Preservative Treatments for Timber

What is Preservative Treatment?

Preservative treatment provides wood with added durability. However, it’s a mistake to assume that all pressure treated wood is the same. Whilst one piece of treated wood may look very much like any other, 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. Only timbers treated to Use Class 4 should be used for ground or fresh water contact, as they will best perform in these environments.


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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 an attack increases significantly if, for any reason, the moisture content of wood rises above 20% – for example, poor installation practice or maintenance, persistent condensation, or damp.


Some degree of sapwood is likely to be present in the timber we use for building, hence the precaution is taken to treat certain timbers with preservative prior to use.


Making the most of a valuable resource

Wood protection technology helps to optimise the amount of harvested timber that can be usefully gained from the tree. Timber preservatives for softwood materials provide tailored levels of protection from decay and insect attack – making the resulting wood more durable and offering reliable longer-term performance.


Making the most of timber resources in this way also reduces waste and reduces pressure on the more naturally-durable, but perhaps more scarce species.

Chemicals and the environment

The preservative industry is highly regulated and manufacturers continue to introduce innovative products that 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

When specifying or purchasing preservative treatment use this as guidance: 


INTERIOR use only (e.g. joists, rafters, studs, tiling battens) – Use Class 2
EXTERIOR use above ground, uncoated (e.g. deck boards, cladding) – Use Class 3U
EXTERIOR use in ground contact and exterior structural support (e.g. posts, deck joists) – Use Class 4


The British standard for wood preservation is set out in the BS 8417 Preservation of Wood: Code of Practice and mirrored in the WPA Code of Practice – Industrial Wood Preservation. 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.


There are other Use Classes available for specialist applications. More information can be found on the WPA website.

Whether you choose UC2 , UC3(u) or UC4. Make sure YOUR choice appears on all the paperwork.


Make sure the supplier has sent you the right treated timber. 

For wood in permanent ground or freshwater contact, or providing exterior structural supportUse Class 4 levels of protection MUST be achieved. Anything less and service life, structural safety, and customer satisfaction will be compromised.


Use Class 4 campaign

The TTF, the Wood Protection Association (WPA), and Timber Decking and Cladding Association (TDCA) have launched a new campaign to promote the accurate description of treated wood to increase the public and merchants’ knowledge on the important differences and purposes between each Use Class. Find out more about our campaign.

TTF Member Code of Conduct

As part of the TTF Member Code of Conduct, TTF Members shall ensure that preservative treated wood is being accurately and unambiguously specified/purchased, for use or resale, and is clearly identified at all points of the chain as fit for the intended purpose.


In addition preservative-treated wood should be produced by or purchased, directly or indirectly, from a treatment provider whose operation has been assessed and approved under an independent and reputable accreditation scheme (e.g. NTR in the Nordic region).


This applies to suppliers of treated wood or treatment services to TTF members and shall be fully in place from mid-2022. Those suppliers need not be TTF members themselves to come within scope.

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 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.

Preservative Treatment: Application Technologies

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 about Application Technologies on the WPA website.