Fire performance requirements
The fire performance for all materials used in buildings, including wood and wood based panel products, are set out in Building Regulations.
These regulations call for fire performance to be in line with British Standard or European Standard Test Methods. The principal standard used for assessing the reaction to fire performance of wood building materials is BS 476 'Fire Tests on Building Materials and Structures.'
Results from BS 476 fire test methods cannot be used to demonstrate compliance with a European product standard. European standards make reference to European fire test methods, the relevant ones for flame retardant treated construction products being EN ISO 11925 and EN 13823.
The test method most widely used for assessing the effectiveness of flame retardant treatments is BS 476 Part 7, which is used to classify a material in terms of the spread of flame across its surface under the test conditions. In general untreated timber or panel products fall into classes 3 or 4, i.e. a rapid surface flame spread. Treatment with flame retardant chemicals can upgrade such materials to class 1, i.e. a low surface flame spread. This level, the highest achievable with this test method, is the performance normally required for wall and ceiling linings.
In certain high-risk areas, such as escape routes, some building regulations require a Class '0' standard of performance, i.e. limited combustibility. To comply, materials must have a class 1 surface spread of flame and a low fire propagation index (less than 12) when tested to BS 476 Part 6. Certain wood based substrates can meet this standard of performance when treated.
British Standard test methods
The principal standard used for assessing the reaction to fire performance of wood building materials is BS 476 'Fire Tests on Building Materials and Structures.
Following the introduction of harmonised European fire test methods, Euroclasses based on performance in these tests. These are known as Euroclass A, B, C, D, E and F. Building Regulations Approved Document B has been revised to include Euroclasses for fire performance alongside the equivalent BS 476 classes.
The EU Construction Products Regulation requires all products demonstrate compliance with harmonised European Product standards (where they exist). Marking a product with the CE mark as a declaration of compliance. This is done in conjunction with a Notified Body.
Once a standard is mandatory, then compliance with Building Regulations is shown by use of the European tests alone, however, prior to this, compliance may be demonstrated by conducting either current national test methods such as BS 476 Part 7, or the European tests.
The final performance of the product is critically dependent upon the care and control exercised by the processor over the treatment process and also upon the quality of the chemicals used.
The WPA operates a quality assurance scheme for fire retardant products and also for the processes by which they are applied. The scheme is called the WPA Benchmark of quality.
How to specify
The WPA Fire Retardant Specification Manual provides detailed guidance about the selection of the most appropriate fire retardant treatment or treated panel product. It includes a Checklist to assist specifiers navigate the product selection process and provides model specification phrases that can be used to define the specific treatment in contractual documents.
The WPA Manual also defines five Commodity Specifications which cover the most typical situations. They are a simplified method by which the appropriate FR treatment is identified by the suppliers of FR treatments and are referenced widely by other standards setting bodies such as NBS and NHBC.
Model specification phrases
(a) Dry interior constructions (maximum 75% relative humidity): [insert material/species] to be treated to [Class 1 Surface Spread of Flame to BS 476 Part 7 or Class 0 BS 476 Parts 6 & 7, or Euroclass B or Euroclass C], using a * WPA Type DI dry interior process.
(b) Interior constructions (humidity resistant): [insert material/species] to be treated to [Class 1 Surface Spread of Flame to BS 476 Part 7 or Class 0 BS 476 Parts 6 & 7, or Euroclass B or Euroclass C], using a * WPA Type HR humidity resistant process.
(c) Full Exterior Exposure: [insert material/species] to be treated to [Class 1 surface Spread of Flame to BS 476 Part 7 or Class 0 BS 476 Parts 6 & 7, or Euroclass B or Euroclass C], using a * WPA Type LR leach resistant process.
* Where specifiers wish to specify the type or identity of fire retardant to be used, appropriate details can be inserted where an asterisk appears in the above sentences.