WHAT IS BBA BULLETIN NO. 4?

BBA Information Bulletin No.4 is a technical guidance document issued by the British Board of Agrément. Its content describes two areas of inverted roof design and specification – the drainage of inverted roofs, and how to accurately calculate a rainwater cooling correction factor for U-value calculations.

The document is referenced in BS 6229:2018 Flat roofs with continuously supported flexible waterproof coverings. Code of practice. The recommendations and guidance of a British Standard code of practice carry weight, meaning the information bulletin does too. In the case of BS 6229, it was developed by the flat roofing sector, for the flat roofing sector.

What does the BBA Bulletin say?

Achieving the correct falls on flat roofs to ensure they drain correctly is a longstanding issue and one that applies to all roof types, not just inverted roofs. BBA Bulletin No.4 was one of the first pieces of official guidance on zero falls, a topic that features strongly in BS 6229:2018. Further guidance on the topic is available in LRWA Guidance Notes 14 and 15, available here.

BBA Bulletin No.4 also sets out industry-standard guidance on the correct application of rainwater cooling correction factors in U-value calculations for inverted roofs. It describes the values that should be used, including rainfall volume and drainage factor, and emphasises that a water flow reducing layer (WFRL) membrane is not a waterproof layer.

Polyfoam XPS keep abreast of all industry guidance and ensure that our U-value calculations are always carried out in accordance with best practice. Contact us with any technical support enquiries or to discuss the requirements of your project, and find out how our products Roofboard Extra, Roofboard Super and Slimline Zero membrane can help you.