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Fire performance of Green, Blue and BioSolar roofs

by Matt Downs

By Mark Harris, Head of Technical and Operations at Radmat Building Products, and Chair of GRO.

The Building Regulations set the Statutory (legal) minimum requirements for the construction of buildings in the United Kingdom, establishing minimum performance requirements to prevent the loss of life.

Mark Harris, Head of Technical and Operations at Radmat Building Products, and Chair of GRO.

In England, Wales and Northern Ireland, the Approved Documents provide guidance on how to comply with these legal requirements, providing individual documents for Structure, Fire Safety, Fall Protection, Access and Use, Thermal performance, Overheating, Glazing, Electrical safety, Site Preparation, Toxic Substances, Ventilation, Sanitation, Drainage and Heat & Fuel, as well as the passage of sound. In Scotland this guidance is provided through Technical Handbooks.

Following the tragedy of the Grenfell Tower fire on 14th June 2017, the Building Regulations have seen extensive review and update, particularly with regards to the fire performance of external walls on buildings over 18m (11m in Scotland) that contain dwellings, with a requirement for the external walls to be constructed of materials that are either ‘non-combustible’ or ‘very limited combustibility’ (fire class A1 or A2 to BS EN 13501-1). The same rules apply to balconies on such buildings.

The fire performance requirement for roofs has not changed since the introduction of classifications to BS EN 13501-5 into the Approved Document B in 2006, classifications that relate to the BS 476-3 classifications that have been in the Approved Document since it was introduced in 1965.

To achieve a fire performance classification to either BS EN 13501-5 or BS 476-3, testing of a roof construction has to be carried out to ENV 1187 by an approved fire testing facility. The resulting classification report is specific to the exact construction tested, so, for example, if you carry out a test that contains 50mm of PIR insulation, that test is not relevant to 150mm of PIR insulation or any other thickness. The same is true for any change to an AVCL, adhesive or waterproofing membrane change.

What happens when a Green, Blue or BioSolar roof is included? 

Demonstrating compliance with the Building Regulations requirements for the fire performance of a roof becomes even more challenging when you have a Green, Blue or BioSolar roof, because the approved fire test facilities can only test a maximum roof build-up thickness of 250mm or 265mm depending on test provider. Both warm and inverted roof constructions including Green, Blue or BioSolar roofs can easily exceed this thickness.

Fortunately, Approved Document B references a Department of Communities and Local Government (DCLG) document published in August 2013. This document, ‘Fire Performance of Green Roofs and Walls’, provides best practice guidance for demonstrating compliance with the requirements of the Building Regulations without the need for test by setting ‘Deemed to Satisfy’ rules.

Section 4.5.2 of DCLG ‘Fire Performance of Green Roofs and Walls’ 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%.’

Fire breaks

DCLG ‘Fire Performance of Green Roofs and Walls’ also provides guidance on the use of fire breaks to support the achievement of compliance with the Building Regulations, recommending the installation of fire breaks to prevent the spread of fire into, or from, a building on to the Green roof. The document recommends that fire breaks should be installed around all openings (rooflights, pipes or vents), and vertical elements on all types of Green roofs ‘consist of paving slabs or non-vegetated strips of pebbles with a depth of 75mm and diameter between 20 and 50mm for a width of 500mm’. It goes on to state that ‘It may be possible to reduce the dimension of 500mm depending on the type of vegetation used and the climatic conditions, however further investigation and evidence would be required to support any reduction from the recommended 500mm fire break.’

There is also a statement that ‘Larger pebble sizes are preferred as this leads to less vegetative growth.’ And a recommendation that ‘fire breaks are provided in 1m strips every 40m across extensive green roofs.’

Parts of this guidance conflicts with other guidance, such as Commission Decision 2000/553/EC which provides Classification Without Further Testing (CWFT) rules for surface finishes to roofs that include the application of loose laid gravel with a thickness of at least 50mm or a mass of ≥ 80 kg/m2 (minimum aggregate size 4mm, maximum 32mm), with the UK accepting that an aggregate size 20mm – 40mm of loose laid gravel falls within this CWFT rule due to our wind scour requirements. This means that any use of a larger diameter gravel as proposed in DCLG ‘Fire Performance of Green Roofs and Walls’ will require wind uplift testing to demonstrate the achievement of fire performance requirements.

When it comes to the size of the gravel fire break, the Green Roof Organisation provides some guidance that suggests the margin width could be reduced to 300mm on small green roofs at external perimeter walls that do not abutt an adjacent building or vertical elevation, but that this would need to be discussed and agreed with local building control officers.

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