Treated Timber has been used for many years now and for a while we were able to get CCA treated hardwood, it’s not available now though for the following reason. The difference is that Softwood is what we call Open Grain timber and Hardwood is a Closed Grain timber, the CCA treatment wasn’t able to penetrate deep enough into the grain of the hardwood to protect it properly to be a good outdoor timber, so they stopped production and just stick with pine.

What is often forgotten is that the CCA treatment will not change the characteristics of pine, Pine is still Pine, it will still bow, twist, warp, crack and split, you may want to treat your structure with an oil to protect it against the elements.

There are other options for treated timber other than the CCA treatment, if you need your timber treated for outside use then consider Creosote and PEC, (Pigment Emulsified Creosote) these are both preservatives that are heavy duty oil-based and although they have been used to treat outdoor timber against insect, decay and attack from borers, I believe that they are being replaced with a product called Cleansote, which doesn’t effect peoples skin as much and can be used successfully on softwood and hardwood. So check with your Hardware to see what they can get for you.
 

 
For more information about treating timber you can check out a site called TPAA. Timber Preservers Association of Australia.

Retaining walls can be built from Treated Sleepers, which make the perfect outdoor timber, easy to use, and come in four available sizes as well as different lengths, these profiles are:

200 x 100 (8″x4″) and 200 x 75 (8″x3″)

150 x 100 (6″x4″) and 150 x 75 (6″x3″).

They also provide 50mm (2″) thick boards but I would only use these for garden boarders and not for higher Retaining walls.

Whereas the Landscape Sleepers are available in two sizes, which are:

200 x 100mm (8”x4”)  and 200 x 75mm (8”x3”). These aren’t as readily available as much now but can still be ordered and purchased through certain Landscape supplies.

All the sleepers will generally come in 2.4m (8’) long. Some timber mills will cut you specific sizes of course but this tends to be more costly.

Second hand Hardwood Railway Sleepers can also be used, you will have to hunt them down as they are not readily available, they produce another type of look to your garden and also come in a variety of dimension, generally the most common being 2.7m long (9ft) and 225 x 125mm (9”x5”)

If using any of the hardwoods then consider the possibility of using some other kind of treatment that you can brush on yourself to protect against termites and dry/wet rot, especially if your outdoor timber will come in contact with the ground

Other types of  timber that is commonly used in landscape projects would be Merbau, Tallowwood, Tasmanian Oak, Brush Box, Jarrah, Spotted gum, Iron Bark and Cedar, all of which have a number of uses and a variety of treatments available to preserve them. These timber types can all be ordered and purchased from any decent timber yard and prepared for you use accordingly.

IMPORTANT   NOTE.

CCA Treated Pine as of March 2006 can NO longer be used for the construction of the following.

  • Picnic Tables and External Seating for park lands etc.
  • Garden Furniture.
  • Handrails for Walkways and Verandas.
  • Childrens Play Equipment.
  • Domestic Decking Boards.

If you are considering building any of the above then they should be built from other timbers like some of the hardwood mentioned. you may have to order it from your supplier as they are often not readily available and they will cost more.

It is still Ok to use Treated Pine for construction in the following.

  • Pergolas.
  • Retaining walls.
  • Fencing.
  • Screens.
  • Structural Timber.
  • Deck Framing.
  • Other applications that are not relevant to landscaping.

All the best with your projects.