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Designing flats above 11m? You’ll need sprinklers

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Geoff Wilkinson advises on sprinkler design in view of the new regulations relating to fire prevention in tall buildings

The government has recently announced that the requirements for sprinklers in blocks of flats is being extended to cover buildings 11m in height and above. For many smaller architecture practices and developers, this will be the first time they have had to consider sprinklers in their projects, so I thought it would be useful to give a quick heads-up on some of the key design considerations. 

Ultimately, the system will need to be designed by a specialist to BS 9251:2014, ‘Fire sprinkler systems for domestic and residential occupancies’, but there are a number of key design considerations architects will need to look at before getting the specialist on board.

Firstly, it’s important to consider which areas will need to be covered. I often speak with clients who think it is only escape routes that need to be covered. In fact, sprinkler protection should be provided in all parts of the residential property, including bedrooms, kitchens and living rooms, as well as shared spaces such as halls, lobbies, stairways, corridors and landings. But there are some exceptions:

  • Bathrooms with an internal floor area of less than 5m2
  • Cupboards and pantries with a floor area less than 2m2
  • Attached ancillary buildings, such as garages that are fire-separated from the building
  • Crawl spaces, ceiling voids and uninhabited roof voids
  • Balconies and roof terraces. 

The next consideration is water supply. A residential sprinkler system will usually be a wet pipe system: ie one that is permanently charged with water. There are two potential sources for that supply, either mains or stored water supply. You will need to determine whether you are taking a ‘mains’ or ‘storage’ approach early in the project to ensure adequate space and costs are allowed for.

Blocks of flats under 18m high and with a floor area under 2400m2 are generally classed in the British standards as a Category 1 system (sometimes Category 2, depending on the requirements of the fire strategy). Typical minimum specifications for these are a nominal flow rate of not less than 40 litres/min and a minimum operating pressure at any sprinkler head of 0.5 bar. But be aware that the incoming pressure will need to be higher to allow for system losses, and a minimum 32mm-diameter supply pipe is likely to be required (note, there have been past problems in some areas as meters for 32mm-diameter pipes are not available and a separate fire main may be required).

A ‘mains’ supplied system is usually the preference, as it eliminates the need to create space for the storage tank, but it may not be suitable in all instances. So it is important to look into this at an early stage by checking with the relevant water authority. In some cases, the water authority will allow a mains water supply boosted by a pump to increase pressure. So, ask your water company to advise on the site-specific available pressures and flows measured at peak demand times. Also check the reliability of water supplies and be aware that water companies are only statutorily required to provide 1 bar pressure and 9 litres/min flow (sprinklers require less than 0.5 bar after pipe losses and less than 40 litres/min flow).

If sprinklers fail for reasons of pressure or flow, it is likely any investigation will look at whether you made all reasonable efforts to ascertain the supply reliability and capacity – and whether it was it reasonable to rely on this. If there is any doubt, it might be best to adopt a storage-based system. 

There are two options: a large tank with sufficient total capacity to supply flow for the specified design time, or a smaller tank with make-up inflow from the water service pipe. These smaller systems must provide automatic mains infilling but tank capacity can be reduced by up to 60 per cent. 

Either way, the effective stored volume must be able to provide the sprinkler system’s demand for the determined sprinkler run time. If it’s a shared system, the storage must also simultaneously meet the building’s peak domestic demand for the duration of the sprinkler run time or have a booster pump with a demand valve that closes the domestic demand in the event of sprinkler activation. This can use up a space that you haven’t allowed for and add a significant cost. 

Other early design factors you will need to consider include:

  • To avoid exposed pipes consider specifying open-web floor joists, which can accommodate the sprinkler pipe
  • The need for structure to support any storage tank
  • The type of heads – in most cases you will want to specify concealed heads that are flush with the ceiling, rather than the traditional pendant design as these are both more aesthetically pleasing and also less prone to damage.

Geoff Wilkinson is an approved inspector and managing director of Wilkinson Construction Consultants www.thebuildinginspector.org

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