Our carefully specified high and low-pressure preservative treatments protect timber from all forms of insects, wood-destroying fungi and other biological decay. Our production process has acquired ISO 9001 accreditation and all treatments are in line with the British Code of Practice, BS 8417:2001+A1:2014 and to the requirements of BS EN 335-1:2006.

We operate two treatment facilities, in Hull and Rochester, which enable us to provide a service to both the north and south of the UK. Our sites are perfectly located for customers to take advantage of our ability to treat timber directly from shipments, through the busy ports of Hull and Sheerness, whilst having good access to local roads and motorways.

We offer detailed guidance on the level of treatment required, based on the application and environment the timber is likely to be used. In addition, we provide warranties of up to 15, 30 and 60 years, depending on the level of treatment, to grant added assurance that timber is impregnated to the correct preservative Use Class.


The high-pressure process is suitable for use in external environments or when timber is to be used in contact with the ground. In these situations, there is a greater exposure to moisture and rainfall which is the essential element for the development of mould and wood rot.

The high-pressure process ensures that the preservative treatment penetrates the wood deeply, to provide long-lasting protection. Our water-based treatments provide excellent durability and mould resistance for almost all end uses and are a popular choice when treating fence posts, decking, sleepers and fencing panels.

At our facility in Hull, customers can choose to add a brown pigment dye to their high-pressure treatments on request. We use the same specification of treatment, the only difference being the colour and presentation of the finished product.

It is important to note that in some cases colours may not present themselves evenly across the wood as the difference between the cell structures of sapwood and heartwood can cause the dye to be absorbed at different rates.  This can lead to a patchy appearance on the surface of treated timber.

WJ Timber Treatments can offer the following warranties on the high-pressure treatment of pine timber:

10-year timber treatment warranty – Use Class 3
15-year timber treatment warranty – National Highways Sector Scheme 4 (NHSS4) in accordance with BS 8417
30-year timber treatment warranty – Use Class 4


Our low-pressure treatment offers long term protection for low to medium risk interior and weather protected exterior applications.

The process is applied using a double vacuum process and is identified through the use of a yellow tracer dye to aid identification of treated stock. The active ingredients provide robust protection against insects and fungi for the lifetime of the building, when applied in Use Class 1 and 2 environments. The low-pressure process avoids penetrating the core of the timber meaning less water is absorbed. As a result, the treated timber withholds its original dimensions and natural moisture content effectively, making it the perfect solution for protecting construction grade timbers, including CLS framing and joinery.

For window joinery and cladding, a coating is required to protect the timber from rain.

How it works

The purpose of preservative treatments is to protect the sapwood from insect attack and fungi growth, through a high or low-pressure impregnation process.  The wood at the core of any timber structure is naturally durable and is therefore very difficult to penetrate with preservative treatments.  In the photograph below, the sapwood (new growth) can be seen as the outer darker area, coloured purple.  If left untreated this area rots over time as it is not naturally durable.  The amount of chemical retained in the wood is what determines the Use Class level achieved during the treatment process.

Use Class System

The timber Use Class system, as defined in BS EN 335-1 is used across the timber industry to determine the level of treatment required in relation to the environment the timber is likely to be applied.  The higher the Use Class number, the higher the risk, and as a result, a stronger solution is needed.  It is important to specify the correct Use Class prior to treatment and the easiest way to illustrate this is with a simple list of examples in relation to each Use Class number:

UC1 Suitable for dry internal timbers such as upper floor joists and truss rafters with the main risk being from insect damage. Treatment to Use Class 1 will manage this risk effectively for the service life of the timber.
UC2 Suitable for internal timbers and would be specified if the timber might be at a slight risk of wetting. For example, tile battens and CLS framing (external walls).
UC3a Suitable for outdoor use but will be coated and above ground. Window frames would be the best example in this instance
UC3b Uncoated, external timbers to remain above the ground would be treated to this level. Consider fence rails as the perfect example.
UC4 If direct soil or water contact is intended, the timber must be treated to Use Class 4. This would include fence posts and certain parts of a decking structure that will be sunk into the ground. Deck substructures have now been increased from UC3 to UC4 in accordance to WPA and WDCA regulations.
UC5 The highest level possible, this is required for marine use timbers such as marine pilings, where ground and water contact is continuous.

“UC” = Use Class


Where can I use high-pressure treated wood?

High-pressure treated wood is designed for garden timbers and fencing as well as interior and exterior construction materials such as framing, trusses, joists and should not normally be used in permanent saltwater or freshwater contact without prior discussion with us.  High-pressure timber treatments are typically applied when Use Class 3 and 4 are specified.

Where can I use low-pressure treated wood?

Low-pressure treated wood is typically designed for protecting construction grade timbers, including CLS framing and joinery.  Low-pressure timber treatments are typically applied when Use Class 1 and 2 are specified.

Is high-pressure treated timber always green?

No. The green colour of freshly treated wood naturally fades to a pale straw colour after 2-6 weeks depending on the time of year and exposure to UV. After 6 – 12 months the wood will ‘silver’ to a natural grey unless measures are taken to avoid this. As an alternative, we offer high-pressure treatment in brown, through our Hull plant, so please enquire for further details.

Is low-pressure treated timber always yellow?

No. In most low-pressure treatments a yellow marker dye is added to the solution so treated timber is recognisable on construction sites.  The treatment itself is clear and can be supplied without the yellow marker dye on request.

Which glues and fixings can I use?

Most standard wood glues will bond with high-pressure treated wood when dry but we can give specific advice if requested.  Our preservatives are not corrosive to metal fixings but care should be taken to use appropriate products. Hot-dipped or stainless fixings should be used for outdoor applications to protect from atmospheric corrosion. To ensure stability, fixings should be added once the timber has been treated. Always use the appropriate fixings such as proprietary deck screws for attaching deck boards to sub-frames. Guidance on the suitability of various fixings for both indoor and outdoor use can be found in EUROCODE 5 (BS EN 1995).

How do I dispose of treated timber waste?

All treated timber waste should be segregated and disposed of through an approved contractor. Treated timber waste should not be made into animal bedding or burned except in approved facilities.

Does it conform to current European Standards?

Our high and low-pressure preservative treatment conform to the service expectations set down in BS EN 599, Use Classes 1-4. The UK interpretation of this document is BS 8417:2003.

How do I handle the treated wood safely?

High and low-pressure treated timber is not classed as hazardous. Once surface dry it can be handled safely and is suitable for children’s playground equipment. As is the case with any wood, treated or not, goggles should be worn when rip-sawing and gloves will protect from splinters.