H-series

The H-series is a system for classifying the hazard level of timber, which determines the type and amount of preservative treatment required to protect it from decay and insects. The higher the number, the greater the hazard and the more protection the timber needs. This system is based on Australian Standards and is crucial for selecting the right timber for a specific application.

H-series hazard levels:

H1 (Inside, above ground): This is the lowest level of treatment. It's for timber used indoors, in well-ventilated areas, and completely protected from the weather. It primarily protects against borers, but not termites.

H2 (Inside, above ground): Similar to H1, this is for timber used indoors and protected from wetting. However, it provides protection against both borers and termites.

H3 (Outside, above ground): This is for timber used outdoors, but not in contact with the ground. It's exposed to periodic wetting and leaching, and is treated to protect against moderate decay, borers, and termites. Examples include decking, fascia boards, and outdoor framing.

H4 (Outside, in-ground): This is the hazard level you asked about. H4 timber is designed for applications where it will be in contact with the ground or in continually damp situations. It's treated to resist severe decay, borers, and termites. Common uses include fence posts, garden beds, and pergola posts that are set in the ground.

H5 (Outside, in-ground, or in fresh water): This level is for timber in contact with the ground or in freshwater. It offers protection against very severe decay, borers, and termites, and is used for critical applications like retaining walls, house stumps, and building poles.

H6 (Marine waters): This is the highest level of treatment, designed for timber that will be in prolonged contact with saltwater. It protects against marine wood borers and decay. Examples include boat hulls, jetty piles, and marine-related structures.

Key points about the H-series system:

Higher number, greater protection: A timber treated to a higher H level (e.g., H4) can be used for applications that require a lower level of treatment (e.g., H3, H2, or H1), but not the other way around.

Application-specific: The appropriate H level is determined by the environment and intended use of the timber.

Chemicals vary: Different chemicals and treatment processes are used to achieve the various H levels, with the higher levels requiring a higher concentration and deeper penetration of preservatives.