Timber treatment: what are the best options?

By Ken Collins 11/05/2010 2


timber treatment 2Some time ago there was a lot of media coverage on the use of CCA (Copper Chrome Arsenic) timber preservative. This caused a number of our clients to ask for alternatives, so we did a little research into what the implications are.

Pine needs to have large amounts of preservative to stop it rotting when in contact with water, and especially in ground.

There are two alternatives to CCA commercially available: Copper Azole based (CuAz) and Alkaline Copper Quaternary (ACQ). Both rely on very high concentrations of copper to act as an agent against fungal and insect attack, and as such both are strongly alkaline.

Unfortunately this also means that these products are very aggressive to mild steel and even galvanised steel. The Building Research Association of New Zealand (BRANZ) did some testing and found that in comparison to CCA treated timber:

  • In timber treated to H3.2 (the treatment level for timber in occasional water contact, often used in exterior wall framing, in wet areas, and outdoors above ground contact like decks), mild steel corrodes at rates of up to 5 times in CuAz and ACQ preservative.
  • In timber treated to H5 (the treatment for timber in contact with the ground, eg floor piles, structural posts, and fence posts), mild steel corrodes at rates of up to 12 times in ACQ preservative. (you can’t get H5 treatment in CuAz).
  • Galvanised steel corroded at a slower rate than mild steel, however the corrosion rate of galv steel in CuAz and especially in ACQ was still significantly higher than in CCA treated timber.
  • 316 Stainless Steel performed well in all the preservatives with minimal differences in corrosion rates.

A copy of their Conference Paper is available here and the more in depth Study Report can be found here.

timber treatmentIn short this means that if you use the alternatives to CCA then all the fixings (nails and screws), bolts and brackets (in-fact anything metal) that touches the timber must be 316 stainless steel or powdercoated. Consideration also needs to be given to the use of stainless steel flashings as well, because even water runoff from ACQ and CuAz treated timber will corrode mild steel, aluminium, and galvanised steel..

What ever you do, there is no current solution to preserving timber without using large amounts of heavy metals and toxic chemicals. But in saying this, you would have to suck on a lot of logs (or wood chips) to leach enough of the chemicals out of the timber for it to have a measureable effect on you. This includes CCA treatment.

So, unless you are building a children’s play area and have particular concerns about children eating the wood, CCA treated timber is still the best to use for the construction of buildings in NZ, especially where you need to treat to H3.2 or above. Not only for cost, but also because of serious durability concerns raised by the BRANZ testing.


2 Responses to “Timber treatment: what are the best options?”

  • The main concern with CCA-treated timber is how you dispose of it, because burning it converts the arsenate to arsenite, which is roughly ten times more toxic (depending on the species it might poison). Therefore ash from burnt CCA wood, which tends to have a telltale green tinge, should be disposed of carefully. Bad idea to spread it on your garden, for example. Above all, don’t spread it on pasture because cattle seem to love the taste of it and will eat it, with disastrous consequences.

    • The burning of any timber treated to H3 or above is not recommended, no matter the preservative used. I certainly wouldn’t want to put the ash from timber treated with very high concentrations of copper on my garden or pasture either. The advice to us has been that even timber treated with very high copper levels needs to be disposed of carefully. In short, to make timber useable as a construction material you need to use some very nasty chemicals. Currently there is no way around it commercially. There was a public backlash over the use of CCA treatment to timber, however what the public didn’t realise is that the alternatives came with their own set of problems too.