fixing centres are used around the perimeter zone of the roof. If Ballast is used avoid shingle around the perimeter. Paving slabs are preferable.
The increasing intensity and amount of rainfall requires higher levels of durability and drainage provision. Drainage systems can easily be swamped – these should not allow water to build up beyond the design limits. Weirs or overflows should be part of designs if only to alert building managers to problems. The weir and overflow design should ensure that all water above a maximum is dumped, as ideally the roof should shed water rather than retain it.
Specifiers would be wise to design to a higher code than set out in the standard. For commercial premises a category 3 or even 4 design from BS EN 12056 is not unreasonable. Consider siphonic drainage systems rather than gravity drainage as these can remove large quantities of water from a roof quickly. However they must be working to optimum efficiency with no blockages or backing up and water must flow freely to the first manhole. This will require a good maintenance regime and fully trained operatives. Always specify at least two outlets to any roof area with an overflow as well. A British Standard for siphonic drainage systems is in preparation but not yet available.
Look carefully at the design of upstands, flashing and cloaks. It may be normal practice to specify 150 mm deep flashings but these may be defeated by strong winds, rain water system backup or drifting snow.
Solar radiation is also increasing in intensity thanks to the reduction of the ozone layer. Manufacturers are taking account of this by subjecting their products to more extreme testing regimes in the hope that any detrimental effects are addressed long before going to market. Long-term exposure particularly to ultraviolet radiation bleaches out the plasticisers that make plastics flexible, making them brittle. This is particularly relevant for any plastics such as single-ply membranes so most high quality versions use additional plasticisers to cope with this reduction.
5. Colour fading and adhesive and sealant deterioration
All colours fade to some extent, with bright colours suffering more than others. Significant levels of ultraviolet radiation will cause glues, sealants and bonding agents to weaken or fail. The specifier should seek detailed information on the long-term testing of any products used. Be wary of simulated tests or paper-based studies. In all large roof areas changes in appearance will occur over time.
6. Thermal cycles
Roofs need to be able to withstand movement caused by seasonal heat differences. If the roof geometry is complex or the choice of materials is varied, allow for the roof to move. Detailing should make allowances for these movements, which otherwise will cause distortion. It is essential for specifiers to understand that many of these movements will occur at the junction of materials and will not to be covered by any one manufacturer’s warranty.
7. Keep to the specification
After considering the environmental details be wary of specification changes. The roofing sector is competitive and contractors might promise cheaper or better materials or frustratingly make a direct approach to the client. These offers should nearly always be treated with suspicion.
Check out contractors by visiting other sites they have worked on or talk to project teams. It is easy to give in to a tempting offer but clients will not be impressed by specification changes that result in a substantially poorer product.
8. Health and safety
No checklist on roofing could be complete without the usual warning on safety because of the particular dangers when working at height. Undertake a full risk analysis and eliminate any obvious risks.
Subject guides similar to this are available from Barbour Index as part of its Construction Expert and Specification Expert services. For further information, contact Barbour Index on 01344-899280 or visit www.barbour-index.co.uk
BSEN 12056 Part 3 Gravity Rainwater Disposal Systems BS 6399-2 Wind Loadings on Buildings
Health and Safety in Roof Work HSE 1998