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Use Class House

Campaign to promote 'fit for purpose' treated timber

Make sure its 4 logo


WPA Industrial Wood Preservation code of practice
Is your treated timber 'fit for purpose'?

On-going campaign


It’s a mistake to assume that all pressure treated wood is the same. Whilst one piece of treated wood may look like any other, the level of preservative protection could be very different.

Use Classes and British Standards

British Standards for wood preservation (BS 8417 and the WPA Code of Practice: Industrial Wood Preservation),  require timber treatment processes are tailored to the timber component’s end use.  The criteria covers the loading and penetration of preservative impregnated into the wood.


Interior and exterior environments are very different. As such, applications for treated wood are grouped into Use Classes, the three main classes being: 2, for interior applications and 3(u) and 4 for external applications.

Timber Use Classes table

Messaging to target markets

For the fencing, garden and landscaping sector, the focus is on promoting the importance of treating wood to Use Class 4. Nowhere is the fitness for purpose of treated wood more important than for applications in contact with the ground, frequently wet and/or providing exterior structural support

For the broader timber supply chain, the emphasis is on differentiating between interior and exterior applications for treated wood – asking the question 'Use Class 2, 3 or 4?' It's a mistake to use wood for an external application that has been treated for an indoor use  - failure is inevitable.

In each case it's important that the seller, specifier and buyer all know what level of timber treatment is required for each construction component.

Please use and share any of the campaign assets below – to learn more and help raise awareness of timber treatment.

Make sure it's Use Class 4 logo
Buyer's Guide to preservative treated wood
Treated Timber Use Class Poster
Specifying, buying and installing UC4 treated timber

Making the most of wood WEBINAR

This short educational training webinar explains the principles involved with the correct specification and use of preservative treated wood.

Campaign Assets
Timber Fence posts in a field
Specifying, buying and installing UC4 treated timber

Focus on fencing

Properly treated timber can be relied upon by following 'the Ground Rules'  

– a trinity of requirements which ensures your fencing is fit for purpose:

  • Specify Use Class 4

  • Use a 'trusted treater'

  • Be prepared to pay for quality

Use Class 4 treated timber: specifying, buying and installing

Download this concise resource which covers Use Class 4 specification, including timber species and preservative penetratation requirements as well as best practice installation guidance. 

Ground Rules

Guidance Notes: 

Understanding Use Classes for preservative treated wood


These Guidance Notes include: a simple explanation of what national standards require for each application; what species are most suitable; what preservative penetration to expect in different treated commodities; the necessary quality control criteria.

Understanding Use Class 2
Understanding Use Class 3C
Understanding Use Class 3
WPA TW 8 Understanding Use Class 4
WPA Benchmark treater logo

Find a Trusted Treater

To be sure you are getting what you ask and pay for, it's important to buy or source from an accredited supplier who has third-party verification of treatment standards.


The WPA Benchmark Scheme provides audited verification of a company’s ability to treat wood to current industry Standards. 


  • If you’re buying treated timber, look for third-party accreditation, such as WPA Benchmark and always ask for product documentation, including proof of treatment.

  • If you sell treated timber, ensure it’s fit for purpose by sourcing from a WPA Benchmark Approved Treater.

  • If you treat timber, show you’re a trusted supplier by attaining WPA Benchmark Approved Treater status.

TDUK logo

Preservative Treatment Action Plan

Since 2017 Timber Development UK (TDUK) and the WPA have been working together to grow demand for preservative treated timber. They identified three priority areas for action, which have since been adopted as TDUK policy:

  1. To address the common failure to specify treated wood correctly;

  2. To address the lack of understanding on how to correctly install and use treated wood;

  3. To ensure independent verification of the treatment process.


This Preservative Treatment Action Plan is now part of the TDUK Code of ConductAs a result, Members must adhere to the guidance provided in BS8417 (2014) and the 2021 WPA Code of Practice: Industrial Wood Preservation for the choice, use, and application of wood preservatives.

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Resource Centre

FAQs - Manuals - Guidance Notes - Training

All of our resources are free to view and download. Please contact us if you have questions about any aspect of wood protection specification, quality assurance or application good practice.

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