A question that is often posed to me is whether to use CCA Treated SA Pine in my decks sub structure or to use Balau or another similar hardwood.
Balau is a very good, hardy and durable wood to use in outdoor wooden decks and other outdoor projects.
It contains toxins that limit insects eating it and it is high in resins and oils which naturally repels water and limits rot. All wood will eventually rot. It is just that some will rot quicker as they are less dense and less oily which means they will absorb water more easily, which remains in the wood, causing fungus to grow which breaks down the fibres in the wood and is commonly called rot. Water doesn’t cause rot. Fungus, as a result of water and sunlight, causes rot.
CCA Treatment is a process of pressure treating SA pine. A vacuum is created in a chamber that contains the pine and a solution of copper, chrome and arsenate is introduced which then takes up the void created by the vacuum sucking the solution into the cells. The copper prevents fungus growing which in turn prevents rot, the arsenate keeps the insects away and the chrome binds the two to the wood so that I doesn’t leach out.
Balau is too hard and dense to treat.
Various categories of CCA treating are used for different applications, H3 is suitable for timber outdoors above ground. A correctly treated piece of pine to H3 will, as per SAWPA guidelines, last in excess of 50 years which is pretty impressive in comparison to a piece of balau that comes with no fixed life expectancy.
Pine is considerably cheaper than balau structural timber. So from an economic point of view it makes sense to use pine in place of other woods wherever possible.
To use Pine as you decking one would need to use a 32mm thick plank as opposed to a 19mm Balau plank making Pine more expensive. Balau is dense and hard as opposed to Pine being softer and more susceptible to cracking, warping and twisting.
Therefore the answer is CCA Treated SA Pine for the sub structure and Balau for the decking.