Ask an Expert

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. Were currently working on Burnham Beeches, Sassafras – a hospitality and residential redevelopment of the historic art nouveau estate. The client would like to use existing on site Victorian Ash (trees) for fences, logged with an onsite saw mill (see attached design and saw). We are concerned about the use of untreated Victorian Ash, especially where the posts will meet the ground. Can you please provide advice as the weather this proposal is viable? What are the issues, timber treatment and details required?

You are right to be concerned about the use of Vic ash (alpine ash and mountain ash) outdoors. Above ground the ash timbers are moderately durable as long as there are no joints that will collect water. Australian Standard 5604 rates them Durability Class 3 above ground and Class 4 in ground. The probable life expectancy of a Class 4 timber in ground is 5 years or less. We would strongly recommend a different material for the posts (treated pine or a durable hardwood) with perhaps ash for the above-ground components. However, your client should be aware that freshly milled ash will shrink as it dries and may be subject to distortion (warping and bowing) due to growth stresses, particularly if cut from relatively small trees.

Q. I would much appreciate if you could advise on the following. I am going to build a 200 mm to 500mm high timber deck of approximately 800 sq. metre on concrete slab. Can the deck be directly laid on movement joint of the concrete slab? What precaution shall I adopt when the deck is on top of movement joint? Your valuable opinion will highly respect.

Presumably the deck joists will be bolted to the concrete. A timber deck will tolerate some movement in the slab, but perhaps it would be wise to position fasteners so that the concrete can move independently. For example, try not to place the fasteners both sides of the joint. However, we don't feel it is a major issue. Movement in construction joints is usually only a few millimetres at most. A more important issue is to make sure water can drain away from under the deck when it rains. The concrete should have a slight fall to encourage drainage and if the joists sit directly on the concrete they must run down the fall, not across it, otherwise they will hold water. If water pools under the deck it will be absorbed into the timber and make the decking swell and warp.

Q. What single span can I get from a 190 x 45mm H3 F7 treated pine joist?

Assuming the joists are in an outdoor deck, 190 x 45 F7 can span 3.6m at 450mm centres. This includes decks that are more than 1m above ground.

Q. I have coastal grey box verandah posts which are opening up should I treat them?

Coast grey box usually has interlocked grain which should prevent deep splits. However, surface checks and shrinkage are likely if the timber is still drying. Painting the posts would help as it will slow down the drying rate, allowing the timber to dry more evenly. Rapid drying creates a moisture gradient between the surface and the core, with the surface trying to shrink while the core is still swollen, causing splits. Exterior stains and oils will have little effect.

Q. The eJarrah currently being sold by Masters has been pre oiled. Should this not stop the bleeding?

Oil helps to repel rain (as long as water "beads up" on the surface), but doesn't provide a very effective barrier to water sitting on the wood for prolonged periods. This is particularly so with horizontal surfaces such as decking where water is inclined to sit rather than running off. So we don't think pre-oiling will stop tannin bleed in the longer term. Maybe you could run a test by obtaining a sample of eJarrah and putting it outside to see what happens when it is exposed to rain.

Q. If I cleaned merbau with deck cleaner would it arrest the bleeding problem? The deck I want to replace is above a concrete slab and I'm concerned that if the timber bleeds the concrete will be stained forever and the house that the deck is attached to will also be stained. Please advise your thoughts as to what I can do to overcome this problem of bleeding.

Removing tannin with deck cleaner is advised by paint manufacturers, but it's not something we've evaluated ourselves. In your case it would have to be done before the deck was built, with the decking laid out on the ground, since the technique involves scrubbing the wood with deck cleaner and then hosing it off. If you tried it on the finished deck it would simply wash all the tannin off onto the concrete below, which is what you are trying to avoid. So maybe it would be better to choose a timber that is known for its low tannin content. A data sheet published by the Queensland Forest Service recommends spotted gum because it "has a lower tannin content than most other eucalypts, therefore staining of paintwork, brickwork, etc. as a result of water running over unpainted timber surfaces, is unlikely to occur".

Q. What species of timber decking is suitable for use in a shower, it is raised off the floor .

If possible it would be best to design the decking to be removeable so it could be taken out to dry occasionally, otherwise mould growth could be an issue. Such decking is sometimes referred to as "duckboards". Western red cedar would be a good choice as it doesn't splinter and is stable when exposed to changes in moisture content. All fasteners will have to be highly corrosion-resistant.

Q. Does forest red gum bleed in the same way as Merbau? When used in external decking, does it break down in seaside environment?

Most of the eucalypts have some tannin content and as a rough rule of thumb the darker the colour, the more tannin. Merbau is particularly noted for its tannin content. We wouldn't expect forest red gum to be quite so prone to bleeding, but you would have to expect some. Manufacturers of timber finishes recommend a preliminary scrub with deck cleaner to minimise tannin bleed, so you might want to try that. Forest red gum is rated Class 1 Durability, so it will stand up to the weather well. A seaside environment isn't particularly hazardous for timber but is very corrosive for metals so stainless steel fasteners would be a good idea.

Q. Hi I am designing a merbau deck in a bal19 area in Adelaide Hills. Have all spans ok. But inquiring regarding spacing of deck boards. There are a lot of trees around thus plenty of gum leaves. I am concerned that over several years there will be build up of leaf litter under the deck ... some of which will be difficult to get at. If spacing too wide, embers can readily drop through igniting the litter. Embers on deck will do little damage apart from small burn marks. Unlikely to ignite merbau decking. What is the industry best practice .. am I right in thinking a close space, say 3mm better than wider space, say 8mm. How much is the merbau likely to shrink? eg another 2mm

We usually recommend a spacing of 4mm to 6mm. Wider spacings can be a nuisance - the deck looks a bit "gappy" and high-heeled shoes can get caught. Merbau is supplied kiln dried and unlikely to shrink more than 1mm, even during hot weather. But if the boards are closer than 3mm, and there's a bit of swelling in winter, dirt can clog up the gaps and prevent drainage. So generally 4mm to 6mm is about right. Maybe the deck can be designed so leaf litter and weeds can be removed, eg. by a trapdoor in the deck, or access under to allow raking out.

Q. Can someone please contact me regarding timber cladding on a new job?

Telephone advice is not available through this service, but if you would like to leave a more detailed question we can give you a written reply. If you particularly need to discuss the matter, a telephone advisory service is available from the Timber Merchants Association in Victoria, or Timber Queensland might be able to help on (07) 3254 1989.


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.