It is now almost universally recognised that in order for a timber fire resisting door to satisfy the integrity requirements of the BS476: Part 22: 1987, or even the more recent EN1634-1 test procedure, it will need to incorporate an intumescent seal in the leaf to frame junction. Because integrity is the way we measure the ability of an assembly to resist the passage of flames, the contribution that an intumescent seal makes too restricting the flow of warm and hot fire gases is often completely under estimated. The criterion of the standard fire resistance test, which is solely that of consists of continuous flaming, or the ignition of a cotton fibre pad close to any gap. The use of continuous flaming gives people a mental image that if the presence of visible flames is prevented, then the objective has been achieved. When the cotton pad test cannot be used due to high surface temperatures on the door, then a gap criterion which measures the dimensions of holes that go right through the specimen from one side to the other is used instead. This further supports the myth that it is only flames that compromise life safety.

A greater analysis of life safety would soon indicate that the tenability within any protected space is definitely compromised by hot gases, particularly smoky gases, whether accompanied by flames or not. In the case of a timber door assembly an intumescent seal is invariably fitted to prevent flaming during the test which gratuitously provides a resistance to the flow of hot gases, thereby meeting the fire safety objective and also satisfying integrity as a concept. As a consequence, the protection that is provided to meet what should be the regulatory objectives is achieved as a bi-product of the fire resistance integrity criteria, it is not targeted specifically. However, this gratuitous smoke control only applies to timber fire resisting doorsets, because the cotton pad integrity test is not used for uninsulated metal doors, nor is there anything to produce flaming, and therefore, they can never provide the same level of smoke and hot gas protection as the timber door will.

This traditional benefit that ‘timber’ doors provide to life safety is in danger of being lost because of changes within the construction products industry. Hardwoods have become less and less acceptable and as a result one hour timber fire doors, which commonly require the use of hardwoods, are becoming less acceptable environmentally and more expensive as a consequence. Aesthetic steel doors are becoming achievable because of the modern metal working techniques that have evolved, particularly with the increase use of robot manufacture, making them more and more compatible in price and appearance in the personnel door market. The criticisms that such doors are not able to be sized readily on site are also being lost by the increasing trend towards doorsets in standard sizes which favour metal door assemblies. Such doors will rarely be made fully insulating because our legislation does not require it and consequently the less onerous form of evaluating integrity, by means of gap gauges, rather than cotton pads, again favour their use. As a result these doors do not require the use of intumescent strips, and therefore, the gratuitous protection that they provide to resist the flow of hot smoke and combustion gases is lost.

This raises important issues in respect of regulatory guidance documents. Are they right in requiring ‘integrity’ only doors? In modern building design we now incorporate refuges for the disabled, in hospitals we used compartments for horizontal evacuation and in many high rise buildings phased evacuation is the ‘norm’. Is it still correct for these doors not to have an element of insulation performance? If they were insulatory, then intumescent seals would be fitted to pass the test and the protection to personnel in a fire would be maximised in respect of exposure to smoke.

However, this would again be solving the problem by default than by design. Has the time not come when we recognise the need to control the spread of hot smoke in buildings? We have, for a number of years, recommended the use of doors that control the spread of cold smoke in regulatory guidance, evaluated against the BS476: Part 31.1 method, in regulatory guidance documents. This only addresses the passage of ambient, or ‘cold’ smoke which may, or possibly may not, represent the exposure conditions attacking a ‘refuge’. The adoption of the medium temperature smoke test, embodied in ISO DIS5925-1 and En 1634-3, should certainly apply to those areas where people take refuge, even if it is the authority’s view that doors onto normal protected routes do not need to have a ‘hot’ smoke control function.

However, we are moving towards the risk assessed world of the new Regulatory Reform Order (RRO) and it will be interesting to know whether risk assessors will be happy to sign-off a building as safe when we do not control the spread of ‘hot’ combustion gases on all doors. Any risk assessor who needs to know more about the role of fire seals in a fire risk assessment can download a guidance document from the IFSA website (www.ifsa.org.uk).

We have the technology to control ‘warm’ or ‘hot’ smoke, perhaps now we may have the will.

By Peter E Jackman IEng MIFireE FBEng AIWSc
Technical Consultant to IFSA