H codes (H1 to H5) are conditional use codes representing appropriate levels of preservative treatments (against biological decay) required to use a specific timber in a specific application and location. Each code (H1, H2, H3, H4, H5) represents a different, required minimum preservative treatment.

N.B. Currently, the heartwood of most hardwood timbers and some softwood timbers cannot be treated to H3, H4 or H5 level of protection. The timber treater is to ensure that their process has achieved the required level of penetration to meet the stated H level specification. 

Depending on the application, preservative treatment to provide protection in H3 conditions can involve one of two levels. The lower retention must not be used in a load-bearing application where routine inspection and maintenance cannot be readily carried out. The lower retention level applies only to vertical (water-shedding) applications that are able to be regularly maintained. In Australian Standard AS 1604.1-2005 these constraints are signified in the brand by the letter ‘A’; that is, H3A.