The LWF Blog

Fire Safety Engineering for Design – Fire Detection & Alarm Systems – Part 159

November 20, 2023 11:40 am

LWF’s Fire Safety Engineering blog series is written for Architects, building designers and others in the construction industry to highlight and promote discussion on all topics around fire engineering. In part 158, LWF discussed video smoke detection (VSD) and its effectiveness. In part 159, we consider the siting and spacing of detectors for a fire alarm system.

There are two main codes of practice for fire detection and fire alarm systems for buildings relevant to the siting and spacing of detectors:

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, and

NFPA 72 National Fire Alarm and Signaling Code

The recommendations of each standard differ quite significantly in relation to the siting and spacing of fire alarm detectors. BS 5839-1 provides specific guidance about permissible distances, whereas NFPA 72 guidance is based on statements as to where detectors should or shouldn’t be placed and gives general spacing information (along with instructions to check with the manufacturer of the equipment).

As smoke and heat rises and settles just below ceiling height in an enclosed space, detectors should generally be placed at this height.

BS 5839-1 recommends:

  1. Heat detectors should be placed so that the heat-sensitive element is not less than 25 mm and not more than 150 mm below ceiling or roof height. The measurement should increase to 600 mm for smoke detectors. For pitched or north-light roofs, detectors should be installed in each apex.
  2. The maximum horizontal distance between any point in the area and the nearest detector (for point type heat and smoke detectors) is as follows:
    • Under flat horizontal ceilings and in corridors more than 2 metres wide, 5.3 m for point-type heat detectors and 7.5 m for point-type smoke detectors.
    • For square-type arrays (detectors set in a square) maximum spacing between smoke detectors is typically 10 m, and for heat detectors the maximum distance is typically 7  m.
    • In corridors where the width does not exceed 2 m, detectors may be mounted along the central line. Smoke detectors can be placed at intervals of 15 m, heat detectors at intervals of 10.6 m, provided the maximum dimension from end walls is 7.5 m for smoke and 5.3 m for heat detectors.
    • In the apex of a pitched or north-light roof, add to the maximum horizontal distance 1% for each degree of slope to a maximum increase of 25%.

In part 160 of LWF’s series on fire engineering, we will continue to discuss the placement of detectors as per BS 5839-1 and NFPA 72. 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 since 1986 to produce innovative and exciting building projects. If you would like further information on how LWF and fire strategies could assist you, please contact the LWF office on 0800 410 1130.

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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