What is Modified Wood?
Wood modification is a term used to describe solid timber that has undergone a chemical, biological or physical process. This essentially creates a new timber product with enhanced performance properties – including durability and dimensional stability. These relatively new technologies are providing a wider choice of material for designers and architects to consider – popular for cladding, decking, windows, doors, landscaping and structural applications.
Current commercially available modified wood technologies can be grouped into three main types
Performance of Modified Wood
Here we focus on durability and features of modified wood that may be different from preservative treated or naturally durable timbers. For further guidance on strength and dimensional stability please contact the relevant modified wood product manufacturer.
Specification of wood preservative treatment and naturally durable wood in the UK is made through a system of use classes and desired service lives.
The natural durability of a timber species against fungal attack is classified in terms of a durability class. This durability class relates to the resistance of the heartwood of the species and has historically been based on ground contact field testing and/or long-term practical experience.
The system by which durability against fungi is measured in preservative treated timber in ground contact is not universally applicable to wood modification as some modified woods perform well out of ground contact but poorly in ground contact.
Where the consequence of failure of a wooden component is high, say due to risk of human safety or high cost of replacement, a specifier may wish to choose wood with a higher durability class than might otherwise be considered so as to comply with their duty of care.
The durability class of modified wood can be measured in accordance with BS EN 350 as if the modified wood were the heartwood of a naturally durable timber. The most durable materials fall into durability class 1 whilst the least durable fall into class 4.
Field test data in support of performance claims is preferable to laboratory testing wherever practical and is essential for Use Class 4 and 5 applications. Where durability classes are used, the wood must be modified across the full cross section of the component. Some aspects of the sampling procedures given in BS EN 350 will not be applicable to modified wood, but should be followed as closely as possible.
The table gives the minimum durability class suitable for some common example end use applications. A modified wood substrate with a superior durability class will usually provide even better service life performance than that shown. Always check with the supplier which durability class rating their modified wood products carry.
Durability Class Recommendations for Wood Components
* NOTE: Some chemically modified wood products have sufficient field test data to demonstrate that they can achieve sufficient durability for use in ground and/or water contact. A minimum of 10 years field data is recommended. Please consult manufacturers for further information.
Performance in a Fire situation
The performance of modified wood products in fire can normally be enhanced using a secondary flame retardant treatment. Such treatments can also meet Building Regulations requirements, where applicable. The suitability depends on many factors including thickness and size of component. Please check for full details of recommended, independently assessed treatments and where they can be obtained. Check with product manufacturer for details and remember that all the specification rules still apply.