A fencing contractor I know and I pulled out a line of six-year-old posts in Claudelands a while back, and every one of them had rotted off at exactly the same point: right where the post met the ground, in a band maybe 100 mm wide. Above that band the timber was sound. Below it, buried, it was also mostly sound. It was the collar right at ground level, wet and dry in turns, that had gone. That band is where most treated timber actually fails, and no amount of paint on top of it was ever going to change that.
H3.2 and H4/H5 are answering different questions
H3.2 treatment is rated for above-ground use in exterior conditions with occasional wetting, which covers most fence rails, framing and cladding, but it is not rated for sustained ground contact. H4 is the treatment level intended for timber that is in the ground or permanently damp, such as posts set in soil, and H5 goes further again for situations with higher decay hazard, such as posts embedded in ground that stays wet or in contact with concrete that holds moisture against the timber. Using H3.2 posts for a fence line because they were what was in stock, or because nobody asked the merchant which grade they were buying, is one of the most common reasons a fence in Waikato’s damp clay and peat ground fails within five to eight years instead of the fifteen to twenty a correctly treated H4 or H5 post should manage.
A cut end is untreated timber, whatever the post was rated
Treatment is a chemical process applied to the timber before it leaves the mill, and it does not travel very far into the wood from a fresh cut face. Every time a post is trimmed to length, notched, or drilled on site, that cut exposes untreated core timber right at the point most likely to sit wet, and unless that cut end gets a proper end-grain preservative treatment before it goes in the ground, the post is effectively starting its clock as untreated timber at exactly its weakest point. This is the detail I see skipped more than any other on Hamilton fencing jobs, because the treatment happened at the timber yard and the cutting happens on site, with nobody carrying the second step through.
Paint and stain do very little once timber is against the ground
A coating sitting on timber that stays in continuous contact with damp soil is fighting a battle it can’t win; ground moisture moves into the timber from below and from the sides in contact with soil, well below where any surface coating can act, and a film of paint or stain at the ground line tends to trap moisture in rather than keep it out, because it stops the timber drying between wet spells. On Waikato’s peat and heavy clay ground, which holds water for far longer after rain than a free-draining gravel site would, that trapped moisture is close to the worst outcome available. Coatings do real work above the ground line, protecting timber from UV and moderate weathering, but right at and below ground level the job belongs to treatment level and detailing, not to what gets brushed on afterwards.
Getting the timber off the ground altogether
The most reliable fix, on a fence, a pergola post or a deck bearer, is a construction detail that lifts the timber clear of standing moisture rather than a better product to paint on it: a post set in a galvanised steel shoe bolted to a concrete pad, rather than buried directly, or a bearer sitting on a damp-proof pad well above finished ground level, keeps the wood dry enough that its treatment level barely gets tested. Where a post genuinely has to go into the ground, H5 timber, a properly treated cut end, and free-draining backfill around the base rather than compacted clay all add years to the life of that ground-line collar.
For fence and gate timber above the ground line that still needs a maintenance coat to look right and shed weather properly, that finishing work is worth putting to a Hamilton painter once the ground-contact detailing is sorted, since painting over a post that is already failing at the collar only hides the problem for a season.
Frequently asked questions
What is the difference between H3.2 and H4 treated timber? H3.2 is rated for above-ground exterior use with occasional wetting, while H4 is rated for sustained ground contact, such as fence posts set directly in soil.
Do cut ends on treated posts need extra treatment? Yes; cutting exposes untreated core timber, so any trimmed or drilled end needs an end-grain preservative applied before it goes into the ground.
Can paint or stain protect timber that sits against damp ground? Not effectively; a surface coating at ground level tends to trap moisture in rather than keep it out, so the fix is treatment level and construction detail, not the finish applied on top.