Published on 01 July 2021 09:46

Fire Safety Considerations When Specifying Insulation for an Inverted Flat Roof

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Key Learning outcomes
  • How Approved Document B applies to inverted flat roofs
  • Understanding of Broof(t4) classification and testing regime
  • The impact of EC Decision 2000/553/EC on testing requirements
  • The importance of selecting appropriate insulation material
Introduction
It is a legal requirement for buildings to be designed to prevent the spread of fire from one structure to another and consideration must be given to this while designing and specifying building materials for use in an inverted flat roof. There has been an increased focus on fire safety in residential buildings following the 2017 Grenfell Tower fire, where combustible materials used in the cladding of the tower are believed to have contributed to the spread of fire. However, this does not mean that only non-combustible materials can be used safely in an inverted flat roof.

1.0 Approved Document B and Broof(t4)

Approved Document B requirement B.4 (2) states “The roof of the building shall adequately resist the spread of fire over the roof and from one building to another, having regard to the use and position of the building.” Table 12.1 of Approved Document B Volume 1 Section 12: Resisting fire spread over roof coverings shows that only roof coverings that achieve the Broof(t4) classification can be used on any part of the roof. Such systems are defined as being “unrestricted with respect to proximity to a relevant boundary”, whereas coverings that meet lower classifications cannot be used within varying distances of the relevant boundary (see table).

Appendix B Sections B15 to B18 of Approved Document B Volume 1 and 2 describe how Broof(t4) is the highest performing classification for a roof system as defined in BS EN 13501-5:2016, assessing the reaction when exposed to external fire. To achieve this, fire cannot penetrate a roof system within 60 minutes and during preliminary testing there must be a flame spread of less than 0.38m across the region of burning.

The (t4) designates that the system has been tested in accordance with test 4 of DD CEN/TS 1187:2012, the most intensive test and the only one which meets UK fire safety requirements. This is a two-stage test using burning brands, wind and supplementary radiant heat to assess the spread of fire across the external surface of a roof, fire penetration and the production of flaming droplets or other debris.
Approved Document B outlines what fire classification is required within various distances of the relevant boundary
Approved Document B outlines what fire classification is required within various distances of the relevant boundary
 
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