Appropriate Sprinkler Design Standards for the Protection of Multi-Occupancy Buildings

The residential and domestic sprinkler standard BS 9251:2021 (Fire sprinkler systems for domestic and residential occupancies code of practice) has been recently updated. This recent update has introduced multi-occupancy opportunities for those buildings that overlap into commercial and industrial fire hazard classifications that are already covered by BS EN 12845 - 'multi-occupancy' or 'mixed-use' buildings.

BS 9251 is listed within Approved Document B and is seen as a life safety standard, however, there is a concern that the standard does not consider the consequences of the commercial and industrial risks, or the fire loads associated with these risks. This could compromise the life safety intent of BS 9251. 

While BS 9251 has a place within the industry for ‘life safety’ and to help achieve conditions where occupants of a building can escape safely in the event of fire before structural failure or collapse, this is generally the only obligation government legislation requires.

The standard is limited in its strength and resilience in providing an aspect of ‘property protection’ associated with the sprinkler system.

Acceptability of sprinkler protection by property insurers depends on commercial and industrial design standards. Sprinkler-protected buildings to BS 9251 are considered effectively un-sprinklered for property-underwriting purposes.

While Table 4 of BS 9251 lists commercial areas of multi-occupancy buildings in which sprinkler heads should be used and the density of discharge’s suitability for Ordinary Hazard commercial and industrial applications, it also lists ‘Areas of Operation’, such as in, but limited to:

• car parking within or beneath a block of flats
• retail shop units
• bar/restaurant/café
• plant rooms
• bin store located within or beneath the flats

There are much less areas listed here than there are in the current LPC Rules: BS EN 12845:2015. There does not appear to be any explanation for the reductions in the ‘Areas of Operation’.

Mixed use buildings require the correct selection of sprinkler protection based on the highest risk classification within the building in question. This is covered within BS 9251 clause 4.1 note 3 and clarified in clauses 4.2.3.2 sub clauses a) and i), and clause 4.2.4 sub clause f), where the requirement of the sprinkler system should be discussed and agreed with the ‘insurer(s) of the building and its contents.’

Where property insurers are one of the Authorities Having Jurisdiction (AHJ), as well as building control, the standard of sprinkler protection tends to be specified as the LPC Rules of Automatic Sprinkler Installations 2015, incorporating BS EN 12845. The standard is based on research and testing and results in a more complete sprinkler system, covering property protection and the life safety requirement.

Car parking facilities within BS 9251 have an assumed fire area of 100m². The same fire hazard within BS EN 12845 has an assumed fire area of 144m² with a sprinkler discharge design density of 5mm/minute. Nevertheless, modern car parking facilities now includes vehicles which are larger, heavier, contain more plastics, and may have lithium-ion batteries and electrical charging stations. As well as that, the building structure configuration all add to the intricacy of fire control.

The fire hazards associated with the increased risks are higher fire loads, fuel leakage resulting in running fuel fires, shielded fires, fire spread, lower ceilings, higher temperatures, and thermal runaway with lithium-ion batteries. These hazards require further consideration for increased sprinkler protection from beyond the LPC Rules and BS EN 12845 requirements, rather than a reduced allowance that was mentioned in the new release of BS 9251. For insurance purposes, it has been advised that car parks should be based on the core guidance for hazard classification assessments, which considers: 

• presence of plastics
• the likelihood of a shielded fire, where the vehicle obstructs the sprinkler from a fire either below or within the vehicle
• presence of flammable liquids
• possibility of pool fires or running flammable liquid fires
• presence of other hazards such as li-ion batteries or charging stations

This would fall into category HHP3 of BS EN 12845, with a sprinkler design requirement of a 12.5mm/minute design density over the fire area.

This approach is backed by insurer losses and recently published car park fires. NFPA has undertaken a research project on this and have already adjusted the next edition considering the recognised concerns to the new risk profile.

Retail shop units are another risk, which implies that the sprinkler design criteria, based on an assumed fire area of 100m2, would be sufficient. However, BS EN 12845 requires a larger assumed fire area of up to 216m2, with a design density of 5mm/minute in order to control fires in storage areas. This allows for storage in retail units, which may consist of a variety of stored products on different storage configurations to different storage heights also benefitting from a water supply duration of 60 minutes. 

Another fire risk example included in Table 4 are plant rooms. They tend to be located either below ground or on the top floor. These risks are classed in BS EN 12845 as Ordinary Hazard group 3, requiring a sprinkler discharge density of 5mm/minute over an assumed fire area of 216m2, whereas BS 8251:2021 recommends the sprinkler design criteria be based on an assumed fire area of 100m2.

Early engagement and collaboration with the two primary AHJs is essential in enabling the right level of sprinkler protection to be afforded to mixed used buildings, in order to understand the extent of sprinkler coverage, the hazard classifications, the ambition of the sprinkler system, and how it will all fit into the fire strategy for the building.

Engineered solutions to reduce cost and water supply footprint should not be the determining factor for specifiers in place of a detailed review of the fire loads associated with the different hazard classifications. This needs to be undertaken to guarantee the correct sprinkler selection standard is confirmed and agreed for the building.

The updates BS 9251 is an improvement from the previous, as it covers more areas of the building, a requirement to adopt BS EN 12845 when commercial compartments exceed 100m2 and an acknowledgement of system resilience for system design. However, it fails to fully consider applications relating to the high rise building and does not cover water supply requirements sufficiently. The added feature for sprinkler protection to commercial classifications does not provide appropriate minimum or comparative levels of protection to that of LPC rules and BS EN 12845.

This means that sprinkler systems designed and installed to BS 9251 will not meet the minimum performance requirements required by property insurers to be recognised as providing property protection. Using expected water supply arrangements for pumps and tanks as well as reduced water storage capacities and expected water supply based on BS9251 for commercial areas of properties could undermine the process’s capability to provide supply durations to allow for the safe evacuation of residents, response time for the local fire and rescue service, and control of the fire for a defined period, as confirmed for commercial and industrial applications. 

When considering sprinkler protection to mixed-use buildings that include commercial and residential classifications, sprinkler protection should be set up using third-party approved products and contractors’ schemes, such as those managed and published by LPCB, FIRAS, and other approval schemes.

Designers and installers must have the required qualifications to the type of sprinkler system that they’re providing. If domestic and residential-only contractors are providing sprinklers to a mixed-use building, they cannot issue compliance certification for the acceptance by property insurers to cover commercial areas. Certification may only be provided against BS 9251 for life safety.

Choosing the right sprinkler standards is essential in ensuring the proposed sprinkler system can accomplish its intended task of controlling and supressing the fire in its early stages, bas on the anticipated fire loads and sizes. The consideration for sprinkler protection of mixed-use buildings must involve an in-depth analysis of the hazards and fire loads associated with the building.

Collaboration with the primary AHJs, a full understanding of the ambition of the sprinkler system (life safety or property protection) from the fire strategy, and confirmation of terms and the acceptance of property insurance depends on which design standard is adopted.

To overcome the issues surrounding mixed-use buildings, the RISCAuthority Active Sprinkler Rules Working Group are developing a new LPC Technical Bulletin to provide a fully comprehensive insurer approved sprinkler standard that meets the expectations of life safety and property protection for these types of buildings.

This new Technical Bulletin concentrates on using the capabilities of robust and resilient water supplies, which will provide a minimum duration of stored water for one hour. Sprinkler protection to the residential areas of these buildings can then use the available increased water flow and provide enhanced sprinkler protection within the individual domestic units and full sprinkler protection to all other risks within the building to commercial and industrial standards, backed by insurers.

One of the key aims of the FPA’s Know Your Building campaign is for sprinkler systems to be a regulatory requirement for commercial buildings such as care homes, schools, hospitals, and healthcare premises.

Source: Fire Protection Association