Timber Species

TOPIC CHECKLIST

timber species - Wooden Stair Case

  • Does it have the right strength qualities?
  • How durable is it?
  • What does it look like?
  • Will it last?
  • Are there legal and sustainable supplies?
  • Is the species suitable for the end use?

Which species of wood should I choose?

 

Consider these points:

 

  • The end use – for example, is the wood to be used as a structural material and then covered with something else, i.e. studding covered by plasterboard, or will it be exposed?

 

  • What strength is required? Does the timber need to have a high bending strength, such as a joist, or a high tensile strength where the timber is stretched in the application?

 

  • Is the wood to be used purely for a decorative effect? Is this to be a dark or light colour?

 

  • Is the wood to be machined? Some species are more easily machined than others.

 

  • Is the wood from a certified legal and sustainably-managed forest source (i.e. FSC or PEFC) or is it from a source that is making progress towards certification (i.e. Verified Progress)?

 

  • Cost. It may look nice, but is it worth the additional cost, if another less expensive and more commercially available timber can do the same job?

 

  • Durability and treatability: is it necessary to use preservatives?

timber species Hard Wood

Moisture movement

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

Durability

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’

Treatability

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’.

Modified timber

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.

 

 
Accoya timber species

Tannin stain

 

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.

 

Further information and advice

Sourcing sustainable timber

 

Available species

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

Sustainable timber

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.