The LWF Blog

Fire Engineering Design and Risk Assessment – Siting of Fire Alarm Detectors – Part 17

September 21, 2016 11:34 am

In this blog series for Architects and those who work in building design and construction, we have been looking at types of fire alarm system and detector recently. Today, we will discuss how the siting and spacing of fire alarm detectors is important for fire protection.


As you will be aware, when a fire ignites and develops, both smoke and heat rise and collect most densely high in the enclosed space, just below ceiling height. For this reason, smoke and heat detectors should also be sited at these levels.


In BS 5839:1 Fire detection and fire alarm systems for buildings. Code of practice for design, installation, commissioning and maintenance of systems in non-domestic premises it states that heat detectors must be sited so that the heat-sensitive element is between 25mm and 150mm below the ceiling. In cases where a shaped roof is in place instead of a flat ceiling, detectors should be sited in the apexes.


Of course, simply dotting detectors around at will is not sufficient protection either. Each detector should have a suitable maximum horizontal distance between itself and its neighbour which varies dependent upon configuration of the space.


Some of the distances and circumstances laid out in BS 5839:1 are as follows:


For Point-type heat detectors, where there is a horizontal flat ceiling and the space or corridor is more than 5m wide, the maximum distance between any point and a smoke detector should be 5.3m (over a maximum area of 50m2) . For point-type smoke detectors, the maximum distance can be increased to 7.5m (over a maximum area of 100m2).


Where four detectors are placed in a square (one detector at each corner of the arrangement) the maximum space between the smoke detectors should be 10m and between heat detectors that is reduced to 7m.


In the case of a corridor which has a width not more than 2m, it is stated that the distance between smoke detectors placed along the centre line is a maximum of 15m and for heat detectors, 10.6m. These distances are given, provided that the detectors which are sited most closely to each end wall are placed within 7.5m for smoke detectors and 5.3m for heat detectors.


BS 5839:1 also lays out how to calculate the correct placement of smoke and heat detectors within the apex of a roof.


Something else which should be borne in mind is the possibility of an obstruction at ceiling height which may interfere with the passage of smoke or hot gases to a placed detector. Most commonly this might be a beam and in cases where the obstruction has a depth greater than 10% of the ceiling height, further allowances must be made. Indeed, the areas to each side of the beam should be considered separate rooms for the purposes of heat and smoke detection.


In next week’s blog, we’ll take a look at ceiling height limits. 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.


Share this post