Timber treatment options

What is preservative treatment?

Not all timbers are classed as durable, for less durable timbers, such as redwoods, a preservative treatment can be used to boost a timbers natural durability and extend its expected in-service life. There are a range of different treatment options available, all of which work in slightly different ways, however the general principle is the same:


When impregnated into the timber the preservative components bond with the wood structure and cannot be easily removed. Most treatments have a greenish tinge initially which will weather to a silver grey with exposure to the elements over timer. As an alternative option it is also possible to add a dye to the treatment which will give the timber a brown shade initially, however this will still weather to the same silver grey long term. The weathering process does not indicate any loss of protection.

Does my decking need preservative treatment?

Timbers are divided into five durability classes as shown on the chart below:


Generally timbers in classes one to three do not require preservative treatment as they are naturally resistant to insect and fungal attacks, with class one having the highest resistance. For classes four and five it is advisable to get the timber treated with preservative to increase its natural resistance  and extend its expect service life.

Certain timbers can fall into multiple durability classes. This can depend on where they are grown, or the grading used for the individual species of timber. A good example of this is Larch. The majority of Larch is durable, however within the grading rules an allowance is made for a certain percentage of non-durable sapwood.

How long will my treated timber last?

Depending on the percentage of preservative used in a treatment cycle the timber will achieve a different treatment rating as outlined below:

- Use Class 3 – External timber used above ground contact – generally gives a 15 year expected service life. Typical uses: domestic decks and any decks installed above ground contact.

- Use Class 4 – External timber used in ground or fresh water contact - generally gives a 25 year expected service life. Typical uses: forest walkways and boardwalks in wet environments.

- NHss4 – National Highways Sector Scheme 4 - generally gives a 30 year expected service life. Typical uses: marinas and pontoons.

Do I need to apply any other treatments to my decking?

During installation you will need to apply an end grain preservative to any cut ends and screw holes to ensure the treatment envelope remains intact, however once the boards are installed there is no need to apply any additional treatments.

Treated timber can be stained or oiled without adversely effecting the preservative properties of the board. This means that you can stain the board or apply any UV protection oils to ensure the boards are the perfect colour for your decking.

Can I re-treat my decking to restore the colour?

In principle there is no reason why decking cannot be retreated, however it is always very difficult to predict the final colour. Retreating timber does not hide what a board may look like. If the wood has become weathered or has blackness or other signs of aging this will only become accentuated, the imperfections often look far worse than before the retreat!

If you have any further questions or wish to discuss your timber treatment requirement please call 01726 844616.