Behaviour of wood in fire
The keyword for timber's behaviour in fire is predictability. Although it burns, this occurs at a predictable speed known as the charring rate. The thermal insulation properties of timber are such that the timber just a few millimetres inside the burning zone is only warm. This is in contrast to high thermal conductivity materials such as steel, which heat up more uniformly giving rise to problems of expansion and loss of strength over the whole section.
Different timbers char at varying rates, largely as a function of their density with the higher density timbers charring more slowly. For structural timbers listed in the code of practice for the design of structural timber, BS 5268-2, this rate of depletion is taken as 20 mm in 30 minutes from each exposed face. Certain of the denser hardwoods (>650kg/m³) used for structural purposes merit rates of 15 mm in 30 minutes, eg, keruing, teak, greenheart, jarrah. Timbers of lower density will char more quickly eg Western red cedar is quoted as 25mm in 30 minutes.
The rate of charring is little affected by the severity of the fire, so for an hour's exposure, the depletions are 40 mm for most structural timbers and 30 mm for the denser hardwoods. This enables the fire resistance of simple timber elements to be calculated. The predictive method is published in BS 5268-4 Fire resistance of timber structures.
The fire resistance of many timber structures, such as timber frame buildings, is achieved by a combination of the internal lining material, the timber structure and the insulation.
The terms reaction to fire and fire resistance describe different behavioural characteristics of a material.
Fire resistance describes the ability of a building element to provide its structural or compartmentation function in the event of a fire.
Reaction to fire refers to a material’s behaviour when exposed to fire and is used to establish what contribution the material will make to the fire’s growth and spread. It is relatively simple to improve the reaction to fire performance of wood-based products using standard treatments and coatings.
Surface spread of flame is the current UK test methodology used to establish the likelihood of flame travelling along the wall or ceiling surfaces to other areas of the building. Materials may be assigned a classification from Class 1 (little risk of fire spread) to Class 4 (some risk of fire spread). Untreated timber and panel products typically fall into Class 3, although fire retardant impregnation or coating systems can raise performance to Class 1.
The Fire Propagation test is used to determine the likely contribution a material will make to the growth of the fire. Essentially it measures the heat released from the material in a fire. A series of performance indices are provided by the test and these are interpreted in Building Regulations Documents as part of the requirements for Class 0 materials. Class 0 is not derived from any single test methodology but is a material having Class 1 surfaces (based on the surface spread of flame test) and suitably low fire propagation indices. Most timber and wood-based materials can achieve Class 0 when treated with a suitable fire retardant impregnation or coating system.
Non-combustibility is a pass/fail test that determines whether a material burns or not. Wood-based products, whether treated or not, are deemed combustible.
Materials of limited combustibility are also defined in Builidng Regulations. There are several means by which this classification can be achieved but wood-based materials do not fully satisfy any of them and therefore should not be used in situations where this is a requirement.
Publications from TRADA available from the Bookshop:÷
Timber and wood-based sheet materials in fire. WIS 4 – 11.
Flame retardant treatments for timber. WIS 2/3 – 3.
