The most readily available timber decking boards are Blackbutt, Spotted Gum, Tallowwood, Yellowbox, Merbau and Bamboo. There are other Australian hardwoods suitable for use, such as Ironbark & 'River Reds', but not as widely available.
We prefer to use Merbau. It's physical properties are equal to or better than the others, but more importantly, it is more stable after installation.
The Australian hardwoods are more reactive to the weather, resulting in more shrinkage/expansion, are prone to cupping, and more susceptible to splitting and weathering.
We understand there is some reluctance to use Merbau, due to doubts over it's provenance.
& importers are required to undertake due diligence to minimise the risk of illegal imports.
Some Clients also prefer the colours & variations in the Australian hardwoods, or to match internal flooring.
Boards are 19mm thick, with 70mm, 90mm & 140mm widths available, with the Australian hardwoods a few mm narrower. Planks are also available in 42mm thick in Merbau, with a limited range in the Australian hardwoods.
The width of decking boards vary by 1 or 2mm over the length of the board, except for engineered boards. We set out every 5 or so boards to a chalkline, to ensure the boards are laid in straight lines & not wavy. The variation in board widths is therefore accomodated in adjusting each gap width over the 5 boards. This is particularly important in long decks.