Tanalised Timber Explained (UK Guide)

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Ever wondered how some wooden fences, decks, and garden sheds seem to last forever, shrugging off the relentless British rain while others rot away in a few short years? The answer isn’t magic, but a clever bit of science known as pressure treatment, and one of the most famous names in the game is ‘Tanalised’ timber.

Chances are you’ve seen it everywhere, from playground climbing frames to the timber skeletons of new-build homes. It’s that wood that often has a faint greenish or sometimes brownish tint, a hallmark of its superpower: the ability to resist rot, fungus, and hungry insects. For anyone in the UK planning an outdoor project – whether it’s a humble raised bed for your veg patch or a sprawling deck for summer barbecues – understanding tanalised timber isn’t just useful; it’s essential.

But what exactly is it? How is it made? Is it safe for your family and pets? And how do you get the best out of it? This guide will unpack everything you need to know. We’ll journey from the forests where the wood is grown to the high-tech treatment plants that give it its long life. We’ll explore its history, debunk some common myths, and give you practical, no-nonsense advice for using it in your own garden. Think of this as the ultimate handbook for building outdoor projects that stand the test of time, and the unpredictable British weather.

What on Earth is Tanalised Timber? The Basics Explained

Let’s start with the basics. The term ‘Tanalised’ can be a bit confusing. It sounds like a type of wood, like oak or pine, but it’s not.

‘Tanalised’ is actually a brand name for a specific type of wood preservative treatment. The name comes from the ‘Tanalith’ preservative that’s forced deep into the timber. Think of it like how Hoover became a common word for a vacuum cleaner, or Tannoy for a public address system. Tanalised has become a household name for pressure-treated wood in the UK.

The whole point of this treatment is to protect wood from its natural enemies. In our damp climate, untreated timber left outdoors is a feast for two things:

Fungal Decay (Rot): Microscopic fungi in the air and soil love to eat the organic fibres of wood, especially when it’s wet. This is what causes timber to go soft, spongy, and eventually crumble.
Insect Attack: Certain insects, like wood-boring beetles, see timber as a tasty meal or a cosy place to lay their eggs.

The Tanalith preservative makes the wood poisonous to these fungi and insects, effectively stopping them from munching on your fence posts or decking boards.

Pressure Treated vs. Tanalised: What’s the Difference?

This is a key point that trips many people up. All Tanalised timber is pressure treated, but not all pressure-treated timber is Tanalised.

Pressure Treatment is the method. It involves placing wood inside a large, sealed cylinder and using high pressure to force a preservative chemical deep into the wood’s cellular structure.
Tanalised refers to the use of a specific, market-leading preservative called Tanalith (currently, the most common version is Tanalith E) during that pressure treatment process.

While other companies make their own wood preservatives, Tanalith has built a reputation for quality and reliability over many decades. That’s why the name has stuck. When you buy Tanalised timber, you’re buying wood treated with that specific product.

The Science in Simple Terms: How it Works

Imagine a dry sponge. If you just dip it in water, only the outside gets wet. But if you put it in a sealed container and force water in under pressure, the water will penetrate right to the very centre.

That’s exactly how pressure treatment works.

The Timber: The process usually starts with a softwood, most commonly pine or spruce, because its cellular structure is more porous and accepts the treatment well.
The Autoclave: The wood is loaded into a huge, strong steel cylinder called an autoclave. The door is sealed shut, creating an airtight chamber.
The Vacuum: A powerful vacuum pump sucks all the air out of the autoclave and, crucially, out of the tiny cells within the wood itself. This is a critical step – it makes the wood thirsty for the preservative.
The Preservative: With the vacuum held, the autoclave is flooded with the Tanalith preservative solution.
The Pressure: Hydraulic pumps then apply immense pressure (often over 12 bar, or 180 PSI), forcing the preservative deep into the timber, right to its core.
The Final Vacuum: The pressure is released, the excess preservative is pumped out, and a final vacuum is applied to remove any surface liquid, leaving the timber ready to be dried.

The result is timber that isn’t just coated in a preservative but is saturated with it. This is why it’s so much more effective than just painting or dipping wood in a protective solution.

A Quick Trip Through Time: The Story of Wood Preservation

Protecting wood from decay isn’t a new idea. People have been trying to make timber last longer for centuries. The ancient Greeks and Romans used substances like olive oil and tar to protect their ships and buildings. But the modern era of wood preservation, the story that leads directly to the Tanalised timber in your garden, really began in the 19th century, driven by the Industrial Revolution.

The Age of Steam and Steel

The Victorian era saw a massive explosion in infrastructure. Britain was building railways, telegraph networks, and canals at a frantic pace. All of these projects required vast amounts of timber for things like railway sleepers, telegraph poles, and lock gates.

The problem was that this timber, sitting in the damp British soil, rotted incredibly quickly. Replacing thousands of miles of railway sleepers every few years was a logistical and financial nightmare. Engineers and scientists desperately needed a way to make wood last.

This led to experiments with various chemicals. One of the most successful was creosote, a black, oily substance distilled from coal tar. Patented in 1838 by John Bethell, the ‘Bethell Process’ involved pressure-treating timber with creosote. It was revolutionary. Creosote-treated railway sleepers and telegraph poles lasted for decades, and you can still see some of these dark, almost black, timbers in use on heritage railway lines today.

The Rise of a Cleaner Alternative

Creosote was effective, but it had its downsides. It was messy, had a strong, lingering smell, and contained compounds that were later found to be harmful to health and the environment. A cleaner, safer alternative was needed, especially for wood used in homes, farms, and public spaces.

This is where the story of Tanalith begins. In the 1930s, a scientist named Sonti Kamesam, working at the Forest Research Institute in India, developed a new type of water-based preservative. It was a mixture of copper sulphate, potassium dichromate, and arsenic pentoxide. This formula was known as Chromated Copper Arsenate, or CCA.

The Wolman company, a German firm that later expanded into the UK, licensed and developed this technology. They branded their CCA preservative ‘Tanalith’. The process was cleaner than creosote, and the treated wood was odourless and could be painted. It was a huge success.

The CCA Era and The Big Change

For much of the 20th century, CCA-based Tanalith was the gold standard. It was incredibly effective and gave timber a very long life. However, by the late 1990s and early 2000s, concerns were growing about the arsenic and chromium components in the preservative, particularly in wood used for residential applications like decks and playgrounds where people, especially children, would have direct contact with it.

This led to new regulations. In 2004, the European Union banned the use of CCA for treating timber for residential and domestic use. The industry needed a new formula, fast.

In response, the manufacturers developed a new generation of arsenic-free preservatives. The most successful and widely adopted of these is the product used today: Tanalith E. It’s a copper-based biocide combined with organic co-biocides (triazoles). This formula is just as effective at protecting the wood but without the arsenic and chromium, making it much safer for use in our homes and gardens.

So, when you buy new Tanalised timber today, you are not buying the old CCA-treated wood. You’re buying wood treated with a modern, more environmentally conscious preservative.

The Treatment Process: A Look Inside the Autoclave

We’ve touched on the “how,” but let’s dive deeper into the nitty-gritty of the pressure treatment process. It’s a fascinating blend of chemistry and engineering that turns ordinary softwood into a super-durable building material.

Step 1: Selection and Preparation

It all starts with the right wood. The process works best with permeable softwoods. In the UK and Europe, this is typically:

Pine (specifically Scots Pine or Corsican Pine): The most common choice. Its cell structure allows for deep penetration of the preservative.
Spruce: Can also be used, but it’s less permeable than pine. It often needs to be incised first – a process where small slits are cut into the surface to help the preservative soak in.

The timber is sawn to its final dimensions (e.g., fence posts, gravel boards, decking) before treatment. This is a crucial point we’ll return to later. The wood must also be dried to a specific moisture content (usually below 28%). If it’s too wet, the cells are already full of water, and there’s no room for the preservative to get in.

Step 2: Loading the Autoclave

The prepared timber is stacked onto special trolleys, or ‘bogies’, that run on rails. These are loaded into the long, cylindrical autoclave. The heavy-duty, pressure-sealed door is swung shut and locked, turning the autoclave into a giant pressure cooker.

Step 3: The Initial Vacuum

This is the magic ingredient. A powerful vacuum pump starts to remove the air from the cylinder. As the air is pulled from the chamber, it’s also pulled out from the millions of tiny, microscopic cells and vessels inside the wood. This creates a powerful negative pressure within the timber.

Step 4: Flooding with Tanalith E

While the vacuum is maintained, valves open, and the Tanalith E preservative, a water-based solution, is pumped in from a storage tank, filling the autoclave completely. The wood is now fully submerged.

Step 5: Applying the Pressure

Now for the ‘pressure’ part of pressure treatment. Hydraulic pumps kick in, increasing the pressure inside the autoclave dramatically. This intense pressure overcomes the wood’s natural resistance and forces the preservative solution deep into the timber’s structure, saturating the sapwood. The level of pressure and the time it is held depends on the type of wood, its thickness, and the desired level of protection (the ‘Use Class’). Computers monitor this phase precisely to ensure the correct amount of preservative is absorbed.

Step 6: Final Steps and Quality Control

Once the required absorption is achieved, the pressure is released, and the surplus preservative is pumped back to the storage tank to be used again. A final vacuum is then applied to extract any excess solution from the surface of the timber, leaving it clean and drip-free.

The autoclave door is opened, and the trolleys of treated wood are removed. The timber now has its characteristic greenish tint. This colour comes from the copper compounds in the preservative. As the wood dries and is exposed to sunlight, this green will fade to a honey-brown and eventually, if left untreated, to a silvery-grey.

Every batch of treated timber is subject to quality control. Samples are taken to check the depth of penetration and the amount of preservative retained in the wood, ensuring it meets the required British and European standards.

Understanding Use Classes: Not All Treatment is Equal

This is perhaps the most important concept for a consumer to grasp. You can’t just buy “tanalised timber”; you need to buy the right tanalised timber for your specific job. The level of treatment is defined by Use Classes, as set out in British Standard BS EN 335.

These classes specify the kind of environment the timber can be used in. Using the wrong class means your project could fail much sooner than you expect.

Use Class 1 & 2: The Indoor Grades

Use Class 1: For internal use, permanently dry. Think roof trusses, internal joists, and floorboards. It’s protected against insects but not rot, as it should never get wet.
Use Class 2: For internal use where there’s a risk of condensation or occasional wetting. Think tile battens in a roof or the timber frame of an external wall. It has a slightly higher level of protection against rot.

You will rarely see these sold at a standard timber merchant or DIY store. They are typically used in the construction trade.

Use Class 3: The Outdoor, Above-Ground Grade

Use Class 3: This is for timber used outdoors, above ground, and exposed to the weather. This is a very common grade.
Examples: Fencing panels, fence rails, gravel boards (if they are sitting on top of something and not touching the soil), sheds, cladding, and decking components that are not in contact with the ground.
Protection: It’s designed to withstand rain and sun but not constant contact with wet ground.

Use Class 4: The In-Ground Grade

This is the heavy-duty stuff.

Use Class 4: For timber that will be in direct contact with the ground or permanently submerged in freshwater.
Examples: Fence posts, decking posts, sleepers used for retaining walls, and agricultural stakes.
Protection: It has a much higher concentration of preservative forced into the wood to give it long-term protection against rot in these harsh, constantly damp conditions.

This is a critical distinction. If you use a Use Class 3 fence post in the ground, it will rot at ground level within a few years. Always use Use Class 4 for anything that goes into the soil. Reputable timber merchants will clearly state the Use Class of their products. If they can’t tell you, go somewhere else.

The Pros and Cons: Is Tanalised Timber the Right Choice?

Like any material, pressure-treated wood has its strengths and weaknesses. It’s important to have a balanced view.

The Advantages

Durability and Longevity: This is its number one selling point. Properly treated timber, used in the correct Use Class, can last for a very long time. Many suppliers offer a 15-year guarantee against rot and insect attack, and its actual service life can be much longer. This saves you the time, money, and hassle of replacing rotten timber.
Cost-Effectiveness: While it costs more upfront than untreated softwood, its long lifespan makes it incredibly economical over time. A fence built with Use Class 4 posts and Use Class 3 panels will outlast an untreated one many times over, making the initial extra investment well worth it.
Versatility: It can be used for almost any outdoor project imaginable. From large-scale agricultural fencing and commercial decking to small domestic projects like pergolas, planters, and bin stores.
Low Maintenance: Tanalised timber doesn’t need to be painted or stained to maintain its resistance to rot. You can leave it to weather naturally to a silvery grey if you like that look. However, applying a protective coating can enhance its appearance and provide extra protection.
Safety (Modern Formulations): Modern Tanalith E is safe for use in residential and domestic settings, including children’s play equipment. The preservative becomes fixed in the wood’s structure once it dries and does not leach out easily.

The Disadvantages

The Appearance: The initial green tint isn’t to everyone’s taste. While it fades over time, it can look a bit industrial at first. Some manufacturers add a brown dye (‘Tanatone’) during treatment to give a more natural, warm colour from day one.
The Chemicals: Although modern preservatives are deemed safe for their intended use, some people prefer to use completely natural, untreated materials, especially for things like organic vegetable beds. We’ll discuss this in more detail later.
Warping and Splitting: The pressure treatment process involves saturating the wood with a water-based solution. As this wood dries out, it can shrink, warp, twist, and split. This is a natural characteristic of timber and not a fault of the treatment. The speed of drying has a big impact – rapid drying in hot sun is more likely to cause issues than slow, gentle drying.
Disposal: Treated timber needs to be disposed of correctly. It is classed as controlled waste and must not be burned on domestic fires or wood burners. The smoke and ash contain concentrated chemicals that are harmful to health and the environment. It should be taken to your local council recycling centre (where it will be disposed of correctly) or disposed of by a licensed waste contractor.

Practical Guide: Working with Tanalised Timber

So, you’ve bought your timber and you’re ready to start building. Following a few simple rules will ensure you get a professional finish and, crucially, that you don’t compromise the protective treatment.

The Golden Rule: Treat Any Cut Ends

Remember how the timber is treated before it’s delivered to you? The preservative forms a protective “envelope” around the wood, with the highest concentration on the outside and penetrating deep inside.

Anywhere you cut, drill, or notch the timber, you expose the less-treated wood in the centre.

This newly exposed end grain is like a sponge; it will soak up moisture and provide a perfect entry point for rot and insects, completely bypassing the protective treatment.

This is the single biggest mistake people make. They build a beautiful deck, but the cut ends of the boards and joists, hidden from view, are left untreated. A few years later, rot starts to appear.

Solution: You must treat any cut or drilled surfaces with an end grain preservative. This is a brush-on product, usually available where you buy your timber, that is designed for this purpose. It contains similar active ingredients to the original treatment. Apply two liberal coats to any surface you have exposed. It’s a five-minute job that can add years to the life of your project.

Health and Safety First

Working with any wood produces dust, and treated timber is no exception. Always follow these simple safety precautions:

Wear a Dust Mask: When sawing or sanding, wear a suitable dust mask (FFP3 rated is best) to avoid inhaling fine sawdust particles.
Wear Gloves: The timber is safe to handle, but wearing gloves will prevent splinters.
Wear Eye Protection: Goggles or safety glasses are a must when cutting or drilling.
Wash Your Hands: Always wash your hands after working with treated wood and before eating or drinking.
Work in a Ventilated Area: If possible, do your cutting outdoors.

As mentioned before, never, ever burn treated timber offcuts.

Fixing and Fastenings

The copper content in modern wood preservatives can cause accelerated corrosion of certain metals. This means you need to use the right screws, nails, and bolts.

Use Hot-Dipped Galvanised or Stainless Steel fixings.
Standard cheap zinc-plated screws will corrode and fail surprisingly quickly when used with treated timber, especially in damp conditions. This will leave ugly black stains around the screw heads and, eventually, the fixing will break.
Spending a little extra on high-quality, corrosion-resistant fixings is a wise investment.

Finishing and Maintenance

One of the great things about tanalised timber is that it doesn’t strictly need any aftercare to remain protected against rot. However, many people want to maintain its appearance or give it extra protection from the weather.

Let it Dry: Freshly treated timber is often still damp. It’s best to let it dry for a few weeks or even months before applying any paint or stain. This allows the wood to settle and ensures the coating will adhere properly.
Water Repellents: The preservative stops rot, but it doesn’t stop the wood from getting wet. Applying a clear water repellent can help to reduce the amount of water the wood absorbs, which in turn reduces swelling, shrinking, and splitting.
Stains and Oils: To maintain the colour or change it, you can use decking oils or garden furniture stains. These products often contain UV blockers that slow down the process of the wood turning grey in the sun. They need to be reapplied every year or two.
Painting: You can paint treated timber, but you must ensure it is completely dry first and that you use a suitable exterior primer and paint system.

Common Questions and Myths Debunked

There’s a lot of information, and misinformation, out there about treated timber. Let’s clear up some common queries.

Is it safe for raised vegetable beds?

This is a hot topic. The official advice from the Timber Decking and Cladding Association (TDCA) and the manufacturers of Tanalith E is that it is safe for this use. The copper-based preservative becomes “locked” into the wood’s structure and is highly insoluble, meaning it doesn’t readily leach into the soil.

However, some organic gardeners are uncomfortable with the idea of having any chemicals near their food. If you are concerned, you have two options:

Line the inside of the raised bed with a heavy-duty plastic liner. This creates a physical barrier between the treated wood and your soil.
Use an alternative material. Untreated hardwoods like oak are very durable but also very expensive. Another popular choice is to use composite boards made from recycled plastic and wood fibre.

Is it safe for pets and wildlife?

Once the timber is dry, it is safe for pets and wildlife. The preservative is fixed within the wood. The only risk would be if an animal were to chew on and ingest large quantities of the wood itself, which is highly unlikely.

Why has my new timber got little white spots on it?

Sometimes you might see small, crystalline white or pale yellow patches on the surface of the timber. This is simply dried preservative resin that has been drawn out of the wood during the drying process. It is harmless and will weather off in time.

Why does the colour vary so much?

The final colour of the treated timber can depend on the species of wood, its moisture content at the time of treatment, and how long it has been since it was treated. It’s perfectly normal to see variations in colour, even within the same pack of timber. It will all weather to a similar colour over time.

My new fence post has cracked! Is it faulty?

Small splits and cracks (known as ‘checks’) appearing as the wood dries are a completely normal and natural feature of timber. They are not a fault. These cracks are caused by the outer surface of the wood shrinking faster than the core. They rarely affect the structural integrity of the post because the preservative treatment has penetrated deep into the wood, well beyond the depth of these surface checks.

The Future of Wood Treatment

The world of wood preservation is always evolving, driven by the need for ever-more effective and environmentally friendly solutions. Research is ongoing into new types of preservatives, including metal-free formulations and treatments using natural products like plant oils.

Another exciting area is wood modification. These are processes that change the actual chemical structure of the wood to make it resistant to rot, rather than just impregnating it with a biocide. Examples include:

Accoya: This involves treating wood with acetic anhydride (essentially vinegar), which permanently modifies its cell structure. The resulting wood is incredibly stable and durable.
Thermally Modified Timber: This process involves heating wood in a controlled, low-oxygen environment. This changes its properties, making it much more resistant to decay.

These technologies are currently more expensive than conventional pressure treatment, but as they become more widespread, they offer a glimpse into a future where we can enhance the natural properties of wood in even more sophisticated and sustainable ways.

Conclusion: Build to Last

Tanalised timber is more than just wood with a green tinge. It’s a product of over a century of scientific innovation, designed to help us make the most of a fantastic, renewable building material. It allows us to build beautiful, practical, and affordable outdoor structures that can stand up to the rigours of the British climate for decades.

By understanding what it is, how it’s made, and how to use it correctly, you can approach your next garden project with confidence. By choosing the right Use Class for the job and always remembering that golden rule – treat your cut ends – you can ensure that your fence will stand straight, your deck will stay strong, and your creations will be enjoyed for many years to come. It’s the simple secret to building things that don’t just look good, but are truly built to last.