Wood modification involves the action of a chemical, biological or heat upon a suitable species to deliver enhanced performance. The principal aim is to improve durability – resistance to fungal decay - and modified wood processes achieve this without the need to use biocides in contrast to wood preservatives.
Most modified woods used in cladding are generally rated as Very durable (BSEN350:2 Class 1) or Durable (Class 2) and capable of delivering a desired service life of 30 years or more. Durability against insect attack is also conferred.
Modified woods are usually more dimensionally stable than unmodified timber. Because modification processes usually change the entire cross section of the timber, products can be machined after the modification process without exposing unprotected areas. Other characteristics like strength and density can be affected but this is not normally an issue cladding applications.
It should be noted that much of the performance assessments of modified wood is often based on laboratory tests and so the degree of performance certainty associated with wood preservatives based on field test data and long term use has yet to evolve. The performance properties of modified woods are known to change in relation to the species, the process and variations in process.
The modified wood processes most suitable for cladding are:
Heat – the thermal modification of wood
Impregnation – modification of the water binding cells by a chemical.
Principal modified wood brands used in the UK for cladding