By Joan Ferrer, Technical Sales Manager, Ravago Building Solutions
Since the tragic fire at Grenfell Tower in 2017, there has been an increased focus on fire safety in residential buildings and building material manufacturers have found themselves under the spotlight during the Independent Review of Building Regulations and Fire Safety led by Dame Judith Hackitt and the subsequent ongoing public inquiry. Current thinking suggests that the combustible materials used in the cladding of the tower contributed to the spread of fire, with air gaps and the lack of efficient fire breaks creating a chimney-effect, sucking the flames up the building.
The Building (Amendment) Regulations, SI 2018/1230 [1], formulated in response to Dame Judith’s review, banned the use of combustible materials in external walls and specified attachments of relevant buildings (predominantly residential) taller than 18 metres and the Government is now considering whether this should also apply to buildings 11 metres or greater in height. However, it would be a mistake to assume that only non-combustible materials can be safely used when specifying insulation for an inverted flat roof.
Background to fire safety requirements
As you know, it is a legal requirement for buildings to be designed to prevent the spread of fire from one building to another and particular attention is required in roofing design. Approved Document B [2]requirement B.4 offers practical advice on how to ensure compliance with the fire safety aspect of the latest Building Regulations, with a heavy emphasis on the Broof(t4) classification.
Broof(t4) is the highest performing classification for a roof system, assessing their reaction when exposed to external fire. The requirements include no penetration of a roof system within 60 minutes and flame spread of less than 0.38m across the region of burning during the preliminary test. Roofing systems which comply with the Broof(t4) classification can be used anywhere on the roof, while those of lower-performing classifications cannot be used within varying distances of the relevant boundary as defined by Approved Document B.
The (t4) designates that the system has been tested in accordance with test 4 of ENV 1187 [3]. Test 4 is the most intensive of the tests in this European Specification, and the only one which meets the stringent UK fire safety requirements. It entails 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. BS EN 13501-5:2016[4] outlines the fire classification levels derived from the fire tests.
In addition, roof covering products (and/or materials) defined in European Commission Decision 2000/553/EC [5] of 6 September 2000, implementing European Council Directive 89/106/EEC [6], can be considered to fulfil all the requirements for the performance characteristic ‘external fire performance’ without the need for testing, provided that any national provisions on the design and execution of works are fulfilled, and can be used without restriction.
The Annex of European Commission Decision 2000/553/EC explicitly states that roof systems that are fully covered by non-combustible coverings are deemed to meet these external fire performance requirements, without testing. It states, “The external fire performance of many roof covering products/materials is well established and sufficiently well known to fire regulators in the Member States that they do not require testing for this particular performance characteristic.” As a consequence, the fire properties of insulation used in an inverted roof have no relevance to fire safety as the material is wholly contained within non-combustible material – there is no pathway along which flames can spread due to protection below from the concrete deck and the gravel ballast or paving slabs above.
While European regulations do not provide clarity on green roofs, assurance of compliance is provided by a guidance document issued by the Department for Communities and Local Government, the predecessor department to the Ministry of Housing, Communities and Local Government. Fire Performance of Green Roofs and Walls [7], published in August 2013, states that “In order for green roofs to comply with requirement B4 it is recommended that for all types of green roof the depth of the growing layer should be a minimum of 80mm and the organic content should not exceed 50%.“
Given the increased emphasis on fire safety and out of an abundance of caution, some systems which use Ravago Building Solution’s XPS products have now been tested and as expected achieved the Broof(t4) classification with little difficulty. The exemption from the testing requirements exists for a reason – it’s simply not necessary to test what we already know from decades of safe usage.
It is highly unlikely that the need to achieve Broof(t4) classification as currently outlined in Requirement B.4 in Approved Document B will change as a result of new Building Regulations or as a result of any potential post-Brexit review of guidance. While there may be further changes to fire safety regulations relating to cladding there simply isn’t the evidence to justify a shift from safe, tried and trusted solutions for inverted flat roofs.
Practical application in inverted flat roofs
BS 6229:2018 [8] describes best practice for the design, construction and maintenance of flat roofs, based on accepted interpretations of the relevant Approved Documents and widely adopted industry best practice. This includes both Approved Document B and Approved Document L[9] which addresses thermal performance.
It is concerning to see cases where stonewool or cellular glass insulation is being promoted for inverted roof applications, largely off the back of unsubstantiated fears about fire safety. There are serious questions to be asked about the long-term thermal performance of these materials in an inverted roof system due to water absorption. XPS has a proven track record of high performance based on decades of use and the use of rigorous water absorption tests such as immersion, diffusion and freeze-thaw. None of the above-mentioned insulation products has been tested to the same standards and none of them is included as recommended insulation within ETAG 031 Guideline for European Technical Approval of Inverted Roof Insulation Kits [10]. When it comes to U-value calculations, what correction factors are you going to use for a product which has never been used before in an inverted roof? The recently revised BBA Bulletin 4 Inverted Roofs – Drainage and U value corrections [11] is abundantly clear that these correction factors are essential to an accurate calculation.
There are clear reasons why XPS should continue to be your material of choice for inverted roof applications. It is a tried and tested solution with proven exceptional long-term thermal performance and the high compressive strength of the boards offers incredible flexibility for roof use, allowing for pedestrian terraces, car parking and green or blue roof systems. Specify XPS with confidence in the safety and performance of your building.
First published in Roofing Cladding & Insulation (RCI) Magazine, 12th April 2021
[1] https://www.legislation.gov.uk/uksi/2018/1230/made
[2] https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/937931/ADB_Vol1_ Dwellings_2019_edition_inc_2020_amendments.pdf
[3] https://eur-lex.europa.eu/legal-content/EN/TXT/HTML/?uri=CELEX:02001D0671-20051125&qid=1612193275450&from=EN
[4] https://shop.bsigroup.com/ProductDetail?pid=000000000030302471
[5] https://eur-lex.europa.eu/legal-content/EN/TXT/PDF/?uri=CELEX:32000D0553&from=EN
[6] https://eur-lex.europa.eu/legal-content/EN/TXT/HTML/?uri=CELEX:31989L0106&from=en
[7] https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/230510/130819_SW3529R_-_Issue_3_-_Green_Roofs_and_Walls_Project_web_version_v3.pdf
[8] https://shop.bsigroup.com/ProductDetail?pid=000000000030371977
[9] https://www.gov.uk/government/publications/conservation-of-fuel-and-power-approved-document-l
[10] https://www.eota.eu/handlers/download.ashx?filename=endorsed-etags%5cetag031%2fetag31-part1-en.pdf
[11] https://www.bbacerts.co.uk/wp-content/uploads/2020/11/Datasheet_Bulletin_No4i8_2020_Final.pdf