A CPD hosted by Architecture Today and Medite Smartply explored how architects can specify flame-retardant wood-based panels with confidence.

Flame-retardant, engineered wood-based panels have opened up endless possibilities for construction projects. But how does the technology work? What benefits do the panels provide? And what do architects need to know in order to specify them with the confidence? These questions and more were answered in this recent AT CPD webinar, in partnership with MEDITE SMARTPLY. You can now watch the video, which covers fire principles, understanding correct terminology, fire testing, certification and CE marking, compliance and regulations, as well specification of flame-retardant wood-based panels.

FR panels
Flame-retardant engineered wood-based panels are available as both structural and non-structural products, depending on the Service Class and end-use application. OSB is a structural panel typically used in permanent construction applications, such as flooring, roofing and wall sheathing. MDF has a wide variety of non-structural internal uses, including joinery, fit outs and furniture.

Fire principles
There are four main stages of a fire: ignition, fire growth, fire fully developed and fire decay. All wood burns, but an untreated wood-based panel contributes more fire load in a shorter timeframe, fuelling a much hotter fire. A flame-retardant (FR) panel, on the other hand, has a much lower burn temperature after ignition, and crucially a slower char rate, meaning it burns far more predictably and gives occupants more time to evacuate the building.

Terminology
FR stands for flame retardant – not fire rated, or fire resistant. Flame retardant treatments improve a material’s reaction to fire, which is the measurement of how a material or system will contribute to a fire’s development and spread. Fire resistance refers to the ability of a material or system to resist and ideally prevent the passage of fire from one distinct area to another.

Testing
Testing the fire resistance of a building element involves determining its behaviour when exposed to a particular temperature. The classes REI 30, REI 60 and REI 90 specify the time a building element must not be breached in case of fire – 30, 60 or 90 minutes. The performance of a building element can be derived from this classification. Evidence is provided in the form of a fire test conducted on the complete building element with a corresponding test certificate according to EN 13501.

Cconstruction products used as wall or ceiling linings or free-standing applications must pass two tests: ignitability and single burning item (SBI) before they can be given a Euroclass rating. Crucially, the SBI test setup and result must relate to the intended end use application. This is described in the classification report as the ‘field of application’. Construction products used as flooring must pass the Radiant Panel Test, which measures the spread of flame along the length of the specimen, as well as critical heat flux and smoke development.

Classification and certification
The fire classification report details the Euroclass rating that the product achieved. Products, other than floorings, are classified as A1, A2, B, C, D, E or F in accordance with BS EN 13501-1. The reaction to fire class for a FR enhanced wood-based panel product will normally be Euroclass ‘B’ or ‘C’. Euroclass B is the highest fire performance that a wood product can be classified and is described as having ‘very limited contribution to fire’. There is an additional suffix used to indicate a product’s suitability for flooring only, represented by a subscript ‘FL’.

The CE marking of construction products is mandatory under the Construction Products Regulation (CPR). For wood-based panels the Harmonised European Standard is EN 13986.  The product and CE marking are supported by the following documentation: technical data sheet, declaration of performance (DOP), fire classification reports, certificate of constancy of performance, WPA FR Benchmark certification, WPA ‘FR Build’ benchmark certification.

The ‘field of application’ is arguably the most important section of a fire classification report, since it describes in detail how the product was tested, and therefore how it will be expected to perform in a real-life fire. It is important that specifiers familiarise themselves with the test setup for a particular product and ensure that the field of application reflects what was tested in order to have full confidence that the product is fit for its intended end-use application.

Flame retardant treatments
There are three main types of flame retardant treatment commonly used to improve the fire performance of wood-based products: FR chemicals incorporated into the product during manufacture, post-treatment impregnation of a finished panel with inorganic salt solutions, and surface coatings.

When a flame retardant is added during manufacture, it will maintain flame retardance throughout the panel even when cut or drilled. Increasing a wood-based panel’s reaction to fire performance after manufacture is permitted under the CPR, however it is a very complex process. Pressure impregnation of wood-based panels will almost inevitably alter the structural integrity of the panels and invalidate the original DoP and associated CE mark. FR coatings may claim to enhance a wood panel’s reaction to fire class, but the Wood Protection Association (WPA) does not certify such coatings, unless they are applied under strict factory-controlled conditions with independent auditing, testing and certification in place.

FR OSB treated during manufacture is the simplest route to compliance with the Building Regulations. Until recently, Euroclass B OSB was not available – but now it is! The FR solution is added during panel manufacture and the final product is CE marked at source which guarantees both structural and fire performance.

Specifying flame-retardant wood-based panels
When specifying a product for construction, the reaction to fire performance must be determined for the application. This is laid out within Approved Document B. Requirement B2 – Internal fire spread (linings) is met by achieving a restricted spread of flame over internal linings. Requirement B2; Section 4 requires minimum Euroclass C-s3, d2 Reaction to Fire performance for walls and ceilings within a dwelling, and Euroclass B-s3,d2 in higher risk circulation areas. England, Wales, Scotland and Northern Ireland require products to achieve a defined performance within certain areas of buildings. In all cases the National test evidence (BS) is accepted alongside the European classification evidence (EN).

When specifying a flame retardant product, the service class environment must be determined. The three types of system may be distinguished by properties which limit or recommend their use in specific circumstances. The variation in these properties is largely due to the nature and complexity of the FR chemicals used in the formulations. FR wood-based panels are not suitable for service class 3 exterior use unless appropriately tested and certified. It is very important that wood-based panels are maintained in the relevant service class to which they were specified.

Benefits of FR timber
The main benefits of using FR timber over other types of material that could be non-combustible are as follows:
• Timber can be made safe by methods including flame retardant treatments
• Sustainable timber is renewable and has far lower embodied carbon than comparable building products, such as steel, concrete or aluminium
• The embodied energy of timber is much lower than other comparable building material
• Timber is a durable and recyclable

Conclusion
It is vital that any flame retardant treatment incorporated into a building provides long-term, predictable performance backed by independent verification. Before specifying enhancement using a flame retardant, two specification fundamentals must be established: ‘what reaction to fire is required? And ‘what is the service environment?’ Then you need to determine the end use application and check that that the documentation received from the manufacturer is independently verified and includes all the necessary information, including the field of application. Finally, once the product is delivered, ensure that what you have received is what was specified.