Clear Guidance on BS 5534A

There is a considerable amount of misinformation in the market place concerning the use of breather membranes (RTU’s: Roof Tile Underlays) in relation to the new amendments to wind pressure requirements of BS 5534 annex A. We hope this information below will help you clarify the legal requirements of how to calculate compliance to the new regulations.

These amendments are currently in place and will be fully enforced as of July 1st 2015. To coincide with this we have also written a guide analysing why these standards have been updated and explaining the issues that must be considered, i.e. the 5 wind zones and testing; standards that must be met; factors for normal conditions; and the benefits of taped laps. The full article can be found here (published in June 2015 issue of RCI Magazine).

The implications of these standards are if a membrane’s specification meets the pressure requirements of a specific wind zone, then you have complied with the requirements.

However, this is often NOT the case. For example: Although a project might be shown as being situated within Zone 2 (975 pa) on the UK Wind Pressure Chart Map, any work in a coastal location effectively puts you in Zone 3 (1150 pa).


If this location is exposed then you would need to raise this standard to (1302 pa) and add to that additional calculations for altitude (add approx. 20% for every 100m above 100m) or tall buildings then you are quickly in Zone 4 (1330 pa), Zone 5 (1600 pa) and above.
In addition, many designers do not consider that although a house might comply with the requirements of Zone 2 (975 pa) the garage has a minimum requirement of 1900 pa, almost double. It is not possible for a standard membrane to be used in these circumstances.
Also there are very large increases when the site is located on the side of a hill. For example: Sheffield is situated at an altitude of 300m, which therefore cannot be described as being in Zone 2 even if it appears so on the map.

The only safe way to navigate these standards is to use a membrane that exceeds the maximum requirement of the 2350 pa upper limit.


Permavent APeX 2XT (2XT = with tape) is an ideal solution for this and comes with contact tape already attached. It is an air permeable membrane that is approved for use under the NHBC warranty without high level ventilation and can therefore be used in all applications without the need to worry or calculate compliance.

All Permavent membranes can incorporate our amazing contact tape top and bottom. Our taped membranes are very easy to use, have a zero impact on labour costs, work in any conditions and never fail. I would encourage you to reply to this email and we will send you a free sample for you to try for yourself, simply remove the film and let the ends come together, no pressure is required and the membrane is welded.

Please check our website where all Permavent membranes results can be found as well as the BS Zonal map, coastal amendment chart and comprehensive notes on the requirements.


For a sample pack or if you like to discuss this further then please get in touch, we can help you.




Following the success of the Queen’s award winning Easy Verge, here at Permavent we are proud to announce the launch of the recently developed Easy VergeTrim. This latest addition to the Easy Roof System has been specially designed to meet the requirements of the Scottish and refurbishment markets.

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