A Guide to Timber Preservation
Timber preservation is the process of treating wood to enhance its durability, especially when it's used outdoors or in damp environments. Without protection, many softwood timbers are vulnerable to fungal decay, insect attack, and moisture damage over time.
Preserving timber extends its service life and improves its structural performance. The right treatment can make timber suitable for a wider range of applications, from decking and fencing to structural landscaping and garden features.
How is timber treated?
Industrial pressure treatment for deep protection
Most outdoor timber is preserved using a high-pressure treatment process. The timber is first kiln-dried to remove moisture, which helps the preservative penetrate more effectively. It is then placed into an autoclave – a sealed vessel where preservatives are forced deep into the wood fibres under vacuum and pressure.
This pressure treatment helps to protect the timber from the inside out. Once treated, the wood typically has a green or brown tint, which fades over time to a silver-grey colour as it weathers. Treated timber can be painted, stained or left to age naturally depending on your project needs.
Understanding Use Classes of timber
Matching the right treatment to the job
Timber treatment levels are classified under the British Standard BS EN 335, using what’s known as the Use Class system. Each class relates to the timber’s end use and the level of risk it faces from moisture and decay.
Timber used indoors in dry conditions, as an example, typically requires no preservation at all. Timber used outdoors, however – especially when in contact with the ground – needs a higher level of protection. That’s where Use Class 3 and Use Class 4 are generally required.
Use Class 3 timber is suitable for outdoor applications where the wood isn’t in contact with the ground, such as cladding or decking boards. Use Class 4 is the highest level of protection and is required for timber that is in contact with the ground or exposed to continuous wetting, such as fence posts, sleepers, and subframes.
The importance of Use Class 4
For structural and ground-contact timber
Use Class 4, or UC4, provides long-term resistance against fungal decay and insect attack in exposed environments. It’s the recommended standard for timber used in structural applications or where it will be in constant contact with soil or water.
At Fountain Timber, we offer a wide selection of UC4-treated products built for long-lasting performance outdoors. Our treated softwood sleepers are perfect for retaining walls, garden terraces, and steps, while sawn posts are available in a variety of sizes and ideal for fencing and structural support. For a clean, uniform finish in rural or agricultural settings, our machined round stakes and rails deliver both durability and ease of installation.
We also stock UC4-rated fencing and gate posts, including square and planed options, as well as Redwood Glulam posts for projects where strength and appearance are equally important. All of these are pressure-treated for a 10+ year service life and ready to withstand the demands of the outdoor environment.
Why factory treatment matters for you timber
Brushed or dipped preservatives aren’t always enough
Not all timber treatments offer the same level of protection. Factory-applied, pressure-treated timber undergoes a thorough and controlled process that penetrates deep into the wood. This is very different from brush-on or dip treatments, which only coat the surface.
While brush treatments may be useful for aesthetic finishes or minor maintenance, they aren’t suitable for structural or ground-contact timber. For exterior applications, always choose pressure-treated timber from a trusted supplier and confirm the Use Class rating at the point of purchase.
How can we help?
With the right treatment, timber becomes a high-performance, low-maintenance solution for outdoor environments. Whether you’re installing a garden fence, building a retaining wall, or designing a timber-framed feature, preserved wood treated to the correct standard ensures your work will stand up to the elements for years to come.
At Fountain Timber, we’re committed to supplying high-quality, UC4-treated timber that’s responsibly sourced and expertly prepared. If you're unsure which product best suits your project, our team is always available to offer advice and guidance.
Updating page...