Above-ground durability of painted and unpainted Australian and imported timbers in an L-joint field test after 35 years

Field tests that assess timber durability above ground are essential for shaping construction Standards that guide building practices. This paper summarises the condition of timber specimens after 35 years in an above-ground L-joint field test under very high-decay-hazard conditions in Australia. Forty-one timbers representing 30 botanical species were tested, including a range of softwoods such as Pinus radiata sapwood, heartwood, and CCA-treated sapwood, and a range of hardwoods including eucalypts and other Australian native species. A selection of imported timbers including Thuja plicata and Pseudotsuga menziesii were also evaluated. Unpainted and painted L-joints were tested, and in the painted joints, the paint film around the joint had broken, allowing moisture to become trapped underneath. The assessment of the samples confirmed that the broken paint coat commonly reduced durability, having this effect in 18 of the 41 timbers. The paint coat significantly increased the durability of four species, despite the moisture trapping. The broken paint coat had no significant effect for the remaining timbers. Results from this field test were used to update the Australian timber natural durability Standard, AS5604. A limitation of that Standard is that durability classifications apply for the whole of Australia, encompassing temperate climatic regions in the south to tropical climatic regions in the north. Future research on timber durability performance models that take climatic variables into account promise improved service life prediction capabilities.