Timber has a large selection of species that are naturally durable and do not need any additional preservative treatments to make them resistant to biological decay.

The vast range is available in many hardwoods and modified species ( including softwoods like Siberian larch and imported western red cedar).

If choose a non-durable species, e.g. pine/spruce,  treatments are available that can be added to the wood under pressure if to make them durable to biological attack.

Most timber decking is used externally above ground uncoated with frequent wetting; this is classified a Use Class 3u  (UC3u) in BS EN335:1.

Every species has a Natural durability class given in BS EN350-2 ( durability classification is based on sapwood being excluded) which when used in conjunction with guidance principles of BS8417 Preservation of wood code of practice we get desired service life of the species. ( see: How long will timber or wood last without rot or decay? Durability

The Five Natural Durability Classes in BS EN350-2 are given below.

  • Class 1Very durable

    • Class 2 Durable

  • Class 3Moderately durable

    • Class 4 Slightly durable

    • Class 5 Not durable


Examples of service life based on the durability classification of the selected species


Timber used uncoated  above ground exposed to frequent wetting, e.g. timber decking

Class 3 Moderately durable desired service life will be 15years

Class 2 Durable desired service life will be 30years

Class 1 Very durable desired service life will be 60years

For marine and freshwater environments use durable to very durable species

For non-durable or slightly durable species these need to be pressure treated under high pressure, the service life will vary from generally 15 yrs to 45yrs. The pressure treatment can be modified to match the biological hazard, the timber treatment products have evolved and are now more environmentally friendly moreover some now include pigments to enhance the look or colour of the wood.

How long will my timber decking last? Andrew Goto
Treated softwood timber decking
How long will my timber decking last? Andrew Goto
Timber treatment image

Common brands for biological preservation include Koopers Microshades, MicroPro (Osmose Naturewood Alkaline Copper Quaternary)Celcure , Lonza Archwood Protection – Tanalith , Tanatone (copper triazole technology), Tanalith Creol.

Preservative treatments have predominately been supplied in brownish/ green tints to show the presence of a preservative treatment in the wood.

A well-treated timber product from a reputable treater or supplier should have a treatment certificate stating the level of treatment and use the class/application as a minimum. This gives the client peace of mind that the treatment has been done to the appropriate standard to provide you with the desired service life.

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