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.
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.
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
WPA Industrial Wood Preservation Manual – updated version available soon!
We have covered timber preservative treatments and their specification here in brief but if you need more in-depth information, please get in touch as our manual is currently being updated.
Alternatively, visit our Resource Centre where we have a wealth of additional information, free to download.
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* 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.
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.