Correct Positioning of Domestic Smoke Detectors

This photo was taken during a recent fire risk assessment at an HMO. The reason for taking the photo was to show the client that the optical smoke detector had been installed too close to the wall. But why does this matter?

Smoke detectors and smoke alarms (the distinction being domestic smoke alarms are self-contained units containing both the sensor and the siren in one device, and a smoke detector being the sensor only) are critical fire safety devices that provide early warning of a fire situation, significantly reducing the risk of injury, death, and property damage.

What most people don’t realise, however, is that smoke alarms can’t just be installed anywhere without careful consideration. Identifying the optimum position for installing smoke detectors is crucial for safety, as well as complying with legislation.

Dead Air Space

A fire alarm should not be placed in a dead air space. A dead air space is basically an unventilated space where air doesn’t circulate. This space has the ability to interfere with smoke alarms.

A dead air space generally occurs where corners of walls meet, or in between exposed floor joists and beam in the ceiling or where the ceiling meets the wall.

Positioning of Smoke Detectors.

Smoke alarms should be installed on the ceiling, as central as possible in the room. However, this is not always possible due to a variety of reasons, such as obstructions and light fittings (large central light fixture, decorative ceiling rose, or ceiling fan), structural beams (deep ceiling joists or structural beams), airflow disruptions (air conditioning vents and heat ducts), sloped or pitched roofs (rooms with a pitched or vaulted ceiling), high-risk zones (e.g., close proximity to kitchen or bathroom doorway which would risk false alarms from normal cooking fumes or steam).

When a smoke alarm cannot be placed in a central position on a ceiling, the following guidelines must be adhered to. Fire Building Regulations in the UK point towards BS 5839 as the key code of practice for the planning, design, installation, commissioning and maintenance of fire detection. (BS 5839-6: 2019 for domestic, and BS 5839-1: 2025 for non-domestic).

BS 5839-6 states that a smoke alarm should not be mounted within 300mm of any obstruction such as walls and light fittings, etc. This is to ensure that they are outside of any ‘dead air’ spaces that occur in corners and spaces where the airflow may be blocked.

Smoke alarms should never be installed near a fluorescent light. Placing a smoke alarm too close to a fluorescent light fitting can cause electromagnetic interference and blocked airflow, which leads to false alarms or a delayed response in an emergency. For this reason, while standard incandescent or LED ceiling light fittings only require a minimum distance of 300 mm, fluorescent lights require 1500 mm due to that extra electrical interference.

For peaked and sloped ceilings, ensure there is a maximum of 600mm vertically down from the apex for smoke alarms. Do not push the alarm into the very top corner where trapped stagnant air can delay detection. In this situation, the smoke alarm can be installed on a sloped ceiling.

In larger rooms that require more than one fire alarm, a 7.5m radius is required to provide even detection. Also, a 7.5m distance between smoke alarms represents the maximum radius or spacing rule under standard fire safety guidelines (BS 5839-6) ensuring that every point in a protected circulation area or room is within 7.5 meters of the nearest smoke detector.

Smoke or Heat Alarms in a Kitchen?

Because the average person will burn the toast from time to time, smoke alarms are not suitable for kitchens, but heat alarms are. Installing a heat alarm in a kitchen provides warning of an increase in temperature caused by a fire but will not be set off by cooking fumes.

Remember, do not install smoke alarms …

·       within 300 mm of walls, room corners or light fittings, etc

·       in close proximity to bathrooms or showers

·       within 1500 mm of a fluorescent light fitting

·       further than 7.5 m from a door to a room where a fire might start

·       further than 7.5 m from the next smoke alarm