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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 have just had a deck installed and the decking is swelling and buckling. A building inspector has reported that the decking might not have been seasoned properly and perhaps is not suitable for external use. I will send some images showing the problem.

We have viewed the images you sent and also read the report by your building inspector. We disagree with their conclusions that the timber may not have been properly seasoned and/or was not suitable for external use. It’s very simply that the deck is close to the ground and has taken up moisture into the underside of the decking. If the timber had not been properly seasoned it would have shrunk, not swelled. Building a deck close to the ground is difficult but Timber Queensland have produced a data sheet showing recommended practice. It can be downloaded by writing TQL 13 in your browser.

Q. We have built a merbau deck which abuts our brick rendered house in Perth. The horizontal decking boards sit over the joists, approximately 75mm above ground. However the outer edge of the deck (that which is at right angles to the surface of the deck, provides the vertical face of the deck and sits on the ground) is intended to have soil against it. The end result is that a garden bed will sit flush with the surface of the deck. My concern is that this merbau will be in the ground and therefore could pose a termite risk. Could you please: - Advise if this is indeed a risk - If so, provide some suggestions of how we could mitigate this risk. I'm not sure if chemical protection would be effective due to lack of access once the soil abuts the timber. Would a paver, mortared against the vertical merbau face be an advisable barrier? Your suggestions and advice will be appreciated.

Merbau is rated Durability Class 3 when placed in contact with the ground and is therefore moderately durable, but would not be expected to last more than 15 years at best - possibly less, particularly if the garden bed that abuts the timber is watered frequently, since damp conditions encourage wood rot and termite attack. There are also some other issues of concern in relation to your deck. For example, when decks are close to the ground problems can occur with moisture rising into the underside of the decking, causing cupping and swelling. For more detailed information on building decks close to the ground we refer you to Timber Queensland's Technical Data Sheet 13 titled "Residential Timber Decks Close to or on the Ground". You can download this publication by writing TQL 13 in your browser. Similar problems arise when decks are built with the sides enclosed - it looks neat but prevents ventilation under the deck. So if it's not too late to make some modifications in line with data sheet 13, we would strongly recommend that you do so.

Q. Hello, I have a large quantity of Blackwood which was originally going to be used for craft work but now due to health reasons will not. I would like to know the likely length of time it would last if it was used outside as fence palings ( 90 x 15 ) The timber would be sawn using a large band saw and could be cut thicker if advisable. It would also be stained with fencing stain or oil. Thank you.

Blackwood is a great joinery timber but is not highly durable outdoors. According to Australian Standard 5604 it's rated Class 3, which gives it a probable life expectancy above ground of 7 to 15 years. The rain runs off palings pretty well, so you might achieve a life span towards the upper end of the range, particularly if you keep the timber well oiled and the palings are not touching the ground at the base. The vulnerable points will be where the palings are in contact with the fence rails, since that is where the wood will stay damp.

Q. I had a deck built about 12 months ago, with Eucalyptus sideroxylon the length of the timber up to 4.2m, some shorter. The timber has buckled and lifted. Could you please ring my builder and discuss this problem with him?

Eucalyptus sideroxylon (common name red ironbark) usually makes excellent decking, so we wondered if site factors were contributing to the buckling you describe. For example, when decks are close to wet ground problems can occur with moisture rising into the underside of the decking, causing cupping and swelling. For more detailed information on building decks close to the ground we refer you to Timber Queensland's Technical Data Sheet 13 titled "Residential Timber Decks Close to or on the Ground". You can download this publication by writing TQL 13 in your browser. Similar problems arise when decks are built with the sides enclosed - it looks neat but prevents ventilation under the deck. Our service doesn't extend to discussions with your builder - perhaps you need to consult someone who can inspect the deck and then suggest a remedy to  your builder. We understand John Thornton (ex-CSIRO) consults on timber building problems in Victoria. John’s number is 0407 050 468.

Q. I was going to use H4 treated pine for bearers for a deck but was only going to have the bearers a few cms above wet clay. Will I have problems from white ants? I am trying to keep the deck height as low as possible.

H4 treated pine bearers will be OK close to the ground - in fact you can put H4 treated pine in the ground. However, when decks are close to wet ground problems can occur with moisture rising into the underside of the decking and causing cupping and swelling. For more detailed information on building decks close to the ground we refer you to Timber Queensland's Technical Data Sheet 13 titled "Residential Timber Decks Close to or on the Ground". You can download this publication by writing TQL 13 in your browser.

Q. We are building a house in Port Macquarie, I would like to use timber cladding, vertical and horizontal profiles. As the area is a termite recognised area, can you suggest a timber that would best suit? I was thinking Western Red Cedar but since reading the FAQ thought that a Class 1 Aus. hardwood might be better. Can you offer any advice? It would all be above ground and on concrete slab, which would receive termite protection (still researching that one).

Western red cedar is a good choice for cladding as it is a stable timber with little tendency to swell and shrink with wetting and drying. Class 1 Australian hardwoods can also be used for cladding and are rated termite-resistant. However, since you are building on a concrete slab it's possible to design the slab as a termite barrier. Your builder may not be aware that the slab is classed as a recognised barrier if (a) the slab edge is exposed for a minimum of 75mm above adjacent ground or paving so termite entry over the edge can be visually detected; (b) any construction joints, eg. between two separate concrete pours, are protected with stainless steel mesh or other suitable barrier; and (c) penetrations through the slab, eg. by service pipes, are protected by a suitable barrier. Termites can't tunnel through a properly cured slab, so it's therefore only necessary to protect these three potential entry points.

Q. I am hell bent on having a silver wattle hardwood paling front fence at my home. I live in Perth and within 1 km of the sea. What do I need to do to facilitate prevention of any potential termite attack on the palings? The palings will rest on an H3 treated timber footing which will be bolted onto a brick substrate covered in render. Look forward to your response.

The most important strategy will be to leave a space between the bottom of the palings and the timber footing, rather than resting the palings directly on the footing. This will allow any attempted attack by termites to be more easily detected. It will also prevent moisture being drawn up into the palings by a "wick" effect. We weren't sure whether you intend to coat the palings. Wattle bark has a high tannin content and the wood less so, but there may still be a potential for tannin staining on the rendered brickwork if rain washes over bare wood and onto the render. Salt air won't harm the timber but will tend to corrode fasteners, so make sure all hardware is non-corrosive.

Q. Can you please advise whether H4 treated pine can be used for decking stumps directly concreted into the ground, If so what if any precautions are needed and what is the expected life.

The predicted life of H4 treated pine depends on local climate as well as the design of the structure. We think you will find our Design Guide #05 helpful. It's titled "Timber Service Life Design" and can be downloaded at http://www.woodsolutions.com.au/Articles/Resources/Design-Construction-Guides. For example posts in ground contact, located in decay hazard zone C with full preservative penetration, have an expected service life of 30 years. Note that embedding timber in concrete can affect its service life if the concrete traps moisture. 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 at preschem.com.

Q. I am a designer of mainly dwellings. I have a client who is interested in cladding a pitched roof in timber to match a slatted wall cladding and in doing so would achieve a uniform, continuous appearance in the external material from roof to wall. We can see many examples of this in European architecture but can't find any products or examples in Australia. Being a non-conventional roofing option, I'm also unsure how it would perform in relation to building and construction codes, or if adequate weather-tightness can be achieved in Sydney's often wet climate. Any advice or help would be so appreciated.

The fully timber-clad look is an interesting one, more common in Europe as you say. It's hard to tell exactly how they do it from the images on the net but we suspect that the timber roof is more in the nature of a decorative screen over a weatherproof roof, such as a metal sheet roof, rather than a weathertight roof in itself. We would certainly recommend this approach in the Sydney climate where very wet weather can be followed by hot sunny weather. This possibly produces greater extremes of wetting and drying than would be experienced in Europe. The only type of timber roof that has a history in the Australian climate is one based on wooden shingles, but even shingle roofs tend to find the Australian climate rather severe.

Q. I am looking into a glulam surface for existing bridge foundations for a bridge 61 metres long and around 4 metres wide. Who do I contact?

Big River produce a special plywood for bridge decking called "Bridgeply". You can find out more via this link: http://bigrivergroup.com.au/product/structural-plywood-bridge-decking/. It might also be worth contacting Outdoor Structures who have some expertise in designing and building vehicle bridges. To visit their website, follow this link: http://www.outdoorstructures.com.au/bridges_5.php. Other than those two, contact details for major glulam producers can be found through the website of the Glue Laminated Timber Association of Australia (GLTAA) whose website is at www.gltaa.com.

WoodSolutions

Did you know?

About 6% of Australia’s 147 million hectares of native forests are public forests potentially available for timber harvesting. Timber is harvested from about 1% of those public native forests each year.