Specifying Reinforced Bitumen Membranes (RBMs)

by Charles Stirling
NBS Technical Author

Although still widely cited for the specification of roofing underlays and membranes, the much loved BS 747 was withdrawn in July 2007. It has been replaced by BS EN 13707 and BS 8747. The European Standard BS EN 13707 includes for the wide range of products found throughout the Community; BS 8747 attempts to provide specific guidance for those membranes more typically specified within the UK.

BS EN 13707 specifies definitions and characteristics for some of the reinforced bitumen membranes previously referenced within BS 747 and typically intended for use as top sheets, intermediate layers or underlayers in built-up roofing systems. It does not cover those membranes for use as underlays below slating and tiling or the separating layers utilized below mastic asphalt, e.g. BS 747, Types 1 and 4.

In defining product characteristics, BS EN 13707 does not provide an indication of the required performance standards for specific end uses. Instead it provides recommendations for specific testing requirements; requiring the specifier to have a comprehensive knowledge of material properties; testing methods and service life. A document more suited to the needs of the manufacturing and supply industries than those of the specifier or end user.

In order to provide more practical guidance, BS 8747 has been produced to assist in the selection and specification of reinforced bitumen membranes tested in accordance with BS EN 13707. In particular, the recommendation of a performance based classification designed to specify a suitable membrane based on two critical parameters: tensile strength (S) and resistance to puncture or indentation (P). Each letter is assigned a numerical suffix which increases with the performance capacity of the membrane.

By way of comparison between BS 747 and BS EN 13707 characteristics, BS 8747 suggests the following:

BS 747, Type 3G membranes are not waterproof and are non-classifiable under this standard.

It is hoped that with increased use of the BS EN 13707/ BS 8747 classifications by manufacturers; roofers and specifiers that the 'SP' system will become as universally understood and utilized as the previous BS 747 designations.

Related NBS information:

Articles:

• Warm: cold; vented: unvented roof construction?
• Green roofs: an introduction
• Thatch roofing

Selected links:

• Roofs

Written March 2009

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