Why do fence posts rot at ground level?
Because the soil line is the one place on a post that stays wet and exposed to air at the same time. Below the surface the timber is often waterlogged with little oxygen. Above it, wind and sun dry it out. The band either side of ground level gives wood-rotting fungi both things they need. Trade sources put timber posts at seven to ten years.
Rot needs three things at once: moisture, oxygen and warmth. Timber buried deep in wet ground is often short of oxygen and decays slowly. Timber well above ground dries out between showers and decays slowly too. The band from roughly 50mm below the surface to 50mm above it has both water and air all year, so that is where wood-rotting fungi work fastest. It is also why a post can snap cleanly at ground level while the timber a foot up still rings sound under a hammer.
Treatment level is the other half of the answer. Use Class 4 is the treatment specification for timber in ground contact, which covers fence posts, decking joists and landscape sleepers. The Wood Protection Association runs a campaign called Make sure it's 4 for exactly this reason, and BS 8417 is the standard that matches treatment to end use and intended service life. Dip treatment is a different thing altogether. Forest Garden's own guarantee terms state that dip treated products must not be installed in direct contact with the ground and must be retreated every year.
Site cutting undoes good treatment. Preservative is driven in from the outside face, so a post cut to length on site exposes untreated wood at the cut end, and the cut end is usually the end that goes in the hole. Jacksons Fencing requires cut or drilled timber to be treated immediately, and its 25 year guarantee excludes in-ground timber cut, drilled, notched or planed after factory treatment. Forest Garden gives the same instruction. Ask any installer whether they carry end-grain preservative on the van, because the answer separates the careful from the quick.
You can slow rot down or design it out. Keep soil, bark mulch and borders away from the post base. Finish the concrete collar proud of the ground and sloped away so water sheds rather than sits. Fit gravel boards, which Jacksons make a condition of their timber guarantee, so the panels never touch wet ground. Or remove the mechanism entirely with concrete posts, which do not rot at all. A manufacturer puts timber posts at seven to ten years against concrete lasting decades, and the fitted price difference is around £10 a post.
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