I once pulled apart a section of wall in Ngaio where the buried poles had gone soft and spongy within six years, while the above-ground rails on the same structure looked fine. The timber had come from the same yard, the same job, probably the same delivery. The difference was the treatment level stamped on the poles, which turned out to be H3 rather than the H5 the ground contact needed. Nobody had swapped it out to cut corners; it was more likely an ordering mistake that nobody caught before the poles went in the ground. Either way, the ground found the weak point within a handful of winters.
What the H-numbers are actually measuring
The H rating on treated timber describes the hazard class it is rated for, not a general quality grade. H3 is for timber above ground, exposed to the weather but not sitting in soil, such as fence rails, deck framing, or the visible face boards of a retaining wall. H4 covers timber in the ground but not carrying heavy structural load, like fence posts. H5 is for timber in the ground under structural load, which includes retaining wall poles, deck piles, and anything else where failure would mean a structure moving or collapsing, not just a board rotting. Each level uses a higher retention of preservative than the one below it, aimed at the specific combination of moisture and fungal or insect exposure that timber faces in that position.
The numbers are not a hierarchy where higher is simply better everywhere. H5 timber is not required for above-ground rails, and using it there mostly adds cost without adding a benefit that matters, since the failure mode H5 protects against, in-ground decay under sustained load, is not the failure mode above-ground timber experiences.
Why the wrong level fails quietly
Timber treated to the wrong hazard class for its position does not usually fail on day one. It fails on the same general timeline as untreated timber would in that environment, just delayed by whatever protection the lower treatment level does offer. An H3 pole buried in wet clay might last six to ten years instead of thirty, which is long enough that the person who installed it is often not the person dealing with the consequences. This is part of why treatment level matters more on structural in-ground timber than almost anywhere else in a build: the failure is slow, hidden below ground level, and expensive to fix once it shows.
Reading a timber order correctly
Treated timber should carry a stamp or tag showing the hazard class, and it is worth checking that stamp against the actual use before it goes into the ground rather than after. On a retaining wall job the poles, the parts that are both buried and carrying structural load, need H5. Face boards or rails above ground on the same wall can reasonably be H3. Getting this split wrong in either direction either wastes money on over-specified above-ground timber or under-specifies the one component where failure actually matters. The Ministry of Business, Innovation and Employment building guidance sets out the hazard class requirements for timber used in different exposure situations, and it is worth checking against before an order goes to the yard, not after the delivery arrives.
Where this connects to other timber structures
The same logic applies to fence posts, which sit in a slightly different category again since they are in-ground but rarely carry the kind of structural load a wall pole does; that distinction is covered in more detail when looking at fence post depth and concrete in clay soils. Getting hazard class right on one structure is a reasonable prompt to check it on others around the same section, since a batch of the wrong treatment level rarely arrives on a job by itself.
Frequently asked questions
What treatment level does a retaining wall pole need? H5, since it is both buried in soil and carrying structural load; H3 or H4 timber is not rated for that combination and will decay well ahead of a structural pole’s expected life.
Can I use H5 timber everywhere to be safe? You can, but it adds cost without benefit above ground, where the exposure H5 is designed for does not apply; matching the level to the position is more cost-effective than over-specifying.
How do I check what treatment level timber actually is? Treated timber carries a stamp or tag showing the hazard class; if a delivery arrives without a visible stamp, it is worth confirming with the supplier before it goes into a structural position.