Why does wood need preservative protection?
All mature trees have an inner core known as HEARTWOOD surrounded by an outer layer of younger SAPWOOD. The sapwood is where the tree stores the nutrients essential to growth. These food reserves remain after the tree is logged and sawn into components.
The heartwood of some species contains naturally occurring chemicals that make it relatively durable – the ability to resist decay and insect attack. The degree of natural durability varies from species to species.
Sapwood on the other hand is a source of food for many species of fungi and insects and is always vulnerable to attack.
The risk of attack increases significantly if the moisture content of wood, for any reason, rises above 20% – for example poor installation practice or maintenance, persistent condensation and damp.
Increasing optimisation of forestry management practice and production yields mean that the timber we use to build with today is more likely to contain high proportions of low durability sapwood than in years gone by.
Making the most of a valuable resource
But that’s where wood protection technology comes in – with the addition of a preservative treatment wood can be made highly durable, offering reliable longer term performance.
Pre-treatment allows the use of the more perishable softwood species and non-durable (sapwood) parts of a tree where they might otherwise be discarded or have a short service life – making the most of the timber resource and contributing to waste minimisation and sustainability. In addition, this reduces pressure on the more naturally durable, scarcer and higher value species.
Chemicals and the environment
The preservative industry is highly regulated and manufacturers continue to introduce innovative products which are better targeted towards the organisms we wish to control and more environmentally benign. Wood protection formulations tend to chemically bind into the timber after impregnation and are not free to readily escape in service.
At the end of its service life treated wood can be re-used or recycled for approved applications or disposed of in a way that minimises any potential for environmental damage. The benefits gained by using treated timber can be balanced against any relatively low negative impacts, especially when looking at the bigger picture – see Building with Wood.
Desired Service Life
and Use Classes
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When specifying a preservative treatment, the level of protection required depends on a number of aspects:
The timber species - its durability and ability to accept treatment
Where the timber component is to be used - eg, fence post, cladding etc
For how long the component needs to perform before it is either replaced or taken down - usually 15, 30 or 60 years.
This last point may be influenced by access to the component or project site in addition to cost restrictions.
The minimum standards for the treatment of wood are set out in British Standard BS 8417 (2014) and the WPA Code of Practice - Industrial Wood Preservation (January 2021). These standards consider all of these elements and gives guidance on the loading and penetration of timber preservative, to ensure treated timber is fit for its desired end use.
Working in tandem with BS EN 335 Durability of wood and wood-based products, it groups the applications for treated wood into Use Classes. The most commonly used classes for preservative treatment being 2, 3 and 4.
The allocation of a commodity to a particular use class assumes that good design, installation and maintenance practices are taken into account. If in doubt increase the use class level by moving up to the higher rated category e.g. UC3 to UC4.
For wood in permanent ground or fresh water contact, or providing exterior structural support, Use Class 4 levels of protection MUST be achieved. Anything less and service life, structural safety and customer satisfaction will be compromised.
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Assessing the risk and consequence of failure
Decisions about the need and level of treatment must also take into account the risk and consequences of failure. One use class can cover a range of components yet the risk and consequences of failure can vary from one to another.
During project specification each timber component should be assigned a service factor code to assess the need for preservative treatment.
Preservative Treatment Types
There are of course different types of preservative treatments for the varying levels of protection required. Here’s an outline of the generic treatment characteristics and their use class suitability.
These products will be more commonly known under their respective brand names, if you have a preference for a specific product you can add this to your specification.
The level of protection conferred by a wood preservative depends on its method of application. Brush, dip or spray applied products will afford a degree of protection but for extended service lives of 15, 30 or 60 years typically only wood pre-treated by an industrial penetrating process can give the required level of protection for the life of the component.
The use of CCA (Chromated copper arsenate) preservatives in the European Union was withdrawn from 1 September 2006.
*Download our information sheet 'Use of Creosote and Creosote-treated timber'
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The most widely used process for applying preservative treatments to both solid wood components and panel products is vacuum-pressure impregnation. This is carried out by specialist companies in large pressure autoclaves under factory controlled conditions.
Industrial impregnation plants are sealed and controlled systems, there is no wastage of preservative or risk to the environment and human exposure during the process is prevented
Find out more in the Application Technologies section
NEW WPA Code of Practice: Industrial Wood Preservation
A valuable and current reference of standards and good practice for treaters, specifiers and
users of treated wood. This new publication supports the ongoing education campaign to raise awareness within the wood supply chain for the accurate specification of preservative treated wood products.
This document replaces the former WPA Manual: Industrial Wood Preservation - Specification and Practice.
WPA Benchmark Approved
preservative formulations, treaters and products
The WPA Benchmark Quality Approval Schemes further verify the quality of a wood preservative formulation, treatment process and treated wood product against specific UK and European performance standards.
The preservative Formulation Approval scheme provides independent assessment of the laboratory and field test data manufacturers use to guide their customers in the use of the preservative. This is specifically the loading and retention levels required when timber components are treated to their desired service life in Use Class 4 (ground contact) situations. The full criteria and scope of this and all the other WPA Benchmark Schemes can be found in the Quality Schemes section.
Approved retentions are derived from data provided by an applicant on that applicant’s responsibility. Each applicant remains responsible for the performance of its product both in laboratory and field trials and in wood treated with the product. Neither the WPA nor any agent thereof have or accept liability for the efficacy of approved product(s) or the performance of wood treated with those products. Sole responsibility for fitness for purpose of a treated commodity rests with the business that places that commodity on the market.
WPA Approved Preservative Retentions* kg/m3
* The approved retention is considered by the WPA to be the minimum amount of preservative to be found in the analytical zone of softwood species after treatment (in accordance with use class 4 desired service life specification in British Standard BS 8417). No variation below the retentions approved by the WPA is permitted if performance in accordance with BS8417 has been specified or is expected by the buyer of the treated commodity.
** Evidence of effectiveness in hardwood species has been presented and accepted. The approved retentions therefore also confer protection to treated hardwood species in use class 4 applications and service life combination (unless a different hardwood retention is indicated).
Read the results from the WPA's on-going field trial of preservative treated British softwoods.
WPA Benchmark Approved Treater and Product (TW) schemes relate specifically to the durability (resistance to biological degradation) of wood and wood based materials pre-treated by an industrial process. Individual treated wood products certificated under the Product scheme are verified as being compliant with BS 8417, the wood preservation code of practice - for either 15 or 30 years desired service life.
WPA Benchmark Approved
Preservative Treaters and Treated Wood Product suppliers
Supplying preservative treatment services and treated products
Members who are National Highways Sector Scheme 4 certified
Every aspect of highways construction and repair is covered by a specific sector scheme. Procedures for treated wood fencing specification and procurement is covered in Sector Scheme 4 (SS4). Companies must be ISO9001 certificated as SS4 compliant before they can bid for treated wood supply contracts on highways projects.
Much of the SS4 Scheme Document published by UK Accreditation Services (UKAS) is based on and makes direct reference to WPA Quality Guidance Note 2 (QGN2) and the specifications and practices set out in the WPA Manual of Wood Preservation.