New fire performance updates for flat roofs

New fire performance updates for flat roofs

On the 30th August 2019 the new edition of 'Fire Safety-Approved Document B' (ADB) came into force and with it there are a number of changes that affect flat roofing. Gone is reference to the National Classification system set out in BS 476-3:2004 as the principle determinant of external fire performance for roofs (with exception of a listing in Appendix B Table B2 that sets out the old classification system for historic projects).

This means that the European Classification system set out in BS EN 13501-5, that has run alongside the National Classification system for many years, is now the main reference for external fire performance of roofs in the UK (Scotland still refers to both). This classification still covers External Fire Penetration and Spread of Flame of roof systems in their response to fire from outside the building, not to be confused with individual comprising components tested for reaction to fire to BS EN 13501-1. The test is not concerned with the behaviour of roofs when subjected to the effects of fire from the underside, i.e. from within the building.

European Fire Standards

To advance and clarify the fire standards across Europe for External Fire Performance, the roofing industry has been going through a changeover from the test and classification of BS 476-3:2004 to the tests of TS 1187 (planned to become BS EN 1187 in the next few years) and classification using BS EN 13501-5, the culmination of which has now come in the new ADB 2019 edition. The standardisation into one European test proved indefinable as many countries within the EU had differing regulations and so four test standards were required to cover the legislation in place within the various member states at the time. As such, the TS 1187 has four tests for roof covering systems: t1 for Germany, t2 for Scandinavia, t3 for France and t4 for the UK (and used in the Republic of Ireland). The results from testing under TS 1187 with BS EN 13501-5 classification are given as European Class ratings BROOF(t4), CROOF(t4), DROOF(t4), EROOF(t4) and FROOF(t4). The easiest way of explaining how these class ratings are achieved is to relate back to the BS 476-3 classification system.
National class European class Minimum distance from any point on relevant boundary (England) Minimum distance from any point on relevant boundary (Scotland)
AA, AB or AC BROOF(t4) Unrestricted and can be used anywhere on the roof Low Vulnerability (<6m)
BA, BB or BC CROOF(t4) At least 6m of the boundary Medium Vulnerability (6-24m) 
CA, CB or CC DROOF(t4) At least 6,12 or 20m of the boundary depending on the building type and use  Medium Vulnerability (6-24m) 
AD, BD or CD  EROOF(t4) At least 6,12 or 20m of the boundary depending on the building type and use  High Vulnerability (>24m)  
DA, DB, DC or DD  FROOF(t4) At least 20m of the boundary depending on the building type and use High Vulnerability (>24m) 
For TS 1187 Test 4 it was recognised that Approved Document B focused upon saving human lives rather than protecting the building itself and therefore as previously set to BS476-3, the highest requirement was A for Penetration - not penetrated within one hour, and C for Spread of Flame - more than 533mm. This meant that Test 4 needed to replicate the penetration test, but did not require the spread of flame full test because the preliminary test allowed the tester to determine if classification D or better was achieved. If the preliminary test specimen did not burn for more than five minutes after removal of the test flame, and spread of flame was not more than 381mm, then class C is achieved. That is why Test 4 just requires the preliminary test and penetration test.
First Letter - Fire Penetration Classification (BS476-3)
A: Those specimens which have not been penetrated within one hour.
B: Those specimens which are penetrated in not less than 30 minutes.
C: Those specimens which are penetrated in less than 30 minutes.
D: Those specimens which are penetrated in the preliminary flame test.
Second Letter - Spread of Flame Classification (BS476-3)
A: Those specimens on which there is no spread of flame.
B: Those specimens on which there is not more than 533mm, spread of flame.
C: Those specimens on which there is more than 533mm, spread of flame.
D: Those specimens which continue to burn for five minutes after the removal of the test flame or with spread of flame more than 381mm, in the preliminary test.

It should also be recognised that the classification is for all types of roof system, but with virtually all commonly used flat roof build-ups the aim is to achieve BROOF(t4) when tested. This is why it is often forgotten that the classification is required to determine how close the roof is to the boundary and therefore adjacent buildings. The advantage of using systems achieving BROOF(t4) is that there is no minimum distance required between adjacent buildings and this result is often referred to as 'unrestricted' or in Scotland 'low vulnerability'. The worse the external fire classification the further away any adjacent buildings need to be as shown in the table.

However, to meet the Building Regulations for fire you need to prove that the 'as installed' system has a valid test certificate. For this reason, late product substitution has a risk of not complying with the Building Regulations as there are hundreds of system permutations and so less common build-ups are unlikely to have been tested. With the aim of reducing the huge number of permutations safely, the Flat Roofing Industry trade bodies (SPRA, LRWA & NFRC) working with BRE have established through fire testing that testing on timber decking is the worst case scenario, thus removing the need for testing on steel or concrete decks.

In roofs that incorporate non-combustible surface finishes as set out in European Commission Directive 2000/553/EC such as, minimum 50mm thickness of stone ballast; or minimum 40mm thick stone; or concrete paving slabs; or minimum 30mm thick sand and cement screed that fully cover the roof, these are deemed to fully satisfy the regulations to BROOF(t4) with no testing. The national designations in the previous Approved  Document B in Appendix A Table A5 for 'Flat roofs covered with bitumen felt' have been removed which means reinforced bitumen membranes with bitumen-bedded chippings or non-combustible tiles on the surface do need to be tested.

Green Roofs

Approved Document B references 'Fire Performance of Green Roofs and Walls' published by the former Department of Communities and Local Government (DCLG) in 2013 and this document is the basis of the Green Roof Organisation (GRO) “Fire Risk Guidance Document”. In summary, the growing medium should be certified for use on green roofs and, where there is no permanent irrigation, organic content should be <20% and peat free. Fire Breaks 300mm wide should be a minimum of 50mm thick 20-40mm rounded pebbles; or a minimum of 40mm thick concrete; or stone paving slabs around all perimeters and 500mm wide if adjacent to an opening window, rooflight or similar that could allow fire to enter the building, with a 1m wide Fire Break across the roof every 40m. Maintenance is very important to prevent vegetation growing over Fire Breaks and to remove wildflower dry thatch in the Autumn.

NHBC follows this guidance but also has its own additional guidance for escape routes in landscaped roofs which it describes as “Protected Walkways” to give pedestrians protection from fire on an adjacent area. They should be a minimum of 1.8m wide and the roof build-up, including walkway finish, should achieve BROOF(t4) or is “classified without further test” (CWFT) as BROOF(t4). However, any void formed under the protected walkway should be filled with a suitable granular fill e.g. 20-40mm stones, whilst maintaining drainage. 

Roof Insulation Fire Performance

Before the latest amendments to ADB, the key fire consideration for flat roofing was Requirement B4 which covered the resistance to fire from the outside of a roof system, where achieving F.AC (to BS476-3) in the past and now BROOF(t4), gave unrestricted distance to the boundary of the building. There had previously never been a limitation on the reaction to fire of any component of that roof build-up. It could be argued that there still is no limitation on Reaction to Fire if you accept that a balcony is not a roof, because all the common roof waterproofing membranes achieve Class E and changing the insulation will not affect the typical BROOF(t4) classification for the system.

Individual construction products are covered within BS EN 13501-1 for which insulation as a separate component will be encompassed and allocated a Class according to their reaction to fire test results with letter classifications from ‘A1’ through to ‘F’.  On the whole, materials manufactured in whole or in part from plastics will achieve an ‘E’ rating, which will include for inverted roofs the insulants Expanded Polystyrene (EPS), Extruded Polystyrene (XPS) and for Warm Roofs - Polyisocyanurate (PIR).

Non-combustible insulants, such as cellular glass (BauderGLAS) along with mineral wool (BauderROCK), are clearly desirable materials to include in a flat roof specification because of fire performance, and it is important to consider and balance the factors for inclusion within a roof system. In general, non-combustible insulants are not as thermally efficient as PIR insulation and therefore extra thicknesses, increased weight and reduced compressive strength can be a limitation in some applications. PIR has the advantage of being highly thermally efficient, which reduces the height and weight of a roof covering build-up whilst also offering good compressive strength meaning greater versatility on a project. Within a Bauder warm roof waterproofing system the insulation, be it mineral wool, cellular glass or PIR, is not directly exposed and is therefore determined by the performance of the cap sheet and its system classification; thus these insulants all conform to Building Regulations in the same way - not one achieving a higher rating than the other.

Select grades of Bauder PIR insulation have also been tested and approved by fire experts Factory Mutual (FM), whose principle global business is the insurance of buildings and loss prevention.  FM currently offers PIR insulation within a flat roof just the same as they would mineral wool without an exclusion or premium adjustment to the building owner provided that they are installed as part of a FM Approved Assembly, i.e. a stated system configuration.  Bauder PIR, as with all other products with FM Approval, are under regular surveillance by the insurer to confirm performance on buildings.

With the exception of a balcony, where if insulated it will need to be non-combustible, inverted roof constructions also generally use plastic-based insulants such as EPS and XPS, though these are only used when they are fully covered with paving slabs, stones, etc and are therefore often deemed to meet Building Regulations without testing (CWFT as defined in European Commission Directive 2000/553/EC).

New requirement of Approved Document B - External Walls and 'Specified Attachment'

In late 2018 Approved Document B was amended to incorporate in Requirement B4 Regulation 7— the Government requirement to ban combustible materials as part of the external wall in buildings containing dwellings or institutions. These “Relevant Buildings” include student accommodation, care homes, sheltered housing, hospitals and school dormitories in buildings where there is a storey at least 18m above ground level. It should be noted that there was a Public Consultation in early 2020 on whether, like Scotland, the applicable height should be reduced to 11m and whether the scope of “Relevant Buildings” should be extended. Regulation 7 also introduced a new term 'Specified Attachment' which was included in the ban of combustible materials along with other parts of an external wall. The definition includes a balcony 'attached' to an external wall. Notable exclusions to the ban are membranes, thermal break materials and any part of a roof (except habited mansards with a slope greater than 70°).

This new term has caused much misunderstanding with what the definition of a balcony is, and it appears to contradict the European Commission Directive 2000/553/EC and Regulation 7(3) if the definition of a balcony is deemed to include an insulated roof. The flat roofing industry has worked with the NHBC to come to a common understanding which also mirrors the new BS8579 “Guide to the Design of Balconies and Terraces”. Attached balconies are differentiated from roof terraces in that they are not over habited conditioned spaces and are usually bolted to, or cantilevering from, the external wall. This also includes most inset balconies. So for the purposes of Approved Document B, balconies are not deemed to be roofs.

For areas established correctly as balconies on “Relevant Buildings”, Regulation 7(2) states that non-combustible materials should be used. However, the waterproofing membrane is excluded from this by Regulation 7(3)(g). Further, ADB B4 Section 10.15 (Vol 1)/12.16 (Vol 2), contrary to many interpretations, does not mention or include for “Specified Attachments”. Therefore, a typical roof waterproofing membrane can be used on balconies.
There is another issue that has been highlighted by the new ADB that has existed for years and now needs to be clarified – “When is a wall a roof?”. The area at issue is where the flat roof waterproofing abuts a perimeter or roof penetration and is traditionally dressed a minimum 150mm above the finished roof level or walking surface (as set out in BS6229).

The definition of a roof is 0-70 degrees and above this it is a wall, yet 7(3) excludes any part of a roof connected to an external wall. In a letter from the Ministry of Housing, Communities and Local Government (MHCLG) to the Flat Roofing Industry (in November 2019) it was acknowledged that the part of the roof dressed up the wall is exempt but with no clarification on the extent i.e. max height. Again, the Flat Roofing Industry working with the NHBC have developed some guidelines which also deal with the supplemental queries about the insulation often used at these points. Insulation on the face of the wall/abutment is usually a thermal break and subsequently exempt from the ban. Therefore, for the area up to 150mm above the finished roof level or walking surface, it can be combustible insulation provided that it is no thicker than 60mm (deemed sufficient thickness for a thermal break) and does not span across a compartment line. For heights above this, the insulation is recommended to be non-combustible and, for the NHBC, typically limited to approximately 1100mm for parapets and lift/stair overruns.

Bauder waterproofing systems

Bauder tested exposed membrane waterproofing systems achieve the highest classification for external fire testing in compliance with Approved Document B and are classed as 'unrestricted'  or 'low vulnerability' due to the fire retardant in the top layer and the mineral chippings on the reinforced bitumen membrane’s surface.
Bauder ballasted membrane waterproofing systems generally achieve compliance with Approved Document B due to the European Commission Directive 2000/553/EC “classification without further testing” (CWFT) rules; or the DCLG document “Fire Performance of Green Walls and Roofs”; or a combination of these and including the exposed membrane testing.

Bauder has a continuous fire testing programme where all the most common Bauder system build-ups have been recently tested, including thick and thin insulation, which means that all thicknesses in between are covered in the same approval to give the most complete cover for certification. Having only one test thickness of insulation is only valid for that thickness and no other because variance in thicknesses can change the test result, just as using generic insulation types is not acceptable as there can be differences due to the core or facings - ‘as built’ should match ‘as tested’. Our BBA certification for fire testing was up to date at publication, but results have been extended by our continuous testing.

This guidance document is principally about the English situation based upon Approved Document B, but the Welsh Part B substantially follows that of the English one and, as yet, there has been no change of the Northern Ireland Technical Booklet E. There are more notable differences with the Scottish Section 2 requirements, some of which have been noted in this guidance.

To ensure our guidance is current,  Bauder has participated in the relevant BSI Technical Committees on roof fire testing and the guide to design of balconies and terraces; has been in proactive dialogue with MHCLG on ADB; and has worked collaboratively with the NHBC to agree common guidelines. If there is some area covered in this document that you require more guidance with, please do not hesitate to ask as it is an ever-evolving topic.


  •  Bauder waterproofing systems are tested to TS 1187 Test 4.
  • Bauder tested exposed roof membrane waterproofing systems achieve the highest classification for external fire testing in compliance with Approved Document B 2019 and are classed as 'unrestricted' or 'low vulnerability.'
  • Bauder ballasted membrane waterproofing systems generally achieve compliance with Approved Document B without testing.
  • ‘As built’ should match ‘as tested’.
  • A balcony is not a roof for the purposes of Approved Document B.
  • The roof/wall thermal break is excluded from the ban.

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