Charred timber cladding comes in a myriad range of wood species, finishes and performance claims. This guidance from the Timber Decking and Cladding Association (TDCA) will help designers better understand charred timber and the claims made for its performance.
Charring the surface of wood with fire is not new. The technique originated in 17th century Japan to waterproof Japanese cedar (Cryptomeria japonica) – the country’s main construction timber. The face of sawn cedar was exposed to fire until its surface became charred. The was followed by the application of a natural oil. This produced a stunning charcoal-black water-resistant surface. Depending on the cross-section size of a sawn board, and burn intensity, a deeper depth of charring and a crackled surface was achieved. It was believed at the time that deeply charred cedar also resisted fungal decay, insect attack and was fire-proof.
The 21st century reincarnation of charred timber produces equally beautiful surface effects but the technique is no longer restricted to Japanese cedar. An extensive range of timber species is available with different burn intensities and a wide variety of surface aesthetics achieved by finishing techniques like brushing, colour staining and surface coatings.
It must not be assumed that protection against fungal decay, insects or fire is a benefit of the charring and finishing processes used today. Various scientific studies conclude that charring does not enhance durability or fire performance. If a manufacturer’s declaration of performance includes such claims then they must be able to provide independent evidence to support the claim. Charring processes should be considered visual enhancements only. If durability or fire performance is important, naturally durable wood species or other wood protection processes need to be considered.
Durability
Durability is the ability of wood to resist biological degradation by fungal decay or insects. In the UK, fungal decay is the main threat to wood used out of doors. It is caused when wood is persistently wet. That’s why it’s important to eliminate water traps when designing a timber clad façade and to choose charred timber that has the appropriate durability for the service life that is required.
The default service life for external timber cladding in British and European Standards is 30 years. As such, timber with the ability to achieve this service life must be the base material for charring and finishing. This means using either:
- a species with appropriate natural durability (as defined in BS EN 350) such as cedar or a hardwood like cumaru or jarrah or;
- a species where the appropriate durability is achieved by treatment e.g. softwoods like pine or spruce, pressure impregnated with a wood preservative or modified woods like Accoya, Abodo and Thermowood.
Fire performance
Claims that charred timber cladding has flame-retardant properties must be treated with caution. Whilst the charring process creates a layer of carbon on the surface of a cladding board it is not capable of delivering a verifiable Euroclass flame-retardant rating and is often removed by a brushing technique to enhance the wood grain and prepare the surface for a factory-applied colour stain or weather protective coating.
Any resistance to ignitability that a heavy charring process is claimed to confer will only apply to the face of the board; the back of the board and cross-cut ends are not affected. If fire performance properties are required then the factory application of a flame retardant prior to final finishing ensures all faces of the cladding board have the same fire performance properties. This performance must also be confirmed by a reaction to fire test certificate from a UKAS accredited test house.
A word of caution; the application of flame-retardant coatings on site by brush or spray is not recommended because the quality control of the thickness of the coating (and hence its effectiveness) is almost impossible to achieve.
It is the responsibility of the business that supplies the cladding to provide a reaction to fire test certificate for the specific wood species, board profile and installation design that is to be used. A test for a different species, board profile or installation configuration is a contravention of Building Regulations and must never be accepted.
The fire testing of timber is almost exclusively based on the test criteria in BS EN 13501-1. This Standard has superseded the national standard BS476 which is no
longer referenced in Building Regulations. Testing results in a product being given a Euroclass fire performance rating. Untreated timber typically has a medium
(Euroclass D) to high (Euroclass E) contribution to fire rating. This can be improved significantly by flame-retardant treatment to Euroclass B (very limited contribution to fire) or Euroclass C (limited contribution to fire).
For more on flame-retardant treatments for wood check out the free guidance on the website of the Wood Protection Association
Studies of charred timber performance
In 2017, Coventry University graduate Matthew White wrote his thesis on charred timber cladding: Charred Timber Cladding: Performance Investigation (Coventry University). White’s work examined various properties including flame spread, resistance to UV fading and exposure testing.
Fire performance tests showed no appreciable improvement in fire performance whilst UV resistance in the test used was good. Durability performance was not covered in the scope of the thesis.
White concludes that ‘charring is a useful surface finish for external timber cladding but should not be used in lieu of industry recommended detailing of timber façade systems, or as a solution to fire protection and improved durability. From a timber cladding specification viewpoint, charring should be treated as a decorative finish’.
Likewise, a US Department of Forestry paper entitled Durability and Fire Performance of Charred Wood Siding (Laura E Hasburg et al) concludes that charring ‘should be thought of as primarily increasing the aesthetics of wood rather than improving the durability or flammability’.
This paper did explore durability in fungal decay tests and concludes that there was no measurable performance difference between uncharred and charred.
Maintenance
Close up Courtesy Accsys
It’s a mistake to assume that charred timber cladding is maintenance-free. Charred timber is more about visual appearance than anything else.
Almost all charred timber cladding comes with a finish that seals the carbon layer or burn-effect to protect the surface from weathering. The initial weathering protection provided by this finish will last several years before it needs to be refreshed. The number of years between maintenance recoats depends on variable
factors such as:
- the degree of exposure of the cladding to UV light and weather extremes
- the type of wood that was charred – some wood types are more dimensionally stable than others, which helps any protective coating last longer
- the type of burn finish, crackled, brushed, colour stained
- any additional layers of coating applied immediately after installation.
Because of these variable factors it is vital that the guidance on installation and maintenance of the manufacturer of the charred timber cladding are followed to the letter and passed on to the owner of the property.
TDCA approved sources
Charred timber cladding made from highly durable wood types such as preservative treated larch or well-known modified wood brands are available from TDCA members.
Article written by Janet Sycamore of TDCA
