InSite | Specifying the use of timber externally

Specifying the use of timber externally

Published date: 29 August 2023

Download our Technical Manual

Download a PDF version of this article.




What specification of timber can be used externally?




Timber has a wide range of uses within construction, however it can be confusing when you’re trying to specify the correct type. It’s important that timber which is chosen is suitable for the location, weather and environment. In order to ensure the correct timber is chosen, both the durability and use class need to be considered. In addition, if the timber is to be treated, it should be suitable for the intended use.


Durability class



The durability class is the ability of the timber of a given species to resist decay and infestation by insects. The timber durability class of a given species gives a good estimate of how long the timber will last and it is measured in years. It is important to note that timber durability classes relate only to the heartwood of any species and not the sapwood which is considered class 5, non-durable for all species of timber.


Heartwood relates to the inner dead part of a stem or trunk and it provides the structural support for the tree. The sapwood is youngest part of the stem or the trunk and it transports nutrients and sap. The durability classes do not however taken into account other factors such as moisture, temperature, competency of the installation.


TRADA defines five durability classes for timber and they are:



Durability Class


Timber Life


Very Durable

25+ Years



15-25 Years


Moderately Durable

10-15 Years


Slightly Durable

5-10 Years


Not Durable

0-5 Years


The above should only be used as a guideline as local conditions and some of the factors discussed above can play a role in timber durability, however in general, more durable timber species will last longer than less durable species of timber.


TRADA have a useful webpage which lists different wood species and their durability class; it can be accessed from the TRADA website.


Please note, although oak is considered to be in a durability class of 2-4 we do not accept its use in all circumstances, particularly where it forms part of the external wall of a structure. For further information, please see Appendix C of our Technical Manual. Your warranty surveyor should be informed as soon as possible if you intend on using oak in your project.


Use class


Alongside durability, the use class of the timber should be carefully considered. The end use of the preservative-treated timber is classified into 5 categories by BS EN 335-1 and is widely used in the construction industry to help determine the level of treatment (more on that below) that is required based on the environment the timber is to be used (or its use class).



Use Class

General Use Situation


Interior, dry


Interior, dry, occasional but not persistent wetting. Condensation of water on the wood may occur


Exterior, above ground, exposed to the weather. Sub-divided into:

3.1 Limited wetting conditions (coated)

3.2 Prolonged wetting conditions (uncoated)


Exterior in ground contact and/or fresh water.           


Permanently or regularly submerged in salt water 

Treatment process


Preservative treatment provides natural wood with additional durability. However, not all pressure treated wood is the same, the level of preservative protection could be very different. That’s because the British Standard for wood preservation – BS 8417, requires that the loading and penetration of preservative, impregnated into the wood, is tailored to the desired end use (the above use classes). Two main types of treatment processes exist, these are high pressure and low pressure.


High Pressure vacuum treatment is good for getting timber suitable for external use and it can provide the timber with a 15-60 year service life. The preservatives are forced deep into the cellular structure of the wood and additional additives can be added to change the colour of the timber or to aid in water repellency.


Double vacuum low pressure treatment is used for use classes 1, 2 and 3.1 (Coated) and can deliver a service life between 30-60 years. The treatment provides an effective envelope of protection around the timber.


As per the guidance in our Technical Manual: BS 8417 should be used when determining the preservative treatment required. Tables 1 to 3 of BS 8417 must be referred too to identify ‘Use classes, ‘Service Factor code’ and durability class of wood for the desired service life.


Please note, it is important that any pre-treated timber be re-treated if it is cut to expose untreated end grain. The treatment should be coloured so it can be proven that the end grain has been treated.



The below table has been taken from Appendix C of our Technical Manual and it shows various timber components, their minimum natural durability recommendations for a desired service life of 15 or 60 years and their use class.


BSI Standards - Preservation of wood


Service life 


For warranty purposes, building elements will require to achieve a minimum ‘service life’ depending where in the structure it is and its function.  Please refer to the table at the end of Appendix C1 of our Technical Manual for further guidance when identifying the minimum durability class of wood.




For timber that is used externally, the most appropriate durability class, use class and preservative treatment to BS 8417 should be specified based on the required service life.



Read more articles on InSite and sign up to receive our blog round-up.

Every care was taken to ensure the information in this article was correct at the time of publication (July 2023). Guidance provided does not replace the reader’s professional judgement and any construction project should comply with the relevant Building Regulations or applicable technical standards. For the most up to date Premier Guarantee technical guidance please refer to your Risk Management Surveyor and the latest version of the Premier Guarantee Technical Manual.
Ref: TS-3089

Read more technical articles on our Resource Hub