The LWF Blog

Fire Engineering Design and Risk Assessment – Firefighting & Internal Water Supplies – Part 32

March 18, 2019 3:10 pm

In LWF’s Fire Engineering blog series for Architects and others involved in the building design business, we have been looking at fire safety engineering and, most recently, firefighting. In part 31, we looked at the recommendations made regarding water capacity for any open water used for firefighting purposes and discussed relaying water to the place of fire origin. In part 32, we look at internal water supplies designed for the purpose of firefighting.

Fire mains in buildings may be classed as ‘rising’ ‘falling’ or ‘horizontal’. Rising mains are those mains where the water source is at ground level (a fire appliance pump may be a source, for instance) and the water must rise through the pipework. Falling mains are often serviced by a roof water tank or descend into a basement and natural gravity will ensure that the water falls through the pipework. Horizontal mains are, of course, where the water source is on the same level as the pipework.

In the UK, it is common for all three – rising, falling and horizontal – to be known as ‘risers’. This is further categorised by the riser being wet or dry. A wet riser is one that is permanently charged with water in the pipework, ready for use. A dry riser is one where the water is sent to the area/s of the fire on demand, but the pipework is usually not filled with water. 

In the case of dry mains, the pressure available at the firefighting branch reduces with increased elevation due to the static head of the water in the rising main and frictional loss. In tall buildings, the losses can exceed the pressure supplied from the fire appliance pump supplying the main and fail to work. For this reason, it is required that in buildings with floors above 50 m from fire service access level, the mains should be permanently charged with water to provide a pressure-regulated flow.

The criteria for internal fire mains in the UK can be gained from Approved Document B ( ) and BS 9999 ( both of which concur that all buildings with a floor height of 18 m may have dry risers along with firefighting lobbies and that all buildings with a floor height of 50 m should have wet risers. Approved Document B also states that shops, commercial, assembly, recreation or industrial buildings with a floor area of less than 900 m2 and a floor height of up to 7.5 m, with two or more basement storeys must have a firefighting lobby (which should incorporate a stairway in flats).

In part 33 of this series, LWF will look at those references which provide guidance on the use of dry fire mains. In the meantime, if you have any questions about this blog, or wish to discuss your own project with one of our fire engineers, please contact us. 

Lawrence Webster Forrest has been working with their clients for over 25 years to produce innovative and exciting building projects. If you would like further information on how LWF and fire strategies could assist you, please contact Peter Gyere on 020 8668 8663.

While care has been taken to ensure that information contained in LWF’s publications is true and correct at the time of publication, changes in circumstances after the time of publication may impact on the accuracy of this information.

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