The short answer
No. Tanalised timber is pressure-treated to resist rot, fungal decay and insect attack, but it is not waterproof. Treated wood can still absorb and release water, swell and shrink with the weather, and develop surface cracks. The treatment protects the wood from decay organisms, not from getting wet.
What Tanalised timber is
“Tanalised” is a brand name used in the UK and some other markets for timber pressure-treated with a wood preservative, in the same way people say “hoover” for a vacuum cleaner. In practice, it is often used loosely to mean any pressure-treated softwood sold for garden and construction use. For the general picture, see our guide to pressure treated wood vs regular lumber.
What it is treated with
Modern pressure-treated timber for general use is typically treated with copper-based preservative systems, often combined with an organic co-biocide. Older treatments used different chemistry, including chromated copper arsenate (CCA), which has been restricted for most residential uses in many countries. Which preservative was used on a given piece depends on the product and the date, so check the supplier’s information or end tag. Do not assume.
Water resistance vs decay resistance
Two separate ideas are easy to mix up:
- Decay resistance: the treated wood is much less likely to be attacked by fungi and insects.
- Water resistance: the ability to shed water and resist swelling. Treated wood has limited water resistance on its own.
Because treated timber is often delivered wet, it can dry out and shrink after installation, then swell again in rain. That movement is what leads to checks, cupping and loosened fixings. A water-repellent finish may reduce moisture uptake and slow weathering (see how to waterproof wood), but it will not make timber waterproof, and trapped water behind a film coating can cause problems.
Use classes
In the UK and Europe, treated timber is commonly specified by use class under the BS EN 335 framework. In broad terms, a lower class applies to timber kept dry indoors, use class 3 relates to timber above ground exposed to weather, and use class 4 covers timber in ground or fresh water contact. Higher classes need deeper or stronger treatment. Match the class on the supplier’s label to the job, and choose the higher class for fence posts, anything buried, or parts that are hard to replace.
In North America, a similar idea is expressed through “above ground” and “ground contact” categories.
Coatings for water protection
The best coating depends on the look you want and the exposure. Common options include:
- Water-repellent preservative or exterior wood oil: penetrates the surface, is easy to reapply, and suits decks and fences.
- Exterior stain: adds colour and some UV protection; penetrating types are generally easier to maintain than film-forming ones.
- Exterior paint: forms a film and can protect well if kept intact, but needs prep and maintenance, and cracks can let water in. See deck paint vs. stain for the trade-offs.
Avoid applying a finish while the timber is still very wet, and follow the manufacturer’s instructions on drying time before coating. Some manufacturers suggest waiting weeks or months; the right time varies with weather and timber moisture. A simple test is to sprinkle water on the surface: if it soaks in rather than beading or sitting, the wood is typically ready to absorb a penetrating finish.
Cut ends and drilled holes
Preservative may not reach the core of every board, so fresh cuts and holes expose less-protected wood. Apply a suitable end-cut preservative to cut ends, notches and drilled holes, following the product’s directions. This matters most for posts and parts in contact with the ground or standing water.
When to finish
Treated timber used outdoors usually needs periodic cleaning and re-coating (see treated wood deck maintenance). Check yearly for cracks, mildew, raised grain and failed finish, and recoat as needed. Keep timber clear of soil and plants where possible, allow airflow underneath decks, and avoid designs that trap water.
Safety and disposal
- Wear gloves, safety glasses and a suitable dust mask or respirator when cutting or sanding treated timber, and work outdoors with good ventilation or dust extraction.
- Secure the workpiece, keep saw guards in place and disconnect power before changing blades.
- Wash hands and clothing after handling, and do not eat or smoke while working.
- Never burn treated timber, and do not use sawdust or offcuts as mulch, compost or bedding.
- Dispose of waste at a facility that accepts treated wood, or follow your local authority’s rules.
- Many wood finishes and end-cut treatments are flammable or give off fumes. Ventilate, keep ignition sources away, and lay used rags flat to dry or soak them in water before disposal.
- Use galvanized or stainless fixings that suit treated timber to limit corrosion.