Guidance for assessing wind resistance in breather membranes
It’s vital to ensure that a breather membrane has the necessary wind resistance for the local conditions of the building.
BS 5534, the code of practice for installing membranes, outlines 5 wind zones across the UK, from the lowest wind speed, Zone 1 (centred around London and the south of England) through to Zone 5 covering the very far north of Scotland. The BBA provides guidance and data on the ability of membranes to resist wind loads, outlining the suitable wind zones for each product under BS 5534.
The BBA’s data is regularly updated, so it’s important to refer to this resource for each new project specification. Some low-cost products have recently been downgraded and no longer meet the requirements for certain wind zones. If you have used a product which was compliant for past projects, make sure the data provided at the time is still applicable for your next project.
For example, the below table shows the current wind resistance for ITP’s Powerlon Ultraperm membrane in comparison to the recently downgraded ratings of a competitor product.
BS 5534 compliance (data taken from BBA website)
| 250mm batten width 345mm batten width | 250mm batten width 345mm batten width | |
|---|---|---|
| UltraPerm Lite 92gsm Low-cost competitor product 95gsm |
Zones 1-4 Zones 1-5 |
Zone 1 Zone 1 |
| UltraPerm Standard 115gsm Low-cost competitor product 115gsm |
Zones 1-5 Zones 1-5 |
Zones 1-2 Zone 1 (previously zone 1 and 2) |
| UltraPerm Premium 135gsm Low-cost competitor product 135gsm |
Zones 1-5 Zones 1-5 |
Zones 1-3 Zone 1 (previously Zones 1, 2 and 3) |
| UltraPerm Max 155gsm Low-cost competitor product 165gsm |
Zones 1-5 Zones 1-5 |
Zones 1-4 Zones 1-2 (previously Zones 1,2,3 and 4) |
It is essential to carry out a reliable assessment of the ability of external wall systems to resist wind load and prevent installation failure. That requires reference to the latest data for any given product. When specifying a breather membrane, ensure that every supplier submits full up-to-date performance statistics, particularly in relation to its suitability for the wind zones outlined under BS 5534.
Always check BBA certificates on the BBA website to ensure that they are still valid with the correct and up-to-date properties and wind zone resistance data. You can find the BBA website at bbacerts.co.uk
