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Ask an Expert

Exterior timber, decking & cladding

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. I am looking for information on KD blackbutt ship lap cladding. particularly acceptable shrinkage levels, Is there an AS that can be referenced?

Shrinkage (if any) will depend on the initial moisture content of the wood and the climatic conditions it's exposed to. The Australian Standard that governs hardwood cladding is AS 2796, "Timber - Hardwood - Sawn and milled products", and the part dealing with moisture content is Part 1: Product specification. Section 8 of AS 2796.1 deals with moisture content as follows: "The moisture content of fascia and bargeboards shall be not more than 18 percent and not less than 10 percent or as otherwise specified in accordance with Clause 2.2. The moisture content of cladding shall be as for fascia and bargeboards or may be greater than 18 percent where the profile and intended fixing method allow for the greater expected shrinkage". Clause 2.2 states that "Where it is desired to have a moisture content specification other than the moisture content range specified for a product then, by agreement between the purchaser and supplier, an appropriate range may be specified or a drying quality may be specified". Drying quality can be specified in accordance with AS/NZS 4787, "Timber - Assessment of drying quality".

Q. We have just laid a new timber deck in spotted gum, it was sanded and one thin coat of natural oil and looked great when finished. The coat was wiped down to ensure there was no excess. Within a week the timber started to show black marks and patches which in some areas resembled spores of some description. One week on, the deck is now two thirds black stained so we hope re sanding will remove the stains before again re-oiling. We have no surrounding trees, so cannot understand what has caused the problem. When wet the stain remains but leaches and runs, which further extends the stained area.

It sounds like a case of "iron stain". Metals react with tannin in the wood to form a blue-black stain in the presence of moisture. On new timber this is more likely than mould, but there is a quick way to find out. Oxalic acid (or a brand-name timber cleaner containing oxalic acid) will remove iron stains. A mild solution of bleach will remove mould. To find out more about this problem write "iron stain on wood" in your browser and a detailed data sheet will come up, issued by the US Forest Products Laboratory. It's often hard to know where the iron stain has come from, but if there has been any new metal work nearby that would be an obvious source.

Q. I am about to construct a Post and rail fence using timber. The posts will hopefully be 3.0 m apart and i wanted to use a 190x45 mm treated pine on its edge as the rail. Is this suitable timber and will it sag?

There's no strict rule about this, but treated pine is inclined to move around a bit if not restrained, particularly if the timber is uncoated which we assume will be the case. The rails aren't likely to sag, but might bow, so you could finish up with some curved slightly inwards and some slightly outwards. Not a good look! So perhaps it would be better to put the posts a little closer, say at 2.0m centres rather than 3.0m. Apart from that, treated pine is quite suitable.

Q. Location, Sydney inner west. orientation North/South. Item, double pedestrian gate approx 2 metres X 750mm, fabricated from western red cedar around 15 years old well maintained. Problem is I am told wet rot. I am reluctantly thinking of replacing the gates with powder coated aluminum. Merbau has been recommended as a superior timber for this application. I would appreciate your advice in this regard. I would also appreciate your advice regarding the best timber to use and it's useful life if maintained and preferred coating.

We would expect western red cedar to last longer than that. We wondered if water was getting into the joints, or if it was possible that a different timber was used. Nevertheless, merbau would be a good choice - it is rated Durability Class 1 outdoors above ground, whereas western red cedar is Class 2. Merbau has a high tannin content and if bare wood is exposed to rain tannin will leach out, leaving a brown stain. This can be avoided by painting the wood all round, prefereably with an enamel paint.

Q. Am looking to restore post and rail fences and mangers in a milking shed built approx 1860 in Adelaide Hills. Can you please advise where I could try for suitable gum to maintain authentic look?

In Adelaide you could try Otto & Co at Stepney, phone 8362 3522. They have a range of materials for post and rail fences which you can inspect at their timber yard. You could also try Redgum and More at Verdun.

Q. I have a brand new Merbau deck that I have recently installed. I then had another company to install a steel pergola over it. They placed some steel over the deck and I noticed it has black stains and looks quite worn out when I mentioned it to them they said it was because it wasn’t sealed properly. Can placing steel over deck for short time can do that. My question is how can I fix it so it looks like brand new again?

Your problem sounds like a case of "iron stain". Metals react with tannin in the wood to form a blue-black stain in the presence of moisture. Oxalic acid (or a brand-name timber cleaner containing oxalic acid) will remove iron stains. It hasn't anything to do with whether the deck was sealed properly although, of course, a paint coating would prevent it. It can happen even on a well-oiled deck. To find out more about this problem write "iron stain on wood" in your browser and a detailed data sheet will come up, issued by the US Forest Products Laboratory.

Q. Please advise Merbau or Jarrah which timber is better for decking? I'm in Perth and the deck will be partially covered by shade cloth roof, but some parts will be totally exposed to weather. Lot of people use Jarrah for decking in Perth. I'm not sure it's because better timber or because originated locally, so better availability. Do you recommend other type of wood for decking?

Merbau or jarrah are both suitable for decking. Jarrah, of course, is a local product in WA but it is also used in other States since it is classed as a durable timber (Durability Class 2 outdoors, above ground). Merbau is rated Durability Class 1, which gives it a slight edge. On the other hand, merbau leaches more tannin than jarrah. Perhaps local timber yards have some displays of decking that have been exposed to the weather for a period, so you can compare their performance.

Q. I live in Fremantle and am interested in an exterior timber cladding on the unshaded eastern and northern sides of an addition to our house. Currently thinking of using either Jarrah or Pacific Teak (Vitex Coffasus) and letting it silver off. The supplier will be pre-coating all 4 sides of cladding with Cutek CD50, and then re-coating at completion of project with another coat. Are these good choices of timber for cladding in Perth conditions for durability and low maintenance? Would you recommend something else? What sort of longevity might we expect? How often would we need to retreat the cladding?

We suggest that vitex will have better weathering properties than jarrah if left uncoated to turn grey. Jarrah is a durable timber outdoors, but perhaps a little less dimensionally stable. Vitex starts off yellowish-brown with a slightly grey tinge as its natural colour, so it's already on the way to turning grey. We don't see any advantage in pre-coating the timber with Cutek. Pre-coating it will only delay the weathering process and won't have any long-lasting effect if it's only a one-off application. Note that the weathered look is only achieved where the timber is directly exposed. It will retain its natural colour under eaves, verandahs or in other protected locations. It's hard to predict longevity, but assuming there are no adverse factors such as garden beds against the wall, watering systems striking the timber, heavy rain splashing at the base of the wall, etc. it will last many decades. Regarding how often to treat the cladding, once it has turned grey you could apply Cutek or a similar product to give it some protection although it's more usual to just leave it uncoated. Also you would need to be careful that applying oil to grey timber doesn't give it an odd looking colour.

Q. I am currently working on a project and we are covering the outsides of a cafe in timber cladding. The cafe is near the beach (not sure if this matters). Anyway, I need a durable outdoor structural timber I guess it will need to be a type 1 ? Could you please suggest some appropriate timber options to meet this purpose? Will a particular treatment be needed?

We feel hardwood cladding would be the best choice in a public place. Boral Timber produces weatherboards from locally-grown hardwood and you can find out more from their website at Also, don't overlook "Weathertex", an Australian-made reconstituted cladding material made from sawmill waste and forest thinnings. For more about this product visit their website at Weathertex is designed to be painted, and perhaps this is the most practical finish in a seaside location. Hardwood cladding can be oiled, but natural finishes such as oils need regular maintenance when fully exposed to the weather.

Q. We are currently doing documentation for a Stockland Shopping Centre in Sydney and we have a few questions regarding specification of timber. The timber is Jarrah, Questions:- 1.Recladding an external concrete wall using timber cladding as feature at the lower portion to 4600mm high. The timber board to be 150mm width. Please advise of the thickness of board required, 20mm or 35mm? Furthermore, is it better to use timber batten or steel furring channel as a substructure in this situation. 2.Relining top of external concrete planter edge with timber to form a timber seat, what timber thickness ? 3.Please advise on grade of timber required ( H3, H4 etc.) for the above. 4.Please advise on the finishes system on the timber. We are after a clear finish look. Note that this is a high traffic shopping centre and the timber will need to be durable and be able to withstand knock and be able to clean easily.

Our answers are as follows: 1. The ratio between width and thickness (sometimes called the "aspect ratio") has a bearing on the likelihood of cupping. A US reference suggests that the width of boards exposed to the weather should not exceed eight times their thickness, so 20mm would be OK as long as the cladding is securely fixed in place. Maybe your supplier can quote a standard thickness between 20mm and 35mm that would give you a bit more security. It's preferable to fix to timber battens. There have been problems with screws shearing off when timber is fixed to steel, particularly when outdoor decking is fixed to steel joists. 2. To form the timber seat we suggest the 35mm thickness, and if possible provide an air space between the underside of the timber and the concrete. It's important to avoid cupping as the hollowed surface would collect water (assuming it's exposed to rain). 3. It's not necessary to specify a Hazard Class - H3 & H4 relate to preservative treatment (eg. treated pine) whereas jarrah doesn't need treatment. 4. If the timber is exposed to sunlight you should choose a clear coating that contains UV filters. If it is under cover, not exposed to sun, a floor varnish will give you a hard-wearing, easy to clean surface. Clear finishes fully exposed to the weather need to be maintained and it is important that the client understands the need for this.


Did you know?

Australia’s native forests, timber plantations and wood products are net absorbers of greenhouse gases, sequestering 56.5 million tonnes of carbon dioxide in 2005, reducing Australia’s total greenhouse gas emissions by nearly 10%.