Construction applications with similar environments and conditions of use are grouped into seven (7) applications:
• 5-year target design life (all above-ground applications)
• 15-years target design life (protected, weather-exposed and in-ground applications)
• 50-years target design life (protected, weather-exposed and in-ground applications).
These categories apply to commercial timbers used in constructing Class 1 buildings (houses), Class 10 buildings (carports, garages, greenhouses, sheds) and associated landscaping. Applications with similar requirements are described in the Guide: Target design life.
Classifying timber applications assumes that normal, good design, construction and maintenance practices are followed, particularly by no creating damp, poorly ventilated situations.
Damp situations develop from deficiencies such as water-trapping design, faults in flashing, sarking and other forms of damp-proofing in and around wet areas such as bathrooms, and in roof and external wall framing. Practices that create damp, poorly ventilated situations include:
- filling sub-floor cavities between false joists on slabs (pugging);
- using moisture-retaining types of wall insulation and providing inadequate free air movement through vented wall, roof and sub-floor cavities.
The specification or use of durable timbers is not a substitute for good building practice.
Weather-exposed or in-ground applications subdivisions fall into broad decay hazard zones, based on those described in the Timber service life design guide (MacKenzie et al. 2005) and rationalised to Queensland local government area (LGA) boundaries.