Wood Modification enhances performance properties - such as timber durability
How is wood modified?
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Wood modification is a term used to describe solid timber that has undergone a chemical, thermal 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.​
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Current commercially available modified wood technologies can be grouped into three main types
Characteristics of modified wood
For many end uses of wood, particularly where there is a risk of wood becoming wet,
durability (resistance to decay and insect attack) is seen as the key characteristic of determining its suitability for use. Most wood modification processes improve durability but that's not usually the only effect.
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Although a range of wood properties may be affected by the modification process, the suitability of modified wood for a given end use has been based on the three main criteria: durability, strength and dimensional stability.
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Durability: Service environments are categorised into a series of use classes in BS EN 335-1. Five classes define different service situations on the basis of the likely biological hazard from the moisture conditions which may prevail.
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Use class, service situations and suitable types of modified wood
*Generic assessment. Please check with manufacturers for product specific performance claims.
Dimensional Stability: Dimensional change under the influence of high humidity.
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Strength: A measure of the ability of wood to resist outside forces, such as compression, tension and shear. Different aspects of strength may be more relevant than others for particular end uses.
Please check with manufacturers for product specific claims regarding effects on strength.
Performance of Modified Wood
Durability Class
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.
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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.
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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.
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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.
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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.
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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.
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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.
Coatings & Quality
Coatings may be applied to modified woods using either factory or manual techniques but bear in mind the long-term performance and maintenance requirements. It is generally accepted the more dimensionally stable a material is, the more likely the frequency of maintenance is reduced. Most modified wood suppliers can supply a factory coating and often the finished product comes with an extended warranty.
WPA member Teknos are a coatings specialist who have dedicated products for wood.
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The properties of modified wood are very dependent on the modification process used. To be certain of consistent performance, a Factory Production Control system (FPC) with third party accreditation specific to each individual product and process should be in place. Where a product is covered by a harmonised European Standard (eg. cladding), FPC and CE marking would be required.
Modified Wood Products from WPA Members
Several WPA members manufacture and \ or distribute certain brands of modified wood. Simply select the brand logo to take you to the corresponding website of a WPA member who manufactures or distributes modified wood products. Refer to the these websites for further details regarding availability, specification and technical data.
A WPA Benchmark Product Approval scheme for modified wood used in ground contact is available. A quality scheme covering the production of modified wood is currently under development.
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Distributed in the UK by
Glenalmond Timber
Manufactured by Accsys
from Medite Smartply