Exterior Cladding
Product Type
Subcategory
Exterior timber cladding is growing in popularity in recent years, as architects and designers embrace the practical and aesthetic advantages that it offers. As a strong, lightweight material, timber offers many different design options and finishes to create a distinctive look on any building.
Timber is a versatile material that brings warmth to any building, evoking a strong connection with nature while also satisfying the need for low-carbon, environmentally aware construction techniques.
All external timber cladding is classified as use class 3 this means that timber is exposed to frequent wetting but is out of ground contact. The external timber cladding assembly should have sufficient natural or conferred durability for use class 3. This can be achieved by either:
- Selection of heartwood of appropriate natural durability.
- Application of preservative treatment to less durable but permeable species
- By using modified wood of appropriate durability.
There are many different types of exterior cladding on the market including softwoods, hardwoods and modified wood brands. These include:
- Hardwoods: Hardwoods used in external cladding are most often chosen where the natural durability of their heartwood is suitable for Use Class 3 applications. They may be temperate hardwood species such as European Oak, American White Oak and Sweet Chestnut as well as cladding made from tropical species such as Cumaru, Garapa Iroko and Red Grandis.
- Softwoods: There are only a few Softwood species available in the UK and Europe where the natural durability of their heartwood is sufficient for them to be considered as external timber cladding the main examples being Douglas Fir, Larch Western Red Cedar and Scots Pine. However, many other species of softwood can have preservative treatments readily applied to confer on them the durability necessary for Use Class 3 applications. Examples would be Scots, Lodgepole, Radiata and Southern Pine or Sitka and Norway Spruce.
- Modified Woods: this is a term used to describe solid timber (predominantly softwoods, but including some hardwoods) that has undergone a chemical, biological or physical process to enhance the timber’s natural properties – usually centred around its durability and resistance to decay. These modified woods are all individually branded products, such as Accoya Wood, Abodo, Kebony, Lignia and ThermoWood, among others. Always check the exact performance criteria with the supplier before ordering.
Specifications for naturally durable species should always exclude sapwood as this is always classified as non-durable and therefore unsuitable for external timber cladding. A greater range of durable tropical hardwood species may be available as cladding, machined to order with longer lead times, so check with your hardwood supplier for more details.
Few suppliers hold extensive hardwood exterior cladding stocks. Most will be machined to order, so it’s important to plan ahead and order your desired cladding in plenty of time. Check the architect’s specification and discuss with your timber supplier. Make sure your customer understands that lead times are involved, and get a written confirmation of the anticipated time to delivery from the supplier.
As described above naturally durable hardwoods will not require additional preservative treatments. Softwoods that do not have sufficient natural durability within their heartwood for use class 3 applications can be preservative treated following the guidance given in the national standard BS.8417. Care should be taken to ensure the penetration and retention levels stated in the standard are fully met.
Treatment plants that participate in Quality Assurance schemes such as the WPA Benchmark or NTR Nordic Treaters scheme will provided buyers with independent assurance that treatment requirements are being met. Buyers should also obtain a treatment certificate to accompany each delivery.
The most common UK standards that relate to timber cladding are:
- BS 8605-1:2014. External timber cladding. Method of specifying
- BS EN 14915:2013. Solid wood panelling and cladding. Characteristics, evaluation of conformity and marking.
- BS 8417:2011. Preservation of wood – Code of practice
Wood cladding is classified as Euroclass D as standard provided it meets the necessary density and thickness requirements. This can be upgraded to Class C or B with the application of appropriate factory applied flame-retardant treatments. Consideration should always be given to the Building Regulations to make sure that any exterior timber cladding specified is correctly risk assessed, treated where necessary and covered by the appropriate certifications.
It is not recommended that brush-applied flame retardant products are specified for exterior use, only those with factory applied timber treatments. If in any doubt, always seek further advice.
When dealing with flame-retardant treated cladding, any decorative coating applied must not downgrade the performance of the fire rated timber – check with product manufacturers.
You can learn more about exterior timber cladding in TDUK’s Timber Cladding Handbook
