When a fire takes hold in the roof, there is a significant risk that the building will collapse. Moreover, while fire can often be contained and damage minimised, this is much more challenging when fire spreads to the roof. In this feature we discuss the implications of roof fires and explore the impact of regulation interpretation on risk management.
In the event of a fire, the minimum performance standards in Building Regulations and guidance contained within Approved Document B (ADB) are in place to protect safety of life. They do not, however, ensure the viability of the building and its contents post-fire.
While it is possible to contain a fire, if it should take hold of a roof then there is a significant risk that the building could collapse. The immediate financial impact of this can be accompanied by wider socioeconomic consequences, such as business or public service disruption.
Fire protection is a key priority, but selective interpretations of guidance contained within certain sections of ADB may present an unnecessary risk. Understanding the threat of fire in roofs and identifying where risk can be mitigated is an important step in prevention, and particular consideration should be given to testing protocols and the reaction to fire classifications of materials.
Flat roofs and fire risk
While all roof types can potentially be subjected to localised fire spread from within a building or nearby structure, flat roofs are at a greater risk of fire due to their accessibility. Arson accounts for 50.5% of fire service call-outs in the UK,1 making it the most common cause of fire and a notable concern for all buildings with flat roofs, particularly schools.
Maintenance and refurbishment of flat roofs, including hot works such as grinding, welding and torch-applied waterproofing, can also represent a fire hazard.
Additionally, the mechanical or electrical failure of plant or machinery commonly installed on flat rooftops can lead to a fire in the roof. With solar panels specifically, there are known incidences of faults causing fires by ‘arcing’.
With these risks in mind, it is crucial that roofs are designed to prevent the spread of fire and minimise potential fuel load.
How the roof contributes to compartmentation
Compartmentation is central to providing passive fire protection. To ensure that effective compartmentation is maintained throughout the building, it is essential that junctions between all elements including the external envelope and compartment walls and floors are addressed, including the junctions with flat roofs as described in ADB.
Where the roof interfaces with the head of a compartment wall, it is vital that fire cannot spread via the roof build-up and bypass the compartmentation to enter new zones – nor go on to take hold within the roof structure itself.
ADB Volume 2 considers this in Section 8.26, stating: ‘To reduce the risk of fire spreading over the roof from one compartment to another, a 1500mm wide zone of the roof, either side of the wall, should have a covering classified as BROOF(t4), on a substrate or deck of a material rated class A2-s3, d2 or better, as set out in Diagram 8.2a.’
Section 8.27 then goes on to provide further guidance on the type of buildings to which this requirement applies.
While Rockwool believes that this guidance is clear, it has been suggested that it could be interpreted in a way that allows combustible insulation to be carried across the head of any compartment wall.
This interpretation arises through the way in which the terms ‘roof covering’, ‘substrate’ and ‘deck’ are defined within the context of flat roofs and BROOF(t4).
To understand Section 8.26 of ADB Volume 2, it is necessary to define the roof covering, substrate and deck within the context of both ADB and BROOF(t4).
BROOF(t4) is a performance classification determined in accordance with BS EN 13501-5, which uses results from test methods detailed within DD CEN/TS 1187.
DD CEN/TS 1187 provides the following definitions:
- Roof covering: ‘uppermost layer of a roof’
- Substrate: ‘product used immediately beneath the roof covering’
- Continuous deck: ‘element with a continuous supporting function’
In a warm flat roof construction, the ‘roof covering’ is the waterproofing element, the ‘substrate’ is the insulation, and the ‘deck’ is the supporting element (for example, profiled steel sheeting).
Applying these definitions to Section 8.26, if the roof covering (waterproofing) is placed onto a substrate, then that substrate (insulation) should achieve a Euroclass rating of A2-s3, d22 or better (see Figure 1 for details).
Similarly, if the roof covering (waterproofing) is laid directly on the deck, as in the case of a cold flat roof, then the deck should achieve a Euroclass rating of A2-s3, d2 or better.
An interpretation of Section 8.26 poses that somehow the insulation should not be considered to be the substrate, and any roof build-up with a BROOF(t4) rating can be used continuously across a compartment wall, provided it sits on a deck rated Euroclass A2-s3, d2 or better. However, Rockwool does not believe this interpretation to be correct, for three reasons:
- The roof covering is the ‘uppermost layer of a roof’. Insulation is a substantially different layer which performs a different function to the waterproofing layer. These layers are distinct and separate elements of a roof construction and must be considered as such.
- Flat roof build-ups containing combustible insulation materials (those with Euroclass reaction-to-fire ratings from B down to F) can achieve a BROOF(t4) classification. Furthermore, as energy efficiency targets have tightened, the thickness of insulation necessary for compliance has increased substantially.
If BROOF(t4) was the sole requirement for determining the suitability of a roof system to pass over a compartment wall, this would therefore allow substantive combustible material to pass over the head of that wall without further restriction.
In the event of a fire taking hold in the roof, this could pose significant risk – and in the context of compartmentation, could provide a path of travel by which the insulation contributes to fire spread.
Rockwool believes that this runs counter to maintaining effective compartmentation, and furthermore is inconsistent with the more detailed guidance contained within ADB concerning on which buildings, if any, this might be acceptable.
3. BROOF(t4) is not a fire-resistance test. It is a classification that only considers the external fire performance of a roof when subjected to burning brands, simulated wind and radiant heat – conditions that are far less onerous than a fully developed fire. It cannot, therefore, offer comment on the behaviour of a roof containing substantive combustible insulation when exposed to a fire from within a building.
The real role of the modern roof
With this in mind, and with more insulation needed to achieve Approved Document L (ADL) compliance for thermal performance, there is an increasingly common trend among developers to use non-combustible insulation across the whole roof. This substantially reduces the fuel load, and avoids the complication of having to use two different insulation materials on the same roof while ensuring straightforward compliance with ADB.
An example is Cobham Free School, a brand-new development in Surrey which featured a demanding flat roof specification.
Not only did the project require compliance with multiple standards and regulations – including BS 6229, BB93, and BB100 – there was a demand for non-combustible zones to provide safe rooftop access and ensure compartmentation.
To streamline the installation process, non-combustible stone wool insulation was used across the flat roof, with over 2,000m2 of Rockwool Hardrock Multi-Fix Dual Density installed by roofing contractor Southern Industrial Roofing.
Using non-combustible insulation across the whole area simplified the design, achieved on-site gains during the construction and ensured effective fire protection for Cobham Free School for the long term.
Managing fire risk in the fifth facade
As non-combustible materials do not contribute to the spread of fire nor emit significant toxic smoke or gases, the installation of non-combustible insulation in a flat roof build-up should be considered best practice. Installing non-combustible insulation across a flat roof treats it as the ‘fifth facade’, matching the established principles of using non-combustible materials in the most safety-critical elements of building construction.
Using non-combustible materials is not just a best-practice measure for ensuring effective risk management in today’s market, it also serves to future proof buildings and specifications against changing client demands and an evolving regulatory landscape.
To help develop industry understanding on flat roofs and managing fire risk, Rockwool has prepared a new whitepaper, ‘Flat roofs: Managing fire risk in the fifth facade’.
For more information, go to https://www.rockwool.co.uk/managing-fire-risk-in-flat-roofs/
- Zurich, ‘Human Impact of Fire’
- Within the context of Approved Document B regarding the junction of a compartment wall and roof.