Stuart Power, Director and Fire Engineer at Salus, provides a technical account on the importance of the design and construction of passive and active fire safety measures whilst giving an insight on the numerous misconceptions regarding high-rise flats and buildings.
In light of the recent devastating fire at Grenfell Tower, Salus have produced this technical note to alleviate the numerous misconceptions regarding high-rise flats. We would like to remind clients and users of such buildings of the legislative requirements and the importance of the design and construction of the passive and active fire safety measures to ensure safety.
Whilst we must not speculate on the cause of the devastation at this stage, it is likely to have been a catalogue of events that led to this disaster and not one single aspect, albeit certain aspects probably contributed more than others.
The provision of sprinklers in flats over 30m in height became a requirement for newly constructed or converted flats in 2006, however the guidance is not retrospective, even if work is carried out to a tower built before that date.
Fire safety is achieved through a package of physical fire safety measures in the design and construction process. Whilst the framework of the building regulations ensure that the building is built to achieve an adequate level of safety the ongoing fire safety is dealt with under the Regulatory Reform ( Fire Safety Order) 2015 which generally covers the common parts in flats. This equally applies to all existing blocks that were built prior to the introduction of building regulations. This legislation ensures a fire risk assessment is carried out and the aim of this is to ensure the chance of a fire occurring is minimised by controlling hazards, that the fire safety measures are appropriate and adequate for the type of building and its occupancy and that the building is well managed with fire safety measures tested and maintained. Fire safety in flats within the UK is controlled more by legislation now than they ever previously have.
Fire strategies for flats are very different to any other building built in the UK in that they are designed on a stay put strategy with potentially a single entrance and exit points depending upon the distance to the staircase, with no common fire alarms to give warning. This is on the basis that it’s only the occupants of the flat of fire origin that are notified by the fire alarm and should evacuate. All other occupants are deemed safe in their respective fire rated box and there is no need to evacuate other occupants because they are deemed safe within their home due to the fire safety measures proposed. If evacuation of adjoining areas is considered necessary then this comes under the control of the fire authority and provisions including limiting travel distance and the smoke ventilation of areas allows occupants to escape in a relatively smoke free environment.
This is a strategy that has a proven history of success. There are lots of fires that occur in flats all over the UK and generally most are contained within the flat of origin or spread is limited.
The principle only works if the design and most importantly the site work is carried out in the correct manner to ensure effective compartmentation which in turn ensures that fire finds it very difficult to spread. Failure in this can cause the kind of fatal consequences seen at Grenfell Tower.
We have noticed that there has been a reduction in this at times, where the importance of this is overlooked and not given the level of control and thought that is needed.
As a reminder the following is as aide memoir to ensure compliance with current regulations and guidance;
We need to be sure that through detailing, design and workmanship that all walls and floors in such buildings are designed and built as compartment elements and there is a correct selection of products used where the elements meet each other and where the elements are penetrated. This is equally applicable to risers especially where services enter and leave a riser. Key areas are ducts for ventilation and pipes which without good compartmentation will results in the stay put strategy being flawed. It is vitally important that the correct product is chosen for the scenario and that the product has been tested and fitted in accordance with the fire test report. Whilst walls are constructed as fire rated it is important to ensure doors achieve the relevant fire resistance which includes the correct selection of ironmongery, smoke seals, letter boxes, closers and frames. There are lots of products which are mixed and matched but it is recommended that the whole assembly has a relevant test certificate not just individual parts in isolation and that ultimately they are fitted correctly.
Gas service and installation pipes or associated meters should not be contained within a protected staircase unless the gas installation is in accordance with the requirements for installation and connection set out in the pipelines safety regulations. See guidance in clause 2.42 of ADB.
External Wall construction
This has achieved the most focus and there are lots of grey areas in the guidance and I have seen on many occasions designers get confused on what is required, as every scenario can be different. To be clear however, it states in the Approved Document that;
‘The external envelope of a building must not provide a medium for fire spread.’
It then suggests that combustible materials and extensive cavities may present such a risk in tall buildings.
The surface spread of flame requirement is the first requirement to prevent the flame travelling up the face of the building. For Buildings up to 18m and more than 1m from the boundary, there is no requirement in this respect unless in the assembly purpose group. For buildings closer to the boundary and for buildings over 18m the requirements are more onerous and generally insist upon a class 0 spread of flame rating to the external surface. This is documented in diagram 40 of ADB.
The second requirement relates to the type of products used in the wall construction and for buildings over 18m it suggests that any material used in the external wall construction should be of limited combustibility unless the insulation is contained within a masonry cavity wall. A material of limited combustibility is defined within Appendix A of ADB and applies to the whole wall construction not just the insulation. A common mistake I have heard is “well its Class 0 so it must also be of limited combustibility” but the class 0 only applies to the face material.
Clearly the insulation used at Grenfell did not comply with this criteria. However there is an alternative method to showing compliance by following the performance criteria given in BRE Report: Fire performance of external thermal insulation for walls of multi storey buildings (BR 135) for cladding systems using full scale fire test data from BS 8414 part 1 or part 2 or a desk top study can be submitted by a UKAS accredited testing body i.e. Exova. This potentially allows the use of combustible insulation depending upon the type and other components in the wall make up but it needs to undergo a robust test with report findings and most importantly the wall must be built to the same specification as the wall tested. A change of any product invalidates the test report.
The third requirement is the use of cavity barriers, this is where there is lots of confusion. However, if there is a cavity there must be cavity barriers at window openings and compartment lines. There is confusion relating to the provision of cavity barriers in cladded systems where there is actually no cavity but where combustible insulation is used and whether there is a need to break the insulation and provide a cavity barrier on compartment lines because if the insulation melts away then there could be a void created all of which requires consideration.
Rain screen cladding causes issues in that it creates a ventilated space and generally if the inner lining is constructed of masonry or concrete then there is a perception cavity barriers are not needed but they are a very important part of ensuring the chance of fire spread is minimised. The exemption only applies to non-residential uses and cavity barriers would only be needed in these purpose groups if the insulation used was not of limited combustibility and the cavities offer a class 0 surface spread of flame rating. This requirement applies to any height of building.
The legislative system does allow for safe tower blocks if correctly detailed, designed and built to ensure robust compartmentation along with the correct information being passed onto the end user with correct and precise management, albeit guidance does need to be significantly improved.