Consider these points:
For structural purposes movement is not usually significant, but if you require stability in varying humidities (e.g. decorative flooring), use a species with small movement. These classifications are not directly related to the shrinkage of green timber
Refers to resistance to fungal decay of the heartwood only. Sapwood in most species is generally not durable and should not be used in exposed conditions without preservative treatment. Classes referred to in BS EN 350-1 are:
Class 1 – ‘very durable’
Class 2 – ‘durable’
Class 3 – ‘moderately durable’
Class 4 – ‘slightly durable’
Class 5 – ‘not durable’
Refers to how easily timbers can be penetrated with vacuum pressure preservative treatments. The four levels of treatability in BSEN 350-2 are ‘easy’, ‘moderately easy’, ‘difficult’, ‘extremely difficult’.
A number of brands of timber are now available that have been modified chemically, such as Accoya™, or by heat treatment, such as Thermowood.
Generally, these products provide the sustainability of softwoods with the stability and durability normally associated with hardwoods.
The different modification processes affect the performance of the timbers in different ways. Consult the manufacturer for specific details.
Tannin is natural in softwoods and hardwoods. For example, oak and Western red cedar will exude tannin as they dry, which may give the appearance of a black deposit. As a result, water running off these surfaces can leave staining, particularly around metal fixings.
Sourcing sustainable timber
Consult your local timber merchant or Timber Trade Federation member www.ttf.co.uk
Swedish grown species: www.swedishwood.com
UK grown species: www.forestry.gov.uk
Technical information: www.trada.co.uk
Timber is the most sustainable mainstream building product. It is naturally renewable. Over 90% of timber used in UK construction comes from Europe, where more trees are grown than harvested (source: TTF Statistical Review 2016).
Softwood and temperate hardwood forests in Scandinavia, Europe, Canada and North America are stable or growing. Growing forests act as carbon sinks; wood products act as carbon stores.
Ask for PEFC or FSC Chain of Custody certification.
See Wood Campus RIBA CPD module Procuring Sustainable Timber for more on timber certification and sustainability and government requirements.