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. I would like to obtain technical advice about how to fix (attach) lapped paling fence 1.8 m high CCA treated rails to 50x75 metal posts spaced at 2.4 m centers. Is no14 type 17 batten screw secured to single skin of the face post considered as an acceptable fixing method?

A no. 14 screw has a 6mm shank diameter and is therefore fairly substantial. It should be adequate to hold the fence rails in place assuming that the post provides adequate fixing. We assume there will be three rails (top, bottom and centre) to share wind load on the fence. Note also that treated pine can have an enhanced corrosion effect on metals in the presence of moisture, so the screws will need to be hot dip galvanised, or of equivalent corrosion resistance.

Q. I live in a mountain cold climate with low cloud etc., very moist. My 40 year old Oregon carport with steel posts and tiled roof was destroyed by a huge falling tree in a storm. It was a very strong structure with no problem ever with the wooden beams. Same as new after 40 years. Insurance company is building me a new one, however, I have reservations about what they propose to build it with. Current discussion with them. 1. They tell me Oregon not really suitable as only green available these days. (?) 2. They suggest usingLosp timber - external in this environment? 3. Shouldn't they be suggesting an alternative timber or better, a hardwood? I don't know where to go from here, but hope you may give me reliable sound advice.

It's true that sizes are limited in kiln-dried oregon and unseasoned or "green" oregon is more readily available, so the answer to your question depends on the sizes required. A company called Simply Oregon in Victoria claims to have kiln-dried oregon up to 275 x 50 in lengths up to 6 m and you can visit their website at http://www.simplyoregon.com.au/. You may find others if you write "kiln dried oregon Australia" in your browser. Having said that, we consider LOSP-treated pine is a satisfactory material for weather exposure as long as it is painted. If it is shielded from the weather (not exposed to rain) ordinary pine would suffice if available in the required sizes. Hardwoods of suitable quality are unlikely to be available in large cross-section sizes and long lengths.

Q. Looking for suppliers of charred/burnt cypress for facade similar to the Japanese Shou Sugi Ban technology. Any assistance would be appreciated.

Strictly speaking shou sugi ban is charred sugi wood, ie. Japanese cedar (Cryptomeria). 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. So it should work satisfactorily on cypress pine although we don't know of anyone who has tried it, nor do we know of any suppliers of charred cladding in Australia, but you might find a joinery works in your area that is prepared to have a go. The process is described on this website - http://pursuingwabi.com/2007/11/05/shou-sugi-ban/.

Q. What timber (say sleepers) i.e. 2400 x 200 x 75mm will be best use in White Ant Prone Area, with in ground contact ... to retain gentle sloping sandy soil lawn of Couch. Timbers that have been suggested are as follows: Redgum, Ironbark and Treated Pine? Are there any others that would be better? Thanks for any help.

Red gum is quite a durable timber but ironbark is even better and is rated Class 1 durability in ground contact. This gives it a probable life expectancy greater than 25 years. It's hard to say how much greater - that depends to some extent on how heavily you water the lawn that the sleepers are retaining, since damp soil attracts termites and provides conditions suitable for wood rot. Treated pine will give comparable performance to ironbark as long as the preservative treatment is strictly in accordance with the Australian Standard. If you search the net you will find that some producers give a warranty on their treated pine sleepers.

Q. I am considering specifying a timber handrail for an exterior application on a footbridge at a busy transport interchange in Sydney. With an appropriately specified timber, what is the maximum design life I could expect?

We have a couple of technical guides that will help to answer your questions, available at this address: http://www.woodsolutions.com.au/Articles/Resources/Design-Construction-Guides. Guide no. 8 deals with the design of balustrades and handrails, while Guide no. 5 deals with the service life of timber in outdoor situations. Handrails are not specifically covered in Guide no. 5, but there are several analogous constructions, eg. fence rail to post, pergola beam to post, and cross-arm to utility pole, the junction between the two components being the vulnerable points. Typical service life depends on the durability class of the timber and/or preservative treatment, and the climate zone.

Q. Need input on appropriate timber finishes for harsh climates in snow prone areas. Appreciate if we could have a more detailed discussion over the phone.

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 need to discuss the matter, a telephone advisory service is available from the Timber Merchants Association on (03) 9875 5000. We can say that the longest-lasting finish for timber will be acrylic paint, but if the building owner is prepared to carry out regular maintenance a pigmented exterior stain could be considered. The design of the building is also important, eg. eaves overhangs that will shed snow beyond the wall line and some strategy to avoid/prevent the build-up of snow against the timber.

Q. An architect doing a residential project looking to use cedar shingles as an external wall cladding. Would appreciate samples and more information.

We understand Tilling Timber handle cedar shingles in Australia and you may be able to obtain samples from them. Failing that there are many images of shingle-clad houses on the net. Installation instructions can be obtained via this link: http://www.tilling.com.au/sites/default/files/Shingles%20and%20Shakes%20Brochure.pdf. Generally shingles are left uncoated to weather off to a driftwood grey colour. For best results the shingles should be fully exposed to the weather - sheltered areas will turn grey more slowly if at all. There is also a risk they will discolour if (a) they are exposed to enough rain to leach out the tannin but not enough to remove it, or (b) an unsuitable finiish is applied such as linseed oil.

Q. We are looking to purchase a home that has timber (spotted gum cladding). The cladding is not treated with any sealant or paint and we have conflicting advice from a builder and the seller as to whether or not spotted gum cladding needs to be at a minimum sealed to ensure its longevity and ensure it's weatherproof?

The longevity issue and the weatherproof issue need to be considered separately. Spotted gum is a durable timber that will last quite well without any coating. It's rated Durability Class 1 outdoors above ground which gives it a probable life expectancy of greater than 40 years according to Australian Standard 5604, "Timber - Natural durability ratings". This assumes water won't be trapped in joints, and ventilation and drainage are adequate. The Standard doesn't define how much greater than 40 years could be expected since that depends on local climate, severity of exposure to the weather, and similar factors. While the timber would be expected to last for 40+ years without a coating, it will turn grey (if it hasn't already) and will develop surface cracks in the longer term. If you write "images of weathered wood" in your browser you will see what to expect. Applying paint or exterior wood stain will control or prevent weathering, depending on how well the coating is maintained. Regarding the weatherproof issue, it's generally recommended that timber cladding should have sarking behind it to prevent draughts and rain penetration, particularly in conditions of driving rain and high winds. This can also be researched on the net by writing "sarking for cladding" in your browser. The builder should be able to confirm that sarking was installed.

Q. COULD YOU PLEASE CALL ME REGARDING ENCASING TIMBER STUMPS IN CONCRETE.

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 need to discuss the matter, a telephone advisory service is available from the Timber Merchants Association on (03) 9875 5000. We can say that it's generally not a good idea to embed timber in concrete without some precautions, even treated timber. A report by the International Research Group on Wood Preservation (IRG) concluded that concrete embedment created a suitable environment for microbial activity. And "under such stable and optimum conditions microbial activity is enhanced resulting in timber decay". In addition there was a suggestion that the high pH of the concrete (alkiline) contributed to a weakening of the timber. So if it's necessary for structural reasons to embed the posts in concrete, the concrete must be a "no fines" mix. No fines concrete is made without the use of fine aggregate. This creates a honeycombed product suitable for drainage and filtering applications, whereas dense concrete retains moisture and keeps the wood damp. There are also "pole bandages" which can be used to provide extra protection for timber posts in concrete. You can find out more about these from the Preschem website.

Q. I am looking to source hardwood timber shingles/shakes for a heritage restoration project. Are there any commercial suppliers of this type of product?

Hardwood shingles and shakes are still produced in Australia by the Billinudgel Wood Working Company and you can visit their website at this address: http://www.billinudgelwoodworks.com.au/shinglesshakes.html.

WoodSolutions

Did you know?

About 16% of Australia’s 147 million hectares of native forests are in nature conservation reserves.