Decay hazard zones
Climate is one of the most significant environmental factors affecting the serviceability and performance of construction timbers. Subtropical to tropical climates, with periods of warm temperatures and high humidity, pose a higher risk of attack by fungal organisms that cause decay than drier climates. There is a strong correlation between climate (rainfall days per year) and decay hazard, so Queensland's geo-climatic zones are classified with a rating for decay hazard, for both above ground and in-ground decay.
Queensland's local government area (LGA) boundaries are each classified with the appropriate decay hazard rating (1 of 4 levels) that applies to construction in those locations. These classifications are based on maps produced in the Timber service life design guide (MacKenzie et al. 2005).
Each LGA is allocated 1 of 4 levels of timber decay hazard for above ground decay and 1 of 4 levels of in-ground decay. Above-ground decay hazard zones are Ag:A, Ag:B, Ag:C, Ag:D and In-ground decay hazard zones are Ig:A; Ig:B, Ig:C, Ig:D.
Above-ground decay hazard zones for Queensland
Ag:A—Least potential for above-ground decay
Ag:B—Lower potential than zone Ag:C and greater potential than zone Ag:A for above-ground decay
Ag:C—Lower potential than zone Ag:D and greater potential than zone Ag:B for above-ground decay
Ag:D—Greatest potential for above-ground decay
In-ground decay hazard zones for Queensland
Ig:A—Least potential for in-ground decay
Ig:B—Lower potential than zone Ig:C and greater potential than zone Ig:A for in-ground decay
Ig:C—Lower potential than zone Ig:D and greater potential than zone Ig:B for in-ground decay
Ig:D—Greatest potential for in-ground decay
A moderately durable (Durability class 3) timber species would not be suitable for in-ground use in the wet tropics (zone Ig:D, the highest potential for in-ground decay), but could be expected to give satisfactory performance if used in the arid zone (zone Ig:A, low hazard) provided it is also protected from termites.