H levels represent the appropriate level of treatment depending on the degree of biological hazard that timber will be exposed to in a specified application and specify the appropriate level of treatment (H1, H2, H3, H4 or H5).

N.B. 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.