H4 Treatment
Preservative level for in-ground timber exposed to severe decay and termite hazard, used for fence posts, retaining walls, and landscape timbers.
Hazard Class 4 under AS/NZS 1604.1 specifies treatment for timber in contact with the ground or in fresh water, where decay and termite hazard is severe. Typical applications are fence posts, landscape timbers, retaining-wall sleepers, deck stumps, and pergola posts. CCA, ACQ, and copper azole are the common preservatives, each at higher retentions than H3. The treatment penetrates the sapwood to a depth defined by the standard. For buyers, H4 should not be used for primary structural posts of permanent buildings without engineer sign-off, and H5 is required for critical or heavy-duty in-ground use such as house stumps or poles. Reseal any cuts or borings with a compatible end-grain preservative.
Values and references on this page are indicative and drawn from common industry sources. Always verify the current AS/NZS standard, manufacturer data sheet or NIAA policy publication before relying on figures for design, specification or compliance work.
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