Fire Spread. Internal Fire Spread (Surface). External Fire Spread

The linings of walls and ceilings can be an important factor in the spread of a fire and its gaining hold. This can be particularly dangerous in circulation areas, where it might prevent people escaping. Two factors relating to the property of materials need to be taken into account: the resistance to flame spread over the surface and the rate of heat release once ignited. Various testing methods are used to establish these qualities. In the UK, a numbered system categorises the levels of surface flame spread and combustibility: 0, with the highest performance (noncombustible throughout), followed by classes 1, 2, 3 and 4.

There are a series of standards that must be complied with relating to allowable class of linings in various locations. For example, for small rooms in residential buildings (4m²) and non-residential buildings (30m²), class 3 materials are acceptable; for other rooms and circulation spaces within dwellings, use class 1 materials; and for busy public circulation spaces, class 0 materials should be used. Rooflights and lighting diffusers that form an integral part of the ceiling should be considered a part of the linings. There are limitations on the use of class 3 plastic roof-lights and diffusers.

Internal fire spread (structure). There are three factors to be considered under this heading:

(1) Fire resistance and structural stability. It is necessary to protect the structure of a building from the effects of fire in order to allow people to escape, to make it safe for firefighters to enter the building to rescue victims and tackle the fire, and also to protect nearby people and adjacent buildings from the effects of a collapse. The level of fire resistance required depends on a range of factors: an estimation of the potential fire severity (depending on the use and content of the building); the height of the building; type of building occupancy; the number of floors and the presence of basements. Fire resistance has three aspects: resistance to collapse, resistance to fire penetration and resistance to heat penetration. Building regulations provide tables that set out specific provisions and minimum requirements of these aspects for different structural elements in different classes of buildings.

(2) Compartmentation within buildings. It is often necessary to divide a large complicated building into separate fire-resisting compartments in order to prevent the rapid spread of fire throughout the building. The factors to be considered are the same as those for fire resistance. Regulations stipulate maximum sizes of compartments for different building types. In general, floors in multistorey buildings form a compartment division, as do walls that divide different parts of multi-use buildings. The use of sprinklers can allow an increase in the compartment size in non-residential buildings.

Careful attention should be paid to construction details of compartment walls and floors, particularly the junction details between walls, floors and roofs, such that the integrity of fire resistance is maintained. Strict rules apply to openings permitted in compartment walls and floors, these being restricted to automatic self-closing doors with the appropriate fire resistance, shafts and chutes with the requisite non-combustible properties and openings for pipes and services, carefully sealed to prevent fire spread.

There is a wide range of constructions, each of which offers a specific duration of resistance. For example, a floor of 21 mm of tongue and groove timber boards (or sheets) on 37 mm wide joists with a ceiling of 12.5mm plasterboard with joints taped and filled, will provide 30 minutes of fire resistance. For 60 minutes' resistance the joists need to be 50mm wide and the ceiling plasterboard 30mm with joints staggered. This period is also achieved with a 95mm thick reinforced concrete floor, as long as the lowest reinforcement has at least 20mm cover.

An internal load-bearing wall fire resistance of 30 minutes can be achieved by a timber stud wall with 44mm wide studs at 600mm centres, boarded both sides with 12.5mm plasterboard with joints taped and filled. The same will be achieved by a 100mm reinforced concrete wall with 24mm cover to the reinforcement. A resistance of 60 minutes is achieved by doubling the thickness of plasterboard on the stud wall to 25mm, and increasing the thickness of the concrete wall to 120mm. A 90mm thick masonry wall will achieve the same 60 minutes resistance (only 75mm is required for non-loadbearing partitions).

(3) Fire and smoke in concealed spaces. With modern construction methods there can be many hidden voids and cavities within the walls, floors and roofs. These can provide a route along which fire can spread rapidly, sometimes even bypassing compartment walls and floors. This unseen spread of fire and smoke is a particularly dangerous hazard. Steps must therefore be taken to break down large or extensive cavities into smaller ones and to provide 'cavity barriers', fire-resistant barriers across cavities at compartment divisions.

Regulations stipulate the maximum permitted dimensions for cavities depending on the location of the cavity and the class of exposed surface within it. Further stipulations dictate where cavity barriers must be installed (e.g. within roof spaces, above corridors and within walls). Generally the minimum standard of fire resistance of cavity barriers should be 30 minutes with regard to integrity and 15 minutes with regard to insulation. Fire stops must also be considered. These are seals that prevent fire spreading through cracks at junctions between materials that are required to act as a barrier to fire, and seals around perforations made for the passage of pipes, conduits, cables etc.

External fire spread. The spread of fire from one building to another is prevented by the fire resistant qualities of external walls and roofs. They must provide a barrier to fire and resist the surface spread of flame. The distance between buildings (or between the building and the boundary) is obviously an important factor, as is the likely severity of the fire, which is determined by the fire load of a building (i.e. the amount of combustible material contained within). Regulations therefore stipulate the required fire resistant qualities of external walls and the proportion and size of allowable unprotected areas (e.g. windows, doors, combustible cladding, etc.) depending on the type of building and the distance of the fagade from the boundary.

For example, the fagade of a residential, office, assembly or recreation building at a distance of 1 m from the boundary is allowed only 8% of unprotected area; at 5m, 40%; and at 12.5m, 100%. In contrast, the figures for shops, commercial, industrial and storage buildings are: at 1 m, 4%; at 5m, 20%; and at 12.5m 50%; and only at 25m, 100%. More complex calculations are required when the fagade is not parallel with the boundary, or is not flat.

Generally, roofs do not need to be resistant to fire from inside the building, but should be resistant to fire from outside, and also resist surface flame spread. Again, the type of roof construction permitted depends on the type of building, its size and its distance from the boundary. Different roof coverings are rated as to their resistance to fire: on pitched roofs; slates, tiles, profiled metal sheet are in the highest category, bitumen strip slates in the lowest. Sheet metal flat roof coverings perform the best, whilst the performance of various bitumen felt roof coverings depend on the types of layers, underlayers and supporting structure.