Despite being made of an inherently combustible material, choosing timber cladding for the exterior of a building does not present a big fire risk. When a large wood surface is ignited, the exterior surface chars. This process forms a protective layer which can prevent further combustion and spread of the fire. This mechanism is actually behind an ancient Japanese technique for wood preservation, called Yakisugi.

Untreated wood has a Euroclass fire rating of D (moderate contribution to a fire), which is appropriate for most applications in the UK. With a fire retardant treatment, the fire performance of timber cladding can be enhanced to a B (very limited contribution), making it suitable for certain applications where a specific fire rating may be required or desired.

Fire safety ratings: the Euroclass system

Euroclass is a fire rating system used in the UK and Europe. It was established by EN 13501-1, a European standard that specifies the classification of construction products and building elements based on their reaction to fire.

• A1 – No contribution to fire at any stage
• A2 – Very limited contribution to fire
• B – Very limited contribution to fire, but less stringent than A2
• C – Limited contribution to fire
• D – Materials with moderate contribution to fire
• E – High contribution to fire, but acceptable for some applications
• F – No performance determination, or fails to meet Class E

There are some additional classifications (‘s’ and ‘d’), which refer to:

• Smoke emission: s1 (low smoke), s2 (medium smoke), s3 (high smoke)
• Flaming droplets: d0 (none), d1 (some), d2 (high)

So, an example of a classification for a flame retardant-treated timber cladding product might be B, s1-d0, as in the example of St Mary’s Catholic Voluntary Academy.

A material receives this rating after undergoing a series of standardised tests and assessments at accredited laboratories. During these tests, real fire conditions are simulated and the material’s performance is evaluated, after which a classification report is issued.

Materials that have an A1 Euroclass rating, and therefore are completely non-combustible, include brick and stone. Untreated wood typically has a Euroclass rating of D, which can be boosted to B with a fire retardant treatment. 

Building fire rating requirements for wood cladding

In England, as of 2022, external surface cladding on ‘relevant’ buildings must achieve an A1 or A2 Euroclass rating. Buildings classified as ‘relevant’ include those with a residential purpose and a storey of 11m or more in height.

Examples of such buildings can include residential flats, hospitals, care homes, boarding school dormitories, student accommodation blocks, hotels, hostels and boarding houses.

An organic substrate like wood cannot achieve an A1 or A2 rating – the highest standard possible is B. This means that, even with flame retardant treatment, it cannot legally be used to clad these buildings.

For buildings not falling under these criteria, timber cladding is still approved for use, sometimes with a requirement for a treatment. This means timber can still be used for non-residential buildings up to (and beyond) 18m in height.

For more information, check out the Timber Decking and Cladding Association’s (TDCA) detailed breakdown on these amended timber cladding building regulations.

How fire retardant treatments work

This process involves pressure impregnation of the wood with natural, non-toxic substances. These prevent oxygen from reaching the cladding. The treatments create a surface on the wood that, once heated, can char and release water, which stymies the fire’s spread and prevents further combustion.

These treatments are non-corrosive and invisible, making no difference to the wood’s appearance, with no staining, streaks or oily films. Burnblock is one example of a fire retardant treatment that we offer with our timber cladding products – take a look at how the process works.

Does thermal modification of wood improve fire resistance?

There is sometimes a misconception that thermally-modified wood products like Thermo-Ayous, Thermo-Pine (popularly known as ThermoWood) and Thermo-Tulipwood have improved flame resistance.

While these modification processes do improve dimensional stability and outdoor durability, they do not themselves improve the timber’s fire safety rating from D, which is the standard for untreated wood.

Case study: St Mary’s Catholic Voluntary Academy

Just three years after the original building was the target of an arson attack, St Mary’s wanted to rebuild and create a biophilic space, designed to increase occupant connectivity to nature. They also wanted to operate as a carbon net-zero school.

Timber cladding was the perfect material for this. More specifically, Western Red Cedar vertical shadow gap secret nail cladding (our DTC14 profile).

During manufacture, the wood was impregnated with a non-toxic, natural treatment which gives it a Euroclass rating of B, s1-d0 under EN 13501-1 European standards.

Duffield Timber: safe, sustainable, visually-stunning timber cladding

We supply a range of solid wood cladding, machined from high-quality, sustainably-sourced species to a range of designer profiles.

Our cladding products can be pre-finished before delivery for superior outdoor performance. This, of course, includes the fire retardant treatments mentioned in this article.

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