Building standards technical handbook 2020: domestic

Over 90% of Scottish fire deaths occur in the dwelling of fire origin. Asphyxiation caused by smoke inhalation is the primary cause of death. If there is an outbreak of fire within a dwelling, early detection and warning to the occupants can play a vital role in increasing their chances of escape. This is particularly important as the occupants may well be asleep and are more likely to react slower.

Most fires in dwellings are accidental and statistics show that the main sources of ignition are: Cooking appliances, smoking related, candles, electrical supply and other domestic appliances.

Occupant characteristics - in the 4 years 2014-2015 to 2017-2018 where fatalities were recorded, on average 30% of fires started in the living room, 17% in bedrooms, and 15% in the kitchen. This means that a significant number of fire related deaths, 62% or 107 deaths over the 4 year period, occurred from fires starting in these rooms. It is therefore important that the outbreak of fire in living rooms and kitchens is detected quickly and the alarm raised as early as possible during the early stages of fire growth.

Tolerable Standards - in January 2019 amendments to the tolerable standards introduced a requirement for all dwellings to be fitted with heat and smoke alarms and carbon monoxide detection by February 2021. Guidance in support of the tolerable standards calls for interlinked heat and smoke detection to be sited as per the guidance contained in this technical handbook, with the allowance that the installation may be powered by sealed for life battery units. However, where an existing partial or full mains operated heat and smoke detection system is being altered it would normally be expected to replace mains wired devices with mains wired units.

Conversions - in the case of conversions, as specified in regulation 4, the building as converted shall meet the requirement of this standard (regulation 12, schedule 6).

Living rooms and kitchens should be fitted with fire detectors because they are the most likely sources of fire in dwellings and result in the greatest number of fatalities and injuries in Scotland each year. Statistics also show that bedrooms and other rooms or spaces within a dwelling also contribute to the overall number of casualties in Scotland and as a result the circulation spaces outside these rooms or spaces should be protected to give early warning of fire.

Therefore, in order to provide a fire detection and fire alarm system that should alert occupants to the outbreak of fire, a Grade D system should be installed in all dwellings, comprising of:

The principal habitable room is the most frequently used room by the occupants of a dwelling for general daytime living purposes.

Where a dwelling has an open plan layout, the open plan area will also be used as a circulation space (which could include a stair and landing). The location and siting of smoke alarms and heat detectors should follow both the guidance above and in Clause 2.11.7 to determine the appropriate number of alarms.

Inner rooms - where occupants’ only escape route is through another room (the access room) they are at risk if a fire starts in the access room. Therefore, every access room should be provided with a smoke alarm to give occupants of the inner room early warning.

Where the access room is a kitchen, the type of detector should be carefully considered to reduce the likelihood of false alarms.

Any inner room at a height of more than 4.5m should be designed in accordance with the guidance to clause 2.9.7.

Common systems - in a building containing flats or maisonettes, a common fire alarm and detection system that interlinks all dwellings and common spaces is not recommended due to the risk of unwanted false alarms. However in a sheltered housing complex, monitoring equipment is recommended due to the vulnerability of the occupants.

Detailed guidance on fire detection and fire alarm systems in dwellings can be obtained from BS 5839: Part 6: 2019.

False alarms are common in dwellings and may result in the occupants disabling the fire detection and fire alarm system. The most common causes of a false alarm are:

Consideration should therefore be given to the type of fire detector in order to reduce the amount of unwanted false alarms. There are 4 main types of fire detector used in dwellings:

Optical smoke alarms should conform to BS EN 14604: 2005 and operate on the principle of detecting the scattering or absorption of light within the detector chamber. Optical smoke alarms are more sensitive to slow smouldering fires such as fires involving soft furnishings and bedding.

Principal habitable room - the most likely source of fire in a principal habitable room is the careless disposal of smoking materials. Polyurethane foam found in some furnishings may ignite and begin to smoulder producing large particles of smoke. Optical smoke alarms are therefore recommended in principal habitable rooms however if the room is used by a heavy smoker, this could give rise to some false alarms from tobacco smoke.

In cases where a principal habitable room is open plan with a kitchen, an optical smoke alarm is recommended to reduce the amount of unwanted alarms from cooking fumes.

Circulation spaces - most unwanted alarms occur during cooking. Optical smoke alarms are less sensitive from fumes caused by toasting bread or frying or grilling food. Therefore, optical smoke alarms are recommended in hallways and stairwells adjacent to kitchens.

Ionisation smoke alarms should conform to BS EN 14604: 2005 and operate on the principle that the electrical current flowing between electrodes in an ionisation chamber is reduced when smoke particles enter the chamber. Ionisation smoke alarms are more sensitive to smoke containing small particles such as rapidly burning flaming fires but are less sensitive to steam. Therefore, ionisation smoke alarms are recommended in hallways and stairwells adjacent to bathrooms or shower rooms to reduce the amount of unwanted false alarms.

Circulation spaces - multi-sensor alarms are recommended in hallways and stairwells adjacent to bathrooms or shower rooms to reduce the amount of unwanted false alarms.

A multi-sensor alarm provides the early warning of fire and can significantly reduce the amount of unwanted false alarms in certain circumstances. See BS 5839: Part 6: 2019 for more detailed information.

Heat alarms conforming to BS 5446: Part 2: 2003 have fixed-temperature elements and operate on the principle of responding to the temperature of the fire gases in the immediate vicinity of the heat alarm. Heat alarms are used where ambient temperatures are likely to fluctuate rapidly over a short period such as in kitchens and are less likely to produce false alarms. Elsewhere, heat alarms should not be used instead of smoke alarms to reduce unwanted false alarms.

The guidance in this clause takes account of the audibility levels in adjoining rooms and the effect of smoke travelling along a ceiling.

Smoke alarms and heat alarms by their definition, include an integral sounder. Smoke alarms are designed to produce a sound output of 85 dB(A) at 3m. Therefore, allowing for sound attenuation through a domestic door by around 20 dB(A), a sound level of between 55 – 65 dB(A) is likely at the bed-head in each bedroom which should rouse the occupants. There is no evidence to suggest that lives are being lost in dwellings due to audibility levels other than when people are incapacitated to such a degree (e.g. by alcohol or drugs), that even higher sound levels would not waken them.

Smoke from a fire in a dwelling is normally hot enough that it rises and forms a layer below the ceiling. As the smoke rises and travels horizontally it mixes with air which increases the size of the smoke particles. This means that ionisation smoke alarms may be less sensitive to the smoke. Where a hallway is very long, the smoke might cool to such an extent that it loses buoyancy and spreads along the floor.

Audibility - smoke alarms should be located in circulation spaces:

A smoke alarm located in an access room (which could include a stair and landing), serving an inner room should be not more than 3m from the door of the inner room.

Smoke travel - a smoke alarm in the principal habitable room should be sited such that no point in the room is more than 7.5m from the nearest smoke alarm and in the case of a heat alarm, no point in the kitchen should be more than 5.3m from the nearest heat detector.

All dimensions should be measured horizontally.

Smoke might not reach a smoke alarm where it is located on or close to a wall or other obstruction. Therefore, smoke alarms should be ceiling mounted and positioned away from any wall or light fitting. In order to reduce unwanted false alarms, smoke alarms should not be sited directly above heaters, air conditioning ventilators or other ventilators that might draw dust and fine particles into the smoke alarm.

Smoke alarms and heat alarms should be ceiling mounted and located such that their sensitive elements are:

Monitoring of wiring or faults reduces the amount of time which a system is likely to be disabled before a fault in the system is discovered. A visual indicator or warning signal should be provided to alert the occupant that there is a fault with the system.

Therefore, at least a Grade D fire detection and fire alarms system should be installed in every dwelling which comprises 1 or more mains powered smoke alarm and 1 or more mains powered heat alarm with an integral standby supply in accordance with BS 5839: Part 6: 2019.

However a sheltered housing complex normally provides accommodation for vulnerable occupants with a diverse range of support needs. Therefore, a fire alarm signal should be transmitted to a remote monitoring service or to a warden who can assist with any evacuation if necessary, or call for assistance.

In order to achieve this principle, a Grade C system should be installed in every dwelling in a sheltered housing complex which comprises central control equipment in accordance with BS 5839: Part 6: 2019, and:

Research shows that significant proportion of battery operated smoke alarms fail to operate during the outbreak of a fire. The main reason for this is that the battery is either faulty or has been removed from the alarm. Therefore, smoke alarms and heat alarms should be mains operated and permanently wired to a circuit which should take the form of either:

The standby supply for smoke alarms and heat alarms may take the form of a primary battery, a secondary battery or a capacitor.

The capacity of the standby supply should be sufficient to power the smoke alarms and heat alarms in the quiescent mode for at least 72 hours whilst giving an audible or visual warning of power supply failure, after which there should remain sufficient capacity to provide a warning for a further 4 minutes or, in the absence of a fire, a fault warning for at least 24 hours.

Interconnection - all smoke alarms and heat alarms in a dwelling should be interconnected so that detection of a fire in any alarm, operates the alarm signal in all of them. Smoke alarms and heat alarms should be interconnected in accordance with BS 5839: Part 6: 2019.

The system should be installed in accordance with the manufacturers written instructions. This should include a limitation on the number of smoke alarms and heat alarms which may be interconnected.

Radio linked interconnection between hard wired smoke alarms and/or heat alarms may be used for a Grade D system. More detailed guidance on the use of radio linked technology can be obtained from, BS 5839: Part 6: 2019.