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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. Are there any timber species that meet BAL40 requirements, particularly for a decking application.

Australian Standard 3959-2009 "Construction of buildings in bushfire-prone areas" requires decking in BAL-40 areas to be of non-combustible material or a system complying with Australian Standard 1530 "Methods for fire tests on building materials, components and structures, Part 8.1". We are not aware of any timber decking that has passed the test specified in AS 1530.8.1, and since timber is classed as combustible the only decking permitted for BAL-40 is slate, tiles or similar.

Q. I am an architect, based in Sydney Australia looking to use the Shou Sugi ban charred timber cladding technique for part of the external façade on a building I am designing. I think Cryptomeria is hard to come by here so I was thinking of using either Western Red Cedar or Douglas Fir and a natural Penofin Verde or OSMO UV Protective clear oil finish. Does anyone know of any suppliers or contractors that have already done this or that would do this in Australia? I’m finding it very hard to get technical information for the specification. Any advice would be much appreciated.

Strictly speaking shou sugi ban is charred sugi wood, ie. Cryptomeria, as you say. However, charring can be carried out on any species, preferably a softwood with prominent growth rings. Charring a softwood allows the softer portion of the growth rings to be brushed out leaving the harder part of the growth ring raised, resulting in a textured surface. Hardwoods don't have such prominent growth rings so some of the effect is lost. Western red cedar would be the best readily available alternative to sugi for an external application. We don't know of any suppliers in Australia who offer this service but you might find a joinery works in your area that is prepared to have a go. The process is described on this website -

Q. I have purchased some finger jointed jarrah from bunnings 19mm thick 5.4m in length and 125mm wide. We have just installed this a couple of days ago and it looks like at parts it's peeling or cracking even near the finger joints, I was just wondering if this is something normal? I am waiting for the 6 weeks to oil it as the lady at bunnings said give it 6 weeks before you do any oiling. Thank for your help in advance.

It's hard to give you a definite answer without seeing the timber, but it doesn't sound right. If the finger joints are peeling back it suggests the glue is not holding properly. We suggest you remove a board that shows the problem and take it back to Bunnings for their comment. If necessary they might want to take it up with their supplier.

Q. I have an alfresco area that's 4.5 m x 5.3 metres and is 90% under cover and has a concrete base that's sloped away from my house. There is a 170mm drop to the concrete from the Spotted Gum floating floor in my kitchen living area. I want to build a spotted Gum deck over it and as I have only 170 mm clearance under the decking boards I am concerned about ventilation and any issues that may cause. Therefore I don't know if I can use a 19mm decking board or should be going to a 25 mm board. I want to use a 130 mm board to match the width of the Kitchen living area floorboards. I have 90 x 45 mm joists installed at 450 mm centres on adjustable stirrups so I can get the height to match the height of the two doors that open onto the Alfresco area. The clearance under the joists is approx 60mm one end and 80 mm the other. Thanks for any advice you can give me re above

We have seen problems where timber decking is laid without much clearance underneath, but it sounds as if the fall on the concrete will allow rainwater to drain away. The joists must follow the slope and not be installed across the slope where they will hold the water back. If water is able to pool on top of the concrete it will be absorbed into the underside of the decking, causing it to swell and/or cup. It's also not recommended to enclose the sides of the deck. Although it gives a neat finish, enclosing the sides cuts off the flow of air and stops the timber from drying out after rain. The fact that 90% of the deck is under cover presumably will limit exposure of the deck to rain. Regarding the thickness of the decking, 25mm boards will be more stable than 19mm boards, since they have a better width to thickness ratio.

Q. Just had blackbutt deck finished 2 days ago parts of it are coming through with black spots some parts worse than others (almost looks like mould), don't think the chippy cut or worked with any metal cuttings over the deck, problem seems to be staining through the wood where moisture has got onto it haven't had a chance to hit it with any kind of treatment yet any ideas of what to hit it with don't want it to get any worse than it already is. Please help.

It sounds like a case of "iron stain" rather than mould. 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, since mould takes a while to grow. However, 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 "USDA iron stain on wood" in your browser and a detailed data sheet will come up, issued by the US Forest Products Laboratory.

Q. I am looking for a Australian hardwood durab 1 decking shotedge 120 x 22 or such - with a raised upper surface. My project is in Redcliffe Brisbane. Could you advise who might supply / mill such a product.

Since we are not based in Queensland we don't have a detailed knowledge of the market, so maybe Timber Queensland would be able to advise you with more confidence. Their phone number is (07) 3254 1989. We can say that Narangba Timbers have several Class 1 Durability species available in wide decking, eg. blackbutt, merbau, etc. You can visit their website at

Q. Council is the owner of a structural decking around a unit complex (Marina East at Holdfast Shores in Glenelg South Australia) that is adjacent a marine environment and has had trouble with the timber decking either rotting, splitting or warping thereby allowing the holding screws to pop out which in turn has caused an uneven surface which in turn has become a tripping hazard for users. The decking while predominantly used for pedestrians does need to take the weight of an elevated work platform (self propelled) to clean the units and therefore any timber chosen should have sufficient capacity to hold this plant. Council wishes to address these issues by replacing the timber with a product that is sustainable (long life, safe and will not have the issues discussed above) and also seeks information on fixing methods for the suggested timber and a general specification for installation if possible. The walkway is generally 4200mm wide and 75mm thick. Can you assist us with a type of timber to use and associated fixing methods (photos can be sent of existing timbers and area if required).

A publication you might find useful is Timber Manual Datafile SS4 titled "Timber Decks. Commercial, Industrial and Marine". It can be downloaded from the net via this link: Although last revised in 2004 it contains relevant guidelines and gives span tables for various load situations. The datafile also includes a check list of items which should be included in a specification for a commercial deck. With regard to screws popping, note particularly the recommendation that alternate nails or screws should be offset on each side of the centreline of the underlying joist. While it looks neat to install all the fasteners in a straight line it's more inclined to split the joist, allowing the fasteners to let go.

Q. I would have preferred to use an Australian hardwood of reliable durability for the stairs but all the carpenters I have spoken to so far seem reticent to move away from treated pine. My logic was that being an engineered product, it might be available as a factory produced kit or complete product to avoid the treated pine.

It's understandable that carpenters would prefer to use treated pine - it's easier to work with and cheaper to buy. However, you are the customer and the choice is yours. Seasoned hardwoods suitable for stairs should be available from most timber merchants, or as you say, kits are available ready to assemble. For example, Prefab Stairs produce kits in spotted gum, a durable Australian hardwood. You can visit their website at

Q. Enquiring about suitability of CLT or engineered timber for construction of exterior stairs.

Cross-laminated timber (CLT) is a ground-breaking material, but perhaps not the best choice for outdoors. The timber used to make most of the CLT in Australia (European spruce) is not classed as a durable species under full weather exposure. If the proposed stairs are temporary, or short-term, CLT could be a viable option. Otherwise the most suitable engineered timber is probably glulam made from a species of recognised durability. For more information about glulam the website of the Glue Laminated Timber Association of Australia (GLTAA) is worth a visit at We assume you have also considered the simpler and more economical option of using plain solid timber.

Q. We are constructing a new deck which is attached to the existing house all out of recycled hardwood bearers and joists. Whilst the decking wood is merbau decking wood which has a preservation coating already, do my old bearers and joists need some kind of treatment? At present they are immaculate, dry and with no sign of decay whatsoever, however being used for outside use and not inside any more will these bearers and joists weather much more than your standard treated pine? If so, what do you recommend I preserve my hardwood in?

It's difficult to answer your question without knowing what kind of hardwood it is. Some hardwoods will give long service outdoors without special treatment while others need protection from the weather. One method of protecting lower durability timbers is to install dampcourse material along the top edge. Purpose-made products include Protectadeck and Joistrip. If you can find out the type of hardwood we could advise you further.


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%.