BKE 3
BUILD ING KNOWLEDGE EX C HANG E
Important changes to regulations for underlays for slating and tiling: BS5534: 2014 +A2 2018
To great fanfare, the code of practice for slating & tiling, BS5534, was recently amended with a wide number of changes over the outgoing version. The standard no longer accepts the use of, for example, mortar bedded ridges, as the strength of the mix cannot be guaranteed. The same restriction also applies to hips bedded on mortar and verge tiles. It is now commonplace to see dry fix solutions to these areas. The use of compliant battens is now also a requirement and a number of quality, graded batten options exist and are all accepted by the industry.
Why then, are underlays of variable wind uplift performance still seen on roofs across the country? The answer is quite simple: Most underlays DON’T meet the standard! When the entire standard requirement is read and calculated, the inadequacy of the commonly used zonal method becomes clear. The zonal method may be used if the conditions below are met:
Geographical wind zone
5 4 3 2 1
Design wind pressure, p for underlay (N/m2)
1600 1330 1150 975 820
To give some examples of an area where these conditions are NOT met, Birmingham, at its highest point, sits at 140m above sea level. Milton Keynes sits at 114m above sea level. The Sheffield area ranges between 29m & 548m above sea level. These figures will provide some idea of how relying on a fixed zonal method will mean that many buildings will be constructed in a manner that does not meet the standard.
A little understood area of BS5534 is the ceiling condition. Here the standard is very clear. Wind uplift resistance requirements increases depending on the ceiling (if installed) condition. For example, if an underlay is installed on a garage roof, in order to be classed as unrestricted, a figure of 2350 N/m2 is a starting point. Depending on all the other requirements, this figure could be higher still, requiring a far stronger membrane than the zonal method suggests: • • •
1600 N/m2 when a continuous ceiling is present 1900 N/m2 when no ceiling or a discontinuous ceiling is present
2350 N/m2 when no ceiling or a discontinuous ceiling is present and a permanent dominant opening is present on an external face of the building
The large white building, top left in figure 1 is a case in point. This building sits in zone 1 but as it’s within 300m of the city boundary, it is classed as rural and not urban. Its ridge height is in excess of 15m. It is constructed on a slope, near a large body of water and it may be assumed that a well-sealed (continuous) ceiling is not present. If you run these conditions through the DuPont™ Tyvek® BS5534, BRE approved wind uplift calculator (
www.tyvek.co.uk/wuc), a wind uplift resistance figure of 1634 N/m2 is recorded, a figure almost 100% higher than shown in the zonal table at 820 N/m2.
The lesson here is to be very wary of some of the claims made, such as ‘fully compliant’, ‘all zones suitability’ and ‘universal application’. Some products require that you fit a counter batten, which will alter all the eaves, ridge, verge & gable details, even on an unsupported, cold roof application. If these claims are not backed up by the independent test data, it is very easy to specify an underlay which will not meet the full requirements of BS5534 and will fail in service.
If the following conditions apply to the building and site concerned: • • • •
ridge height not greater than 15m roof pitch between 12.5° and 75° site altitude not greater than 100m
no significant site topography, ie. site should not be more than halfway up a hill or escarpment/cliff with more than 5% gradient
Site topography also includes site location relative to a ‘large body of water’ - this is not only coastal areas but inland waterways, rivers, canals and lakes. This item alone covers most cities, town and villages across the UK.
(Our thanks to Bradley Cameron, DuPont Protection Solutions for this BKE article)
DUPONT PROTECTION SOLUTIONS
TEL: 0844 406 8722 9
WWW.CONSTRUCTION.TYVEK.CO.UK
There are, however, tried and tested, BBA approved underlays which meets not only the full requirements of BS5534, but BS5250, Approved Documents I & C and technical standards across the UK. DuPont™ Tyvek® roofing underlays have been successfully installed across the UK for over 25 years and offer superior durability and performance and importantly, peace of mind to the homeowner, the roofing contractor and specifier.
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