NOTE: The British Board of Agrément has issued an Information Bulletin (No. 2) relating to good site practice when using permeable roof tile underlay’s. This highlights:
An underlay is not a total waterproof barrier and if used as a temporary waterproof covering, some rain penetration may occur.
In certain conditions, particularly if there is persistent heavy rainfall combined with subsequent severe freeze/thaw conditions, an underlay should not be exposed for more than a few days.
In the UK, during mild weather conditions, a maximum exposure period of 3 months should be adhered to for Roofshield. In the winter months, or in cases of more extreme weather, this period should be reduced. Guidance on this is given by the BBA in an Information Bulletin on good site practice.
A full copy of this BBA Information Bulletin No.2 – Permeable Roof Tile Underlay Guide to Good Site Practice is available from the BBA web site, www.bbacerts.co.uk.