H5 Treatment
Heavy-duty preservative level for critical in-ground timber, including house stumps, retaining walls, and poles where failure would have major consequences.
Hazard Class 5 under AS/NZS 1604.1 covers timber in critical in-ground or freshwater applications where decay or termite attack would have serious structural consequences. Typical uses are house stumps, building poles, cooling-tower fill, freshwater piles, and heavy retaining-wall components. Preservative retentions are higher than H4 and are usually achieved with CCA or copper azole, with deep penetration into sapwood. The chemistry resists the more aggressive soft-rot fungi found in wet soils. For buyers, H5 is the minimum specification for pine house stumps and similar primary structural members. Always check the end-tag and never field-modify H5 timber without resealing cuts; on-site notching exposes untreated heartwood and can void the protection.
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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