Wood preservatives

Whilst some timber species have inherently durable properties, other cheaper species don’t.  One method of increasing the life expectancy of less durable species when used in ground contact or in exposed locations, is to use preservative treatment.

 

Using our FAQs and simple guides, let us help you understand the key issues surrounding this topic.  Visit out Wood Species guide to find out the durability ratings for all commonly used species.

 

For testing, inspections and training on preservative treatment, call our technical helpline on +44 (0)1494 569601.  All commercial services are provided by TRADA’s service provider, BM TRADA.

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How do I decide whether a wood product requires preservative treatment?

Consider the following when judging the need for preservative treatment, the type and the level of treatment required:

  • the in-service conditions
  • service factors
  • desired service life
  • the natural durability of timber species.

Refer to WIS 2/3-16: Preservative treatment for timber - a guide to specification for guidance.

Key sources for information are:

  • to establish the ‘Use Class’ of the timber in its service environment BS 335:2006 Durability of wood and wood-based products. Definitions of use classes. General 
  • to determine whether the sapwood and/or heartwood requires additional protection BS EN 350:1994 Durability of wood and wood-based products. Natural durability of solid wood and BS 8417:2011 Preservation of wood. Code of practice.

In some applications, i.e. timber frame construction, where decay consequences are perceived as high risk, insurers require structural timbers to be preservative treated even though they are considered to be in Use Class 1.

Timber in ground contact (Use Class 4) is considered to be permanently wet and at high risk of decay. Examples of timber products included in this category are fencing, deck posts and service poles.

The heartwood of durable timbers can be used in Use Class 4 for a 15-year service life. Very durable timbers should be specified if a 60-year desired service life is required. See BS EN 350-2 Durability of wood and wood-based products. Natural durability of solid wood. Guide to natural durability and treatability of selected wood species of importance in Europe for guidance on species selection.

Refer to the following for further advice in achieving desired service life:

  • BS 8417 Preservation of wood. Code of practice 
  • BRE’s Digest 429: Timbers: their natural durability and resistance to preservative treatment
  • Commodity Specifications (C1 to C12) in WPA’s Manual: Industrial Wood Preservation
  • NBS Z12 Preservative and Fire Retardant Treatment.

The preservative treatment products and cycles typically used will not have a significant impact on the mechanical properties of timber.

The performance of water-borne and solvent treatments does not significantly differ. The liquid simply provides the delivery vehicle for the active ingredients.

Solvent-based treatments have the advantage of not raising the moisture content of the timber so do not impact dimensional stability, or cause raised grain.

Health and safety needs to be considered when handling freshly treated products, with greater care needed for solvent products.

Some treatments may contain additional ingredients that are included to slow down the weathering process. However, without a maintained coating, the timber will still weather to a silver grey colour if left exposed to the elements.

Treatments may also help to prevent mould growth.

See WIS 2/3-60 Specifying timber exposed to weathering for guidance on coating treatments available to protect external timber from weathering.

End grain is particularly vulnerable to decay. See WIS 4-28 Durability by design for additional advice on mitigating this through design.

Manufacturers of pressure applied preservatives produce end grain treatment products, or can recommend particular treatments.

Brush applied preservatives can be used, and should be applied liberally by brush or by dipping.

Depending on the end use, a suitable end grain sealant may be required to stop the treatment from leaching out of the timber. If posts are cut to length, the cut end should not go into the ground, and should ideally be capped or sealed as well as re-treated to provide adequate protection.

Domestic quantities of treated wood products that are no longer usable, e.g. off-cuts, broken timbers, sawdust or treated timber components are not considered hazardous waste. The wood may be disposed of in landfill sites, burned in commercial and industrial incinerators or boilers in accordance with national and regional rules and regulations.

Large quantities of treated wood waste may require the services of a waste contractor for removal and disposal. Information and guidance can also be found on the Environment Agency website.

NOTE: Timber treated with CCA wood preservative or creosote must be considered differently. Incineration must be in compliance with the Waste Incineration Directive (see www.defra.gov.uk).

A number of sources of information are available to help you specify an appropriate preservative treatment. These include:

  • The WPA Manual Industrial wood preservation specification and practice 2nd edition. This includes specifications for a range of commodities including fencing timber, timber framed housing and decking timber.
  • BS 8417:2011 Preservation of wood. Code of practice.
  • Preservative manufacturers are also able to provide guidance on specification of treatments using their products.

See also WIS 2/3-16: Preservative treatment for timber - a guide to specification for general guidance.

Most glulam beams are made from slightly durable softwoods, and sapwood may often be included. These will need preservative treatment similar to that of solid section timber. However there are some practical considerations to take into account when specifying preservative treatment, in particular the levels of penetration and retention that can be achieved.

Glulam beams are less suitable for pressure treatment with water-based preservatives than solid wood, since the wetting may cause excessive deformation or checking. Treatments may be applied as surface applications combined with a surface coating, although this is less effective than pressure treatment and relies on frequent maintenance to achieve a good service life.

Modified woods or durable timbers can be specified as an alternative to preservative treatment.

For further guidance see WIS 2/3-16: Preservative treatment for timber - a guide to specification and BS 8417 Preservation of wood. Code of practice.

The Wood Protection Association and manufacturers’ advice can also be sought.

A certificate of treatment should be available for preservative treated products, and this should include the following information:

  • type of preservative applied to timber
  • method of application
  • intended Use Class
  • company that applied the treatment.

Most types of commercially available treatments are invisible and there is no specific requirement to mark timber as treated. Copper organic treatments give timber a green colouration, though brown dyes are added to some products.

For some treatments, spray-on reagents can detect the presence of certain components, but these are not able to determine the loading, and consequently whether the appropriate critical values have been achieved.

TRADA membership

As a member of TRADA you can enjoy free access to our entire online collection of technical guidance on not only decking but all other construction related topics.

You can also use our Technical Helpline (01494 569601) for free at any time during office hours. (Please note that advice is limited to 30 mins per call. Commercial consultancy by BM TRADA can also be provided where written or professional opinion is required on specific products or projects.)

 

Introductory guidance

You can register for free and download some of the guidance below:

 

Training

We run introductory and intermediate training courses on a range of timber topics. In-house courses can also be provided.

 

Consultants and commercial services

Exova BM TRADA is able to provide a wide range of commercial services including product testing, certification, CE marking, and product design advice.