The long awaited updated BBA Bulletin 4 has now been published, providing clear guidance on drainage of zero fall roofs and how to account for the rainwater cooling effect in U-value calculations.
The published document confirms our long-held view that a Water Flow Reducing Layer (WFRL), such as Ravatherm XPS X MK, has a crucial role in an inverted flat roof, and the importance of using a minimum moisture correction factor when calculating design lambda as detailed in BS EN ISO 17456:2007. Ravago Building Solutions participated in the compilation of this new bulletin alongside fellow members of the Liquid Roofing and Waterproofing Association (LRWA) on a sub-committee to advise the British Board of Agrément (BBA).
To learn more about the importance of the WFRL in an inverted flat roof, please access our RIBA accredited CPD module. This free learning resource discusses how a WFRL is essential to the thermal performance of an inverted flat roof through minimisation of heat loss due to the rainwater cooling effect. It also includes instruction on how to correctly perform a U-value calculation in line with the guidance contained in BBA Bulletin 4.