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. We are seeking recommendations on the most suitable external grade hardwood timber for decking use that would take a lime wash oil/stain. looking forward to receiving your advice.

No doubt a lighter-coloured wood will give best results under a lime-wash stain. The species that have the most favourable combination of light colour and durability are probably spotted gum (light-brown to darker brown) or tallowwood (yellowish-brown). The deeper red of ironbark, jarrah, blue gum, etc. may be problematic.

Q. We've used blackbutt timber capping piece over a copper clad balustrade. The tannin stain from the blackbutt is appearing on the copper sheeting and also staining the stone floor below. Can you please advise how best to stop the timber from tannin and removing the stained surface on the copper and the stone (cobble and sandstone).

The best way to stop the timber from staining is to seal it all round with paint. Tannin staining can only occur if rain strikes a bare timber surface and washes tannin out of the wood. Enamel (solvent-based) paint will provide the best seal. On the other hand, it will eventually stop of its own accord once the tannin near the surface of the wood has leached out. To remove tannin stains, oxalic acid or a proprietary cleaner with an oxalic acid base should be satisfactory. Cabot's Deck Clean and similar products contain oxalic acid, which you can check by reading the product label.

Q. Hi I am hoping you can help me I had a merbau timber deck layed and was told to use cutek CD50 clear oil on the deck I has gone grey which is not what I wanted I wanted to keep the natural grain I was hoping you could tell me how to restore the timber back do I have to have it sanded I am not happy asking my local hardware store as I don't think they know enough about this timber thank you for you time Christine thiele

It sounds as if the oil has worn off, leaving the wood to turn grey. Clear oils need fairly frequent maintenance to keep the natural wood colour. However, it shouldn't be necessary to sand the decking to get back the natural colour, as there are special deck cleaning products that do this. Cabot's have a product called Deck Clean which is widely available, and other brands include Sikkens Deck & Wood Cleaner, Intergrain Reviva, etc.

Q. I have a few timber queries. Firstly, what would be a suitable timber cladding for an outdoor fully exposed bench top. The bench would have a 5 deg angle to shed water. Are there any timber sheet options? Or would timber boards offer better longevity? Also, I am looking at laminated timber to use as both curtain wall mullions and the framing for a glazed roof. These elements are not required to be structural members, however will be required to support glazing up to 1500 x 4000mm. Both elements will be partially exposed to weather. Would laminated timber be suitable for this application? Would there be a high risk of bowing or warping? What kind of sealant/ finish would you recommend with both scenarios? Both projects are based in Melbourne.

Sheet products like particleboard, plywood and MDF don't do too well under full weather exposure, specially in a near-flat situation. Building the benchtop like a deck, with spaces between the boards, would give satisfactory performance but is probably not an option. In our opinion the best solution would be a laminated benchtop. However, note the importance of correct installation, in such a way that the top can shrink and expand slightly without distress. The point is explained in detail here: Even experienced tradespeople are often unaware of this factor, and if the job is architect-supervised correct installation will need to be checked. Regarding a suitable finish, decking oil is probably the most practical. Varnishes tend to show scratches and they fail by cracking and peeling. However, the client must understand that oils need regular maintenance. Regarding the curtain wall mullions and framing for the glazed roof, is it not possible to keep them inside the building envelope? Or perhaps clad the external surfaces with capping, eg. copper? If the wood needs to be exposed, again decking oil is an option, and there are also some exterior clear coatings on the market that contain UV absorbers and thus achieve a reasonable life. In these areas presumably scratches and mechanical damage won't be an issue.

Q. What size floor joist for an open roofed deck with 5 m single span. anticipate 450 mm centres

Joists of treated pine will need to be 290 x 45 for a 5.0m span at not greater than 450mm centres. We assume no unusual loading such as a spa, just normal foot traffic.

Q. What is the recommended timber for paling fence posts. (in Northern suburbs of Melbourne )

Traditionally, paling fences in Victoria were made from local hardwood. Today it is more common to use treated pine for the palings, sometimes combined with hardwood posts. Old fences with hardwood palings, which you might still see in earlier subdivisions, usually had only two rails, but we consider three rails are essential for treated pine palings in fences 1.0m or more in height. This allows the palings to be fixed close to the top, close to the bottom and in the centre, thus avoiding twisting. More detailed information can be found on the websites of fencing contractors, including options such as capping, plinth boards, etc. A brochure produced by the Australian Timber Importers Federation is also available via this link:

Q. I have a timber verandah around my home , in sydney , with finger jointed pine posts with stainless steel wire balustrades going through the timber posts , all the posts seem to be rotting away in the lower half where the wire is drilled through , what sort of timber should I use so that this does not happen again?

It sounds as if water is getting into the holes and causing decay. Preservative treatments don't always go right through the cross-section of the timber, so cuts and holes should be treated with a supplementary preservative from a can or aerosol. Given the difficulty of treating holes thoroughly it might be better to replace the affected posts with a naturally durable timber that doesn't rely on preservative treatment. Suitable timbers include ironbark, tallowwood, spotted gum and other seasoned timbers with a Class 1 Durability rating out of ground, depending on what is available in your area.

Q. I have recently had a yellow stringybark deck layed and after weathering it for 8 weeks a intergrain was used to stain it. After 5 months and with rain and sunshine there appears to be alot of blackness coming from within the timber. Is this a type of ionisation and if so what is the best method of rectifying this. Regards

Your problem sounds as if it is "iron stain", caused by tannin in the wood. If the decking was weathered for 8 weeks the tannin would normally wash out, unless it was a period when it didn't rain. Iron stain occurs when metal comes into contact with most hardwoods in the presence of moisture. If there has been any cutting or grinding of metal in the vicinity, the black stain will show up as spots. In other cases it shows up as a general blue-black patch. Oxalic acid (or a brand-name timber cleaner containing oxalic acid) will remove iron stains. 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 have some old oregon flooring leftover from my recent renovation. It was used in my lounge and dining rooms. I'm planning to build a small deck outside my house in Newtown NSW. I'm hoping to be able to use up the oregon I've kept, but am not sure whether it can be used outside. Would you kindly advise? I will probably paint it but haven't decided for sure yet. What do you advise please? Can I use it & how should I finish it please? Thanks a lot.

Oregon is not a great choice fully exposed to the weather. If your deck was under a verandah so it rarely got wet, oregon would be OK. However, it's not highly resistant to wood rot outdoors. On the other hand, since you have the material available at no cost perhaps you would be happy with a deck that only lasted maybe 7 years and then needed replacing. If the deck is high off the ground, such that failure could cause serious injury, we would advise against it.

Q. Is it legal to build external wheel chair ramp from lvl's and what are the guidelines in Queensland?

Most Australian LVL is made from pine and therefore needs preservative treatment if used in situations where it will get wet. Note that H2 termite treated LVL is not adequate - treatment must be to at least H3 level. Some producers recommend fitting dampcourse or other protective strip material to the top edge when LVL is used in applications such as deck joists. Providing these precautions are taken, and all manufacturer's recommendations are complied with, we consider LVL a suitable material externally. Regarding other guidelines for constructing a wheelchair ramp, if it's a commercial building where disabled access is required by the Building Code of Australia, ramps must comply with Australian Standard 1428.1. Private houses are not required to provide disabled access, but AS 1428.1 would be a good guide if disabled access is needed in a private home.


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.