by Anthony Lymath
This is the final part of an eight-part series of articles, Climate Change Adaptation in Buildings, examining the impact of climate change on the built environment, and the responses that can be made to those changes for both new-build and retro-fitting. Previous articles have covered an overview of the issues, excess heat, flooding, subsidence and drought.
Unpredictable weather patterns include the capacity for storms. Extreme low pressure areas generate high wind speeds, particularly during the traditionally-stormy season of autumn (the Octobers of 1987 and 2013 being two notable examples). The reason that storms are more frequent during the autumn is that the jet stream migrates southwards as northerly temperatures cool, and the increasing temperature difference increases the speed of the jet stream. Low-pressure systems tend to form over the Atlantic Ocean and move across the UK, with consequential impacts on the country’s infrastructure. According to the Association of British Insurers, the October 2013 storm could cost in the region of £130m, although the October 1987 storm cost the equivalent of £2bn at today’s prices1,2. Although wind speeds can vary according to terrain, either by height or by topography, the maximum recorded wind gusts are 228km/h (142mph) in Scotland, and 189km/h (118mph) in England, though the highest ever recorded wind gust was 278km/h (173mph) on the summit of Cairngorm in Scotland3. Research is still continuing to establish a link between climate change and extreme wind events, although BRE Digest 499 Designing roofs for climate change. Modifications to good practice guidance4 notes that “there is a higher degree of certainty that winter storms will increase but extreme events, such as the storm of 1987, may also occur more frequently”. NHBC NF3 Climate change and innovation in house building. Designing out risk states that “increases in winter rainfall and wind speeds are predicted and severe storms may occur more often”5. In addition, a recently-published study in the USA by Diffenbaugh, Scherer and Trapp has suggested that such a link could indeed exist, in North America at least6. Furthermore, design wind speeds have been doubled for certain building types and tripled for others since they were first published in British Standards in 19447. If no firmer evidence currently exists, then this at least suggests an increase in the intensity of wind speeds experienced in the UK in the last 70 years. Whether or not such a link between climate change and storms is found, however, the effects of severe winds can be catastrophic.
The UK is also prone to tornadoes and, perhaps surprisingly, it is claimed that there are more tornadoes per square kilometre than in the USA.
The UK is also prone to tornadoes and, perhaps surprisingly, it is claimed that there are more tornadoes per square kilometre than in the USA8. The most recent notable tornado struck Birmingham in 2005, reaching intensity T5 (T0=lowest; T10=highest) and causing £40m of damage along its 12km path, although the strongest recorded tornado in the UK was in Portsmouth in 1810, reaching T8 strength (wind speed 213–240mph)9.
Wind speeds of 90km/h (56mph) can impact on human safety and speeds of 126km/h (78mph) can cause damage to buildings, which in turn can endanger human life. Uprooted trees, loosened roof coverings and unsecured walls (particularly on construction sites) can all threaten health, cause extensive damage to property and disrupt services. Power transmission cables can be brought down, water supplies contaminated and transport services interrupted. High wind gusts can cause uplift on roofing due to the drop in air pressure caused by the moving air currents; coupled with rain, then even the under-layers can be vulnerable to damage. Temporary propping during construction works, particularly gable walls around roof level, if insubstantial can be insufficient to prevent toppling of freestanding masonry.
Siting and orientation of buildings at the earliest planning stages of a project can often go a long way to militate against the risk of wind damage. In addition, height, massing and roof form can also play a significant part. Angling roofs so that they slope down to face the prevailing wind direction can assist in reducing uplift-related problems, and taking advantage of adjacent shielding, whether in the form of natural landscapes or other buildings, can also be prudent. Excessive roof overhangs should be avoided, particularly on elevations facing the prevailing wind direction, and the proximity of nearby hazards should be evaluated, e.g. mature trees and overhead distribution cables.
Temporary works, such as gable or parapet walls at or around roof level, should be propped securely to the requirements of BS 5975:2008(+A1:2011) Code of practice for temporary works procedures and the permissible stress design of falsework10.
Consideration should be given to specifying single car-width garage doors in preference to double car-width – the increased size can enhance wind deflection and, in extreme cases, push the door out of the guide rails. As soon as the door has been blown in, the interior becomes more vulnerable and in particular the roof can be subjected to additional uplift action, thereby increasing the risk of the roof being damaged.
Roof coverings should be secured adequately and in accordance with the manufacturer’s instructions: nailing and clipping requirements for roofing tiles and slates, for example, to BS5534:2003(+A1:2010) Code of practice for slating and tiling (including shingles)11 and BS EN 14437:2004 Determination of the uplift resistance of clay or concrete tiles for roofing. Roof system test method12 for site exposure to wind and rain. Ridges and verge tiles are particularly important in this respect, as it is the edges of roofs which are the most susceptible to wind uplift. Similarly, BS EN 16002:2010 Flexible sheets for waterproofing. Determination of the resistance to wind load of mechanically fastened flexible sheets for roof waterproofing relates to flexible roofing sheet13 and restraint straps, resist wind uplift, are covered by BS 8103:2009 Structural design of low-rise buildings. Code of practice for timber floors and roofs for housing14.
In addition to BS EN 1991-1-4:2005+A1:2010 Eurocode 1: Actions on structures – Part 1.4: General actions – Wind actions15 and the UK National Annex to BS EN 1991-1-4:2005+A1:201016, Approved Document A of the Building Regulations covers design for wind loading. Diagram 6, and Diagram 7 Table c, contain the wind speed data for buildings, including a weighting for site slope and elevation. Interestingly, the maximum design wind speed is 25m/s or 90km/h (56mph), which as has been seen can be significantly exceeded in a storm event.
Flying debris can be an issue with glazing – consider avoiding large areas of glazing on elevations which face the prevailing wind direction (which in the UK is generally towards the west, and coincidentally would assist in reducing early evening summer solar gains).
Tall buildings which are sited in vulnerable positions, for example coastal regions, will need additional consideration given to claddings. Rainscreens will need to have durable fixings to prevent the wind from blowing panels off, as happened during the October 2013 storm in Ipswich where the town’s tallest building, situated on the waterfront, suffered extensive damage when a large quantity of the polystyrene-based façade cladding panels were blown off17.
Buildings which require continuity of services such as power, for instance financial institutions, secure facilities or healthcare buildings, will require the provision of services and other infrastructure which can be protected against interruptions to power supplies. Careful site design and back-up generators, etc. will minimise risks accordingly.
Regardless of personal opinion on the validity or otherwise of the theory of climate change, the fact remains that during extreme weather events buildings have a tendency to fail. This can be due to a number of factors, and under certain circumstances can be due to conflicting requirements on the building envelope. However, as has been identified, there are numerous systems and products available with which to combat weather and climate extremes. In order to assist in producing buildings that are better able to withstand the vagaries of the weather over their design lives, the NBS offers a number of subscription products from which systems and products may be specified to mitigate the effects of climate change.