Twenty years ago the UKs top performing vapour permeable underlay, Roofshield, was not even launched. In those days we were working under the Code of Practice BS 5534 1990 Part 1(25 pages - now 173 pages) and if you were looking for a roofing underlay you had the choice of traditional bitumen felt, single layer non-woven polypropylene membranes, Flash spunbond or reinforced polyethylene’s with micro perforations.
The only vapour permeability requirements were in fully supported applications like boarding and required to have a vapour permeability of 36g/m2/day in accordance with BS3177. For underlays not fully supported the recommendations were to use BS747 bitumen felt and if using a polyethylene film of 0.13mm the batten gauge should not exceed 100mm.
20 years on and the new BS 5534
Fast forward to 2015 and vapour permeable membranes are now used on most projects in the UK. Over this time, vapour permeable underlays (now typically with MVTRs greater than 1000g/m2/day) have allowed ventilation requirements for roofs to be reduced to ridge-only, or in the case of a high performance air permeable membrane such as Roofshield, eliminated altogether.
Alongside this evolution in the roofing technology, the climatic factors affecting roof design have also changed, with extreme weather conditions occurring more frequently than in the past. The increasing likelihood of violent storm conditions across the UK has demanded a more rigorous approach to roof design, amid a growing recognition that the old principles are no longer fit for purpose.
The recently published BS5534 effective from the end of February 2015 reflects this new reality with far reaching and more rigorous design standards for both the fixing of roof coverings and the robustness of underlays of all types. Under the new standard, traditional methods such as mortar bedding of ridges and hips can no longer be relied upon to resist wind loadings, therefore designers and roofers will need to familiarise themselves with the new requirements, and seek updated fixing schedules from tile and slate suppliers.
For the underlay classification the country will be split into 5 wind zones as shown on map pic above:
Wind zones 1 and 2 comprise the majority of England and Wales, wind zone 3 the Border areas and major population centres in Scotland and Northern Ireland, with the Highlands and Islands making up wind zones 4 and 5. Underlay manufacturers must now clearly state in which zones their underlays are suitable, and if any special installation conditions apply in a given zone. These specifics may include, for example, taping of lap joints, variations in batten gauge or reduced batten spacing in higher wind load zones.
The new changes to BS 5534 should be embraced by the industry and recognised as a raising of standards and producing less problems on roofs due to wind damage.
Based on fully independent 3rd party testing, Roofshield will continue to demonstrate its performance by being compliant throughout the whole of the UK.
For zones 1-3, no special measures are required for open rafter and fully supported applications, beyond ensuring laps are extended (if necessary) to coincide with slate or tile battens. For zones 4 and 5, designers and roofers should consult the A Proctor Group to review the specific application, however for the majority of fully supported applications (such as onto timber sarking, commonplace in these geographic areas) no special measures will be required. If special measures are required, a detailed specification will be issued giving the options for compliant installation.
While significant changes to the long established standard are a source of some upheaval and understandable concern to the construction industry, embracing these changes can only lead to the raising of standards, reducing the likelihood of subsequent problems and expensive remediation.
By continually improving our product ranges to reflect the changing criteria and scope of these standards, the A. Proctor Group aim to make the transition as painless as possible for our customers.
For further information on Roofshield, and the implications of the new BS5534, please visit www.proctorgroup.com.