Timber is natural, sustainable and environmentally superior with inherently high levels of CO2 capturing performance compared to other construction materials.

Despite this, it often receives a negative reaction from the building industry due to its perceived limitations when it comes to its reaction to fire. This is where using the correct Fire Retardant (FR) treatment can make a difference. With the correct treatment, timber has excellent FR properties and its reaction to fire is extremely low.

What’s the difference between Reaction to Fire and Resistance to Fire?

• Reaction to Fire is how the material will allow a fire to spread and is a consideration in buildings where evacuation time is critical.
• Resistance to fire is how the material will perform when on fire, i.e. how long will it retain its structural integrity. This is particularly important for floors and ceilings where the material may lose its integrity causing it to collapse.

Setting the Standard

Industry regulations are in place to protect people’s safety, health and welfare. However, sometimes it can be confusing. Below is a quick guide to help explain the jargon.

• FR treated solid wood (timber cladding) and plywood must be supplied with a CE mark as a legal requirement under the Construction Products Regulation (CPR) EU no. 305/2011.
• The relevant harmonised standards are BS EN 14915:2013 (solid wood) and BS EN 13986:2004 (plywood).
• If a product has a CE mark, it means that the method in which it has been produced and the properties of the product meet certain criteria as defined in the harmonised standards.

FR treatment process

As with the above standards, there are stringent processes that must be followed when treating timber. The manufacturer or person significantly altering the product (this would be a timber treater in the case of FR treatment) must have a clearly defined manufacturing process. This process must be audited by a notified body which will initially grant certification. They will also be responsible for ongoing surveillance to ensure the accreditation is maintained.

The FR chemical manufacturer will provide the treater with a fire classification report, by species, which is issued and audited by a notified body under AVCP* system 1. The notified body can then audit the treater under AVCP* system 1 against the FR classification report. They can then grant the FR treater with a CE certification for their process. This comes in the form of a Certificate of Consistency of Performance (CoCOP) and allows the treater to issue their customers with a Declaration of Performance (DOP) with each supplied batch of treated timber. The DOP ensures full traceability of the FR treated timber when it enters the supply chain.

DOP’s are species specific. Different species have different cell structures and/or densities which can have a significant effect on the quantity of chemical required during pressure impregnation. As a result, the FR treatment process varies depending on the species in terms of treatment time and pressure. It is therefore essential that the species details are provided by the treater’s customer and the treater has the appropriate DOP to allow them to treat the specific species in question.

Understanding COP and DOP Documents

Typical COP and DOP will contain the following information:

• Product: details of the timber species, e.g. Larch; what it is, e.g. solid wood panel, tongue and groove profiled; and what fire retardant treatment has been used e.g. Burnblock. Classification is valid for the end use as cladding or as support for cladding elements.
• Thickness: what is the nominal thickness of the material i.e. 21,5mm.
• Density: what is the timbers nominal density range i.e. 650 - 750 kg/m3.
• Intake: how much dry FR material must be absorbed in line with the approved factory control process. Dry amount of FR 36.5 kg/m3.
• Substrate: details of the substrate that must be used to mount the timber onto any substrates in classes A1 and A2-s1, d0 of at least 9 mm thickness and with a density equal to or greater than 653kg/m3.
• Fixation: how cladding should be fastened, e.g. mechanically fixed to the substrate.
• Air gap details of whether an air gap (typically 40mm) behind the panel is required.
• Mounting: how panels can be mounted, e.g. horizontally, vertically or both.
• Joints: how panels can be joined e.g. vertical, horizontal joints or both.
• Reaction: reaction to fire classification: what rating has been provided for the above conditions, e.g. B-s1,d0

Where first letter (B) refers to its fire rating

Where “s” refers to its smoke rating, with 1 being the least smoke produced. Although s1 is highest achievable on timber (some uses will only allow minimum smoke production, e.g. London Underground due to safety considerations)

Where “d” refers to the amount of flaming droplets produced when subjected to a flame, with 0 being the lowest number produced, i.e. no droplets produced

Assessment and Verification of Constancy of Performance *AVCP Explained

AVCP is a harmonised system defining how to assess products and control the constancy of the assessment results. This system safeguards the reliability and accuracy of the Declaration of Performance.

• Assessment of performance is a one-time activity
• The Verification of Constancy of Performance is an on-going process
• System 1, e.g. FR treater – Product certification comprising the issuing of a constancy of performance with determination of the product type and continuous surveillance by a notified product certification body.

Further reading references

Trada wood Information sheet (WIS 4-11) – Wood-based panel products and timber in fire
Trada wood Information sheet (WIS 2/3-3) – Flame- retardant treatments for timber products
14915: 2013 + A1:2017: Solid wood paneling and cladding - Characteristics, evaluation of conformity and marking.
EN 13986:2004+A1:2015: Wood based panels for use in construction – Characteristics, evaluation of conformity and marking.