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Preservative treatments & finishes

Like to know what wood to use building a pergola or framing a barn? How about the difference between tung oil and polyurethane as a floor finish? How to revive decking or deal with merbau stains? Or how to meet the building code or bushfire standards? Or ask about the environmental advantages of wood or forest certification?

Q. Hi If I rip an H3 treated pine plank down its length to get battens of my required size, will each of the battens have adequate termite treatment? I am looking for H3 battens 35 - 50 mm X 20 mm X 2700 mm. I need approximately 465 metres of battens Your advice will be much appreciated. Regards Paul

If you write 'rip treated timber along length' or similar words in your browser you will find that treatment companies tend to void any warranties if this is done. The reason is that the preservative doesn't always penetrate the full cross-section, so ripping it lengthwise might expose an untreated surface. Having said that, if you are working with timber that is approx. 100 x 20, and ripping it into 50 x 20 there's a better chance that it will have been thoroughly treated as it is relatively thin. However, be aware of the possible consquences.

Q. We are building a new home and our verandah posts are 200x200 merbau. I need to know what type of oil to treat the timber with. Is Lindseed oil fine or because of the already 'oily' Merbau is there a specific oil we should be using. Appreciate your opinion please. Regards Andrew

It's best to use a proprietary product rather than straight linseed oil. Many brand-name finishes have a linseed oil base, but they also incorporate anti-mould chemicals. Plain linseed oil is inclined to turn black in damp situations since it provides a food source for mould. We suggest decking oil or one of the many similar products marketed under names such as 'outdoor furniture oil', 'exterior timber oil', etc.

Q. Hello, I've just had my hardwood floors repolished and am not happy with the result. Along the edge of every board the polyurethane has retreated from the edge which has resulted in an uneven finish. The tradesman insisted that this is because of a previous wax finish and that they used an additive in the polish to minimise this problem. Is he correct in that the best that can be done is an uneven finish? Or is there a solution? My concern is that prior to the re-polish, the finish was even and right to the edge of every board, why is it only a problem now? Would very much appreciate your help. Thanks in advance. Jason

We weren't sure if the previous finish, which was evidently satisfactory, was also polyurethane or whether it was a wax finish. If the floor was previously coated successfully with polyurethane there doesn't seem any reason why wax would suddenly be a problem. On the other hand, it is true that traces of wax will cause polyurethane to be rejected, or fail to cure. If the previous coating was polyurethane, and the floor has simply been re-coated with a similar product, there shouldn't be any rejection of the new coating. In this case perhaps the top coat has been applied too quickly, before the solvent in the coating beneath completely evaporated. We suggest you find out what brand of coating the contractor used and discuss the matter further with the manufacturer. Most major coatings companies have a customer help line for technical advice.

Q. Hi I would like to know whether H4 CCA treated pine posts can go directly into the ground for a patio, without being on stirrups? I've been building patios for 20 years and now I'm told H4 cannot be used as it needs to be on stirrups to prevent rotting. Is this correct? I have contacted the local council in Perth but they told me to find out the information for myself. Is there any written information I can show the council regarding this matter? Thanks, Paul

There seems to be a misunderstanding on the part of the Council. Hazard Class H4 is the appropriate one for in-ground timber, while H3 is for above-ground timber, eg. posts on stirrups. You can refer the Council to Table 1.1 in AS 1604.1 for confirmation. Of course, these recommnendations assume that the timber has been correctly treated in accordance with Australian Standard 1604, Specification for preservative treatment, Part 1: Sawn and round timber. No doubt your supplier can provide a statement or warranty to this effect. In addition to selecting the correct Hazard Class we recommend that the original, uncut end should be placed downwards in the ground, since the preservative penetrates readily into the end grain. Cutting to length sometimes exposes an untreated core if the preservative hasn't penetrated the complete cross-section.

Q. Hi there, I need to replace my wooden verandah posts (and their collapsing galvanised saddles) with new wooden posts. I would like to add a stone clad pillar surround to the bottom half of the wooden post. I've previously attached this stone to a 'stoneboard' sheeting product (designed for this purpose) over a timber frame. I was hoping to use a similar method. I am concerned about termites and rot and don't want to embed my post into the ground in concrete, nor do I want to create a nice warm moist box around my post that might encourage termites. My house is timber framed and clad in cedar boards. Can you advise of any design and/or standards that might be applicable for wood preservation in my pillar and post project please? Thanks.

You have correctly analysed the potential hazard for timber in your pillar and post project, where the timber could be exposed to decay and termite attack. We suggest treated pine for all the enclosed timber and for the post itself. The appropriate treatment level would be Hazard Class H4, as recommended for timber in ground contact.

Q. Hi - my strata block had timber decks installed outside each apartment at the end of 2000. Hardwood was used for stairs, floor decking and joists, but treated pine for all else. Since 2012 we've had significant rotting in all upright posts, which are footed in concrete using metal brackets, and hand rails. All treated pine has been painted and maintained. All hardwood is in good condition still. My question is to whether this is normal to have such significant rot after only 12-14 yrs, to the point that all posts and hand rails now need to be replaced and we now need metal poles to hold up some corners of the decks? Someone told me H4 grade pine should have been used and we could have expected around 25 yrs life out of the treated pine with H4, as opposed to the short duration we've experienced. I'd appreciate any advice you can give. thanks Tim Wallace

We do not consider that 12 years is a satisfactory life for treated pine. Treatment to H3 level should have been sufficient for timber out of ground contact, as long as the treatment complied with Australian Standard 1604. Any timber actually in contact with the ground, or in concrete, should have been treated to H4. You don't mention the type of treatment, but AS 1604 covers all processes. Given the premature failure of the treated pine, the contractor might wish to have some samples analysed to see whether the treatment complied with Australian Standard requirements. Laboratory analysis can be carried out by Symbio Alliance and other similarly equipped testing laboratories.

Q. How do I identify if old decking and its framing has been CCA treated ? I live in Cheltenham, Melbourne.

Usually timber treated with copper-based preservatives (CCA, ACQ, etc) retains a greenish tinge, but if it has been exposed for a long time it might have weathered off leaving the timber looking grey. If the timber is pine, and it has achieved a long life without decaying, then it was almost certainly treated since pine doesn't have a high resistance to wood rot. To be absolutely certain there is a chemical test using chrome azurol as a colour reagent. It turns dark blue in the presence of copper. You will find more information on the net about this if you write "chrome azurol CCA test" in your browser.

Q. What is the effectiveness of seasoned radiata timber being treated with a termiticide when the timber frame is erected?

Pre-treatment is preferable as the solution then penetrates several millimetres into the wood. Also it's not possible to treat all surfaces of an erected frame, eg. the underside of a bottom plate sitting on a concrete slab. However, a generous flood coating of a boron-based preservative, for example, will be beneficial - preferably in conjunction with a barrier system such as Termi-Mesh or similar.

Q. Are H4 Sleepers safe for a childrens playground?

The answer depends on the type of preservative that has been used to achieve H4 level. CCA-treated timber is not allowed for playgrounds on account of its arsenic content and the possibility of hand to mouth transfer of traces of arsenic on the surface of the wood. However, there is no problem with arsenic-free preservatives such as ACQ or copper azole, the latter marketed as Tanalith E.

Q. Could you guide where I can find a Group 1 or Group 2 timber to be used as internal lining in a small area within a smoke lobby.

Plain untreated timber does not comply with the requirements for Material Groups 1 or 2. Timber products generally fall into Material Group 3. However, timber veneers, bonded to a fire retardant treated substrate, achieve Material Group 1 or 2 status. For example, Briggs Veneers have a Material Group 2 product (see and Austral Plywoods have an FR panel that meets Groups 1 and 2 (see


Did you know?

A government report showed there is no evidence proving that harvesting timber from native forests has reduced overall forest biodiversity or led to the extinction of any species of plant or animal.