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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 note that in your Brochure on Balustrades it is stated that the mimimum softwood dimension is 42 x 19 mm or 25 mm diameter for balusters. I am proposing to use finger jointed LOSP pine 42 x 32 which after trimmming approx 2 - 3 mm at the end I can rebate into the treated pine frame used for framing treated pine lattice. The thickness of the baluster at the rebate area will thus be approx 29 mm and the baluster width will be 42 mm. I have searched everwhere to try and determine whether this will comply with the BCA standards. Can you give me your thoughts.

We weren't sure if we understood your question correctly, but it seems that if the minimum dimension for balusters is 42 x 19, then your trimmed size of 42 x 29 would be more than adequate. Make sure there is provision for drainage in the bottom channel, because if water collects in it the life of the timber will be shortened. Feel free to contact us again if this doesn't answer your question.

Q. I am replacing the stringers in my boat, I'm looking at using LVL, Fishing forums on internet don't give much information on LVL they seem to lean toward plywood, My stringers are size from 80mm x 40mm x 4mt, 160mm x 40mm x 4mt, 300mm x 40 x 4mt, My questions, Is LVL suitable for use for boat strings?, Is LVL rot proof?, Will the glue used to laminate the wood react to the epoxy resin I will use to seal the wood, when I fibre glass the complete stringer to the hull?

LVL is available with an anti-termite treatment, but we are not aware of any LVL that is available off the shelf with an anti-rot treatment. It might be possible to have it custom treated, but you would need to arrange this with the timber supplier and/or treatment plant. If you are unable to obtain treated LVL you would be relying on the epoxy to provide a complete seal against moisture. Perhaps in this case a durable kiln-dried hardwood would be a better choice.

Q. Could you advise me how I could completely weatherproof an outdoor mirror with a wooden backing board and an 8 cm wooden frame? Is there some sort of marine grade treatment? Previously I varnished the frame, however after 12 months or so, the frame rotted and fell apart.

For wood to achieve a long life outdoors it's a question of selecting the right kind of wood, as well as a suitable finish. If your previous frame rotted after only 12 months we assume it was a non-durable wood, or one classed as "perishable". For your new frame we suggest western red cedar which is reasonably durable outdoors. It will still benefit from pre-treatment with a water repellent such as Cabot's Bar D-K or similar. The timber should be cut to length and treated before the frame is assembled, with particular attention to the end-grain. Once the water repellent has completely dried, marine grade varnish can be applied. Again, this is best done before the frame is assembled so all surfaces can be sealed.

Q. I need to relay the verandah floor on my edwardian property, with T&G for the correct period look and feel. Most locally appear to be ~80x19 Jarrah and would have been top nailed originally I believe. The area is about 1.6 x 5m and the board would be parallel to the short side (i.e. 1.6m long) The sub-floor framing does have a small fall on it for drainage and it is covered but it does get rain on it. I know I need to leave a 10-12mm expansion gap around the edge but should I be leaving small gaps between each board or clamping them tightly together? Does this advice depend on whether the flooring is new or from recycled timber? Also what size fixings would you recommend, the joist are new KD hardwood? Should I use damp proof course between the boards and joists?

On a modern house we would recommend spaced decking that allows the wood to swell and shrink in response to wetting and drying. However, we understand the desirability of retaining the period look. There may be a small difference between new and recycled timber as far as moisture content is concerned. Recycled timber might be slightly drier, say 9-10% moisture content, while new timber should be around 12%. Perhaps your suppliers can confirm this. Whether you use new or recycled timber, the boards shouldn't be cramped tightly together. There may be shrinkage during the hot summer months, depending on the local climate, but you should anticipate some swelling when the boards are rained on, so we would like to see a slight gap of around 1mm if recycled timber is used. This can be achieved with a cardboard spacer or similar, as long as the boards are straight. It will be somewhat more difficult if they are not straight. The boards could probably be lightly touching if new timber is used. We don't recommend damp course material under the flooring as it may serve to retain moisture in the event that water finds its way through the floor. It might be worthwhile using stainless steel or hot dip galvanised nails to avoid corrosion - the timber code stipulates 50 x 2.8mm bullet-head nails if hand-driven. And for nailing into KD hardwood joists you might want to drill small pilot holes to avoid splitting.

Q. I would like to fix or bond 12mm end grain marine ply AA grade into U channel stainless section. The section is part of the frame of an outdoor chair and is 400mm long. Previously the seat had wood slats fixed by 10mm stainless screws into the end grain of the slats. With age the wood on the ends sitting in the SS channel has rotted and failed hence the option of replacing the slats with marine ply. Also does marine ply need a finish for protection or can the hardwood age to grey safely?

We would be inclined to replace the rotted slats with a more durable timber, rather than marine grade plywood. Unless preservative treated, the species commonly used to make marine plywood are not highly durable. Marine plywood species are selected for other qualities such as impact resistance, closeness of grain, etc. This point is referred to on the Austral Plywood website at Also, plywood exposed to the weather without a finish is inclined to develop surface crazing (fine cracks). If the slats are replaced with a highly durable hardwood such as ironbark, tallowwood or similar they should achieve a long life even if allowed to go grey without a finish.

Q. I have installed prefinished cedar eaves. After 4 years they are showing signs of failing. What can I do to restore before they rot and perish?

We assume that the "cedar" you have is western red cedar. If so that's a very short life for this type of timber. As a Durability Class 2 timber when used outdoors above ground, cedar has a probable life expectancy of 15-40 years depending on maintenance and weather exposure. It's commonly used for roof shingles, fully exposed to the weather with no finish at all. So there is some unusual factor operating in your case. Perhaps the design of the eaves is such that water is trapped in the timber, or in joints. If the wood has already started to rot you may need to think about replacing it. However, before doing so it will be necessary to solve the construction problem that has caused the timber to fail prematurely.

Q. I'm considering constructing some window sashes complete with glazing Q: would either of these timbers be suitable, dark red meranti or jarrah ? Thank you for you advice.

Dark red meranti and jarrah are both suitable for window joinery, but jarrah has a higher durability rating. Dark red meranti is rated Durability Class 3, while jarrah is Durability Class 2 outdoors above ground, according to Australian Standard 5604. That is not to say that dark red meranti won't give adquate service, unless there is severe weather exposure, in which case jarrah might be the better choice. Note that if you intend to use an oil or stain finish, the glass should be installed with wooden glazing beads bedded in silicone - putty performs best when protected by a paint finish.

Q. Re 7mm CD structural pine plywood: would this be a suitable material to clad a garden shed in an area that can have high humidity. Am thinking of boiling up a mixture of sump oil and Ormanoid (bitumen based waterproofer) to paint onto both sides of the plywood as a maintenance protective coating.

Sump oil is a good traditional treatment for timber outdoors, but we would prefer to see you using preservative treated plywood for greater reliability in the longer term. A better plywood cladding product is Shadowclad, which is pressure preservative treated and grooved to simulate vertical boarding. You can find more information on the net if you write "Shadowclad" in your browser.

Q. I am working with an architect as he needs Western Red Cedar for prefab buildings. He would like to know the fixing requirements for timber wall cladding, and the relevant wind loadings. are there any charts? Please let me know if possible.

Various suppliers have their own installation instructions, but it looks as if you are a supplier so we assume you want some "generic" details. One of the most comprehensive cladding guides that is not company-specific was produced by the National Association of Forest Industries as part of their datafile series - now out of print but available for download at this address: Regarding wind load, this is really an issue for the bracing in the wall frame rather than the cladding. Guidance on wall bracing can be obtained from Australian Standard 1684.

Q. I am hoping you can provide me with some guidance please into what is the best method to attach slats 70mm x 19mm as an outside screen. I had a screen as a feature around my garden area and the timber twisted and came away from the structure. I didn’t build this I had a builder come in and do it. I’m getting quotes now for another screen to be erected and does it matter whether you use nails or should the slats be screwed in. I have also gone to a 90mm x 19mm timber in northern box. I’d appreciate your advcse as it has been a costly exercise and I want to get it right his time.

Your new slats sound like a decking profile. Decking is often fixed with nails, preferably twisted nails for greater holding power. However, decking is fixed with two nails per joist and the joists are spaced not greater than 600mm apart. So whether nails are adequate or not depends on how closely they are spaced, and also the type of nail. If the supports for your slats are further apart than 600mm it might be a good idea to use screw fixing this time for greater security. It's not too difficult to use screws these days with "self-drilling" screws available that drill their own hole and tap their own thread.


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.