Is non-combustible insulation required over Compartment walls?

We have written a few times in this column about the fire performance requirements within the Building Regulations, specifically Approved Document B Regulation 7 and some of the ‘grey’ areas within that are creating debate and confusion within the industry.

One such area is the debate about whether or not a combustible flat roof insulation board can be used over a party wall. The confusion comes from Requirement B3: Internal fire spread and specifically the diagrams in Section 8: Compartmentation/sprinklers.

Confused as to why a section of the regulations not directly related to roofing is being classed as a ‘grey’ area?

Starting with 5.11 in ADB V1 (8.25 in ADB V2) this section covers the Junction of compartment wall with roof, stating the internal fire stopping requirements of the junction between roof and wall. 5.12/8.26 states that for 1500mm on either side of the wall the roof should have a covering classified as Broof(t4) on a substrate or deck rated class A2-s3, d2 or better. 

5.13/8.27 refers to the use of class B-s3, d2 or worse materials, stating they can be used as a substrate to the roof covering and any timber tiling battens, fully bedded mortar or other suitable material for the width of the wall, stating a series of circumstances in which they can be used.

Beginning to see the confusion here? 5.12/8.26 is referring to the ‘covering’ being classified as Broof(t4), which is a roof system classification that does test the upper layer for both spread of flame and penetration but requires the testing of a complete roof build-up, whilst 5.13/8.27 is talking about pitched roof materials.

Three diagrams follow this text, each annotated as stating the roof covering to be designated as Broof(t4), but with diagram b having a statement that ‘Thermoplastic insulation materials should not be carried over the wall’, but, and it’s a big but for flat roofing, diagram b is annotated with a section x – x.  Section x – x that follows immediately clearly shows this to be a pitched roof construction not a flat roof.  

So, it is our belief that when MHCLG clarify Approved Document B it will clearly show that combustible insulation can be used in a roof or roof terrace application over a compartment wall provided the roofing system has achieved a Broof(t4) classification to B EN 13501-5.