A key part of this year’s revision to the BS 5534 code of practice for installing tiles and slates on pitched roofs (BS 5334:2014+A2:2018) is to improve their weathertightness and durability, including the underlay.
The UK’s wind zones
To ensure that the underlay is securely fixed, contractors need to pay attention to the UK’s wind zones, which indicate the strength of wind in different parts of the country. BS 5534 includes a map of the five different zones along with recommendations.
In order to comply, the following criteria must be considered:
- Is the site more than 100 m above sea level?
- Is the ridge height more than 15 m?
- Is the ceiling condition, continuous, non-continuous or is there none?
- Is the pitch between 12.5 and 75 degrees?
- Is the site on a slope or large body of water?
However, according to DuPont, which manufactures Tyvek® building membrane, the zonal method does not take into account the entire standard.
It is very common to see claims that if an underlay can resist 1600Pa of pressure, an underlay can be used anywhere, which this is incorrect. For example, the standard states that if the ceiling is non-continuous, a wind uplift resistance figure of at least 1900 Pa is required and if there is no ceiling and a dominant opening, a minimum figure of 2350 Pa is demanded.
Introducing Tyvek® Wind Uplift Calculator
To help installers to calculate wind uplift, DuPont has launched a BRE-calibrated, online tool to ensure total compliance with BS 5534.
Free to use and available at tyvek.construction.dupont.com/wuc, the calculator uses a series of dropdown menus, allowing the end user to select the various options that BS 5534 is based upon and therefore calculate the actual wind uplift resistance figure required for specific, postcode based, location of the site.
The online tutorial provides an example of a site in Bristol (Zone 1 and 820 Pa wind uplift resistance) and by using the tool provides a BRE calibrated figure, in excess of 1600 Pa, which means that had the zonal method been applied, an underlay with only half the wind uplift resistance required may have been selected.
For more information on the Tyvek® Wind Uplift calculator, please contact the Tyvek® Building Knowledge Centre on 08444 068722 or email firstname.lastname@example.org.