There is no honest single lifespan for all treated timber. How long it performs depends on the treatment specification, timber species, Use Class, installation, drainage, maintenance and the conditions it actually experiences.
Treated timber is specified around a desired service life
The Wood Protection Association uses desired service life categories that are commonly 15, 30 or 60 years when treatment is specified. These are design/specification targets for correctly treated timber in the stated use — not a blanket guarantee that every green-treated board will last that long.
Above-ground timber has an easier job than ground-contact timber
A board or rail that can drain and dry is exposed differently from a post sitting in wet soil. This is why the correct Use Class matters. Read UC3 vs UC4 Treated Timber.
How long do treated timber fence posts last?
Our UC4 Treated Softwood Fence Posts are specified for ground-contact use with a claimed service life of 15 years. That figure applies to this product specification; it is not a blanket lifespan for every treated fence post.
The ground line is the section most repeatedly exposed to moisture. Correct installation still matters, and compatible ground-line protection systems can add an extra barrier around this vulnerable area where suitable.
Installation can shorten or extend service life
Water traps, poor ventilation, constant wetting and exposed cut ends can all work against the treatment. Good detailing helps timber dry between wet periods and keeps unnecessary moisture away from vulnerable end grain.
A green colour does not tell you the lifespan
Colour is not proof of the treatment level or desired service life. Check the product specification and intended application instead.
Protect any cuts you make
Cross-cutting, drilling and notching can expose less-protected timber. Use a suitable end-grain preservative as described in our cut-end treatment guide.
Source: Wood Protection Association treated timber guidance.
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