The Use Class system
Softwood species decking components classified as semi or non-durable must be pressure treated, before installation, with an approved preservative to provide the service life required. The amount of preservative protection required is governed by its intended end use situation and these are categorised according to a Use Class system which is detailed in a British/European Standard BS EN 335-1.
- Use Class 3 : includes timber deck components such as balustrades and deck boards that are not in contact with the ground.
- Use Class 4 : includes timber deck components such as posts, beams or joists (occasionally deck boards) that will be in contact with the ground or fresh water*.
*As from January 2020, the TDCA recommend that any decking timbers providing exterior structural support should be preservative pressure treated to Use Class 4 - regardless of whether they are in ground contact or not.
This is to ensure the enhanced durability and safety of deck substructures and is in line with BS 8417 (Preservation of Wood: Code of Practice) which is to adopt this change in due course this year. See our blog article here.
BS8417 is the principle standard governing timber treatment in the UK.
We strongly advise you to ask your timber supplier to confirm in writing the Use Class your timber is treated to and the desired service life specification that applies either 15, 30 or more years, the industry default being 15 years.
Brush-applied wood preservatives or dip treatments are not suitable for the long-term protection of timber used for decking and other outdoor landscaping applications and should not be used.