Photo: Daniel Pesic
Medite Premier FR
Stephen Perse Foundation
© BASF Wolman GmbH
Flame Retardants for timber slow the rate at which a fire can develop - allowing more time for escape
When does wood need Flame Retardant protection?
All building materials experience negative impacts from exposure to fire: steel buckles, concrete spalls and wood burns. Wood has high structural strength and slow charring rate, so its general behaviour in a fire situation is fairly predictable.
However, in areas where there is a higher than average risk of fire, for example in public buildings such as cinemas, theatres, libraries, schools, offices, hotels and hospitals, building regulations are in place governing the use of performance rated, flame retardant building products.
Flame Retardants (FR) generally work by reducing the surface spread of flame, heat and smoke release, providing vital extra time for a safe escape. Factory application of a flame retardant treatment prior to installation ensures all faces of the timber or board product can be protected, offering a safer, low maintenance solution.
Because of this, the application of flame retardant products on the construction site, by brush or spray is NOT APPROVED by the WPA as quality control is almost impossible to assure.
Flame Retardants for Timber - An Overview
with signposts to WPA Guidance Notes
The fundamentals of flame retardant treatment for wood with references to relevant WPA resources for further detail.
This document replaces the WPA Flame Retardant Manual (issued in 2018)
Comprehending how a fire will behave in and around a building starts with understanding the stages in the development of a fire. Being familiar with the terminology used helps to interpret the subsequent fire performance ratings given to building materials and aids correct specification.
Reaction to fire
In the early stages of a fire, it is the 'reaction to fire' properties of the various individual materials exposed to flame that are the critical factors:
Ignitability – how readily will a material ignite and catch fire?
Spread of flame – once ignited, how quickly will flames spread across the surface of that material?
Heat release – once alight, how much heat energy will be generated by the burning material, which will contribute to the further growth of the fire?
Flaming droplets – will the burning material disintegrate and produce burning droplets or debris which might fall onto and ignite other surfaces?
There are four recognised stages in the development of any fire: ignition, spread & growth, flash point into a fully developed fire and eventual decay.
Flame retardant treatments enhance the 'reaction to fire' properties of wood and wood - based materials. Reducing ignitability and consequent spread of flame slows down the development of the fire and allows significantly more time for the occupants of a building to escape and for the fire to be extinguished.
The key differences:
Reaction to fire : the measurement of how a material will contribute to the fire development and spread, particularly in the very early stages of a fire when evacuation is crucial.
Fire resistance : the measurement of the ability of a building/construction element to resist, and ideally prevent, the passage of fire from one distinct area/building compartment to another.
Resistance to Fire
Once the fire is more developed, then containment becomes the top priority with the use of compartmentalisation as a common strategy in buildings. At this stage, it is the 'fire resistance' ratings of the building structure and its design elements such as walls, floors and fire doors that become the critical factors.
'Reaction to fire'classifications are completely different to 'resistance to fire' ratings yet the two terms are often misunderstood and taken to mean the same thing.
Another term which is misunderstood and misused but is commonly referred to within fire performance ratings is 'combustibility'. The official definition used in EN standards and building regulations is as follows:
For a material to be classed as either non-combustible or of limited combustibility it must achieve Class A1 or A2 in testing (see table below). A limited lateral spread of flame classification does not infer any resistance to combustibility, it is solely a measure of the rate of spread of a flame across the surface under defined conditions.
Using this classification system, any material rated Euroclass B or lower is therefore classed as combustible, albeit to varying degrees. Untreated wood-based materials normally have a Euroclass D or E rating. Depending on the system and loading used, this may be enhanced to Class B or C by the addition of a flame retardant.
Whilst it is not possible to enhance any organic substrate, including wood-based materials, to a Class A rating, flame retardant treated wood-based materials enhance safety, add value and are fit for purpose for many applications and in compliance with Building Regulations.
Fire Performance Ratings of Building Materials
Data obtained from reaction to fire testing, results in that material being given a Euroclass performance rating. Almost all fire testing carried out in the UK today is to Euroclass (EN 13501-1) reaction to fire performance criteria.
The Euroclass system also includes classification categories for smoke production (class s1 to s3) and creation of flaming droplets (class d0 to d2).
NOTE: The resistance to fire rating of a building element (in accordance with EN 13501-2), is normally expressed in ‘minutes of fire containment’ (e.g. a fire door with a 30-minute fire resistance rating).
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Service Environment and Treatment Types
Once you have established the Euroclass rating required of your project material, you also have to consider where and how it is being used in order to specify the appropriate treatment. Three service classes are defined in the structural design code BS EN 1995-1-1.
To correspond with these codes, the WPA categorises flame retardant products into three types:
INT1 (formerly DI: Dry Interior) for dry interior applications
INT2 (formerly HR: Humidity Resistant) for humid interior or fully covered exterior applications
EXT (formerly LR: Leach Resistant) for all interior and exterior applications.
These products will be more commonly known under their respective brand names - listed in the WPA Benchmark Approved Formulations. If you have a preference for a specific product (ie. in terms of suitability for species or component) you can add this at specification.
NOTE WPA FR Types use the same framework as those found in BS EN16755: Durability of reaction to fire performance - classes of fire retardant treated wood products in interior and exterior end use applications.
Cladding Compliance Toolkit
RISCAuthority is an annually funded research scheme administered by the Fire Protection Association and supported by a significant group of UK insurers. They publish extensive guides and recommendations for risk management in the areas of fire and security.
Their Cladding Compliance Toolkit is designed to provide a simplified overview of the fire performance requirements of external cladding systems (including Rainscreen and ETICS type cladding, but not curtain walling).
It consists of downloadable flowcharts for England, N Ireland, Scotland and Wales with signposts to additional resources.
Performance Credentials, Audit Trail and Fitness for Purpose
In terms of specifying a flame retardant treatment or wood-based flame retardant treated product it is vital to verify the product’s performance credentials – ie: what fire performance properties are claimed by the manufacturer? All performance claims should be independently verified and reflect the performance required.
Make sure you get what you have specified. If performance in accordance with a European Standard is required, evidence should be obtained to support your product choice, such as a Fire Classification Report (in accordance with BS EN 13501). This details timber species, component thickness and intended use. In addition to this the product will need either a UKCA or CE Mark –mandatory under the Construction Products Regulation (CPR) and a Declaration of Performance (DoP).
Always check that the description of the material given in the Classification Report can be taken to apply to the material to be used in the project. Different species and cross-sectional sizes do affect fire performance ratings and require an Extended Application Report.
3 easy steps to ensure flame retardant timber treatments are fit for purpose:
Specify the Fundamentals
What fire performance is required - Euroclass B or C?
What is the service environment / treatment type?
Internal dry (INT 1)
Internal humid (INT 2)
or External (EXT)
Specify a named product if desired.
Verify Product Performance
What fire performance properties are claimed by the product manufacturers being considered?
All performance claims should be independently verified.
Such data can be found in the certificate for each WPA Approved flame retardant.
The Audit Trail
Make sure you get what you specified - ask for the following:
Classification Report(s) from an Approved or Notified Body in accordance with BS EN 13501
UKCA or CE Mark and a Declaration of Performance (DoP)
The most widely used process for treating both solid wood components and panel products with flame retardants is vacuum-pressure impregnation. This is carried out by specialist companies in large pressure autoclaves under factory controlled conditions.
For panel products such as OSB particle board, plywood and medium density fibreboard (MDF) a WPA approved flame retardant can be incorporated as an integral part of their manufacture.
Formulations for surface application are only approved by the WPA when they are applied in a factory under third-party quality assured processes to achieve the performance set out in fire test Classification Reports.
The application of flame retardant products on the construction site, by brush or spray is NOT APPROVED by the WPA as quality control is almost impossible to assure.
Find out more in the Application Technologies section
WPA Approved Suppliers
- find FR formulations, treaters, products and distributors
The WPA Benchmark Quality Approval schemes cover formulations and process treaters.
Most WPA members are working towards a Benchmark accreditation which provides industry recognised, independent verification of a product or treatment's performance claims – learn more about WPA Benchmark.
For WPA members who supply Benchmark accredited flame retardant process treatments or finished products, using a WPA Benchmark Approved Formulation is a mandatory requirement.
Click the formulation product logos below to view the WPA approved FR formulation certificates. They include details of certificated materials – timber species and thicknesses.
WPA Benchmark Approved FR Formulations and associated WPA member treaters
Click product logos to view formulation certificates
*NOTE: Formulations approved for EXT and INT2 types are also suitable for INT1 applications
WPA FR Build
The WPA FR Build scheme is operated by WPA for the Structural Timber Association (STA). It provides third-party certification of the factory process used to apply STA approved flame retardant on timbers used in the construction phase of a building. This scheme does not confirm compliance of the chemical formulation used with STA performance requirements - that is the responsibility of the chemical manufacturer.