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 am building an outdoor aviary for my reptiles and was wondering what type of wood you would recommend to use, it will be exposed to the weather all year round. I would be using lengths of 4x2 and for the frame and wire mesh for the covering, any help would be good thanks.

Preservative treated pine should be safe for a reptile enclosure since they don't chew or gnaw at the wood as far as we are aware. Use material that has been kiln-dried after treatment, so it has a dry surface. If you are uncomfortable about using preservative-treated timber, a naturally durable hardwood would be satisfactory, eg. one of the ironbarks, tallowwood, etc.

Q. I'm after technical information on a low-level deck, likely on a concrete slab. It's a small area and we would like the external deck to look similar to the internal timber floorboards. However I know the Australian Standards say you aren't supposed to have a deck below approx 400mm from the ground. But I have seen a lot of photos of people doing this, even reputable architects - so I would like to confirm the appropriate technique!

Building a timber deck at ground level can be problematic. When decks are close to wet ground moisture rises into the underside of the decking, causing cupping and swelling. Digging out the ground under the deck sometimes makes the situation worse, since it can hold water after heavy rain. Building on a concrete base is usually better, as long as water can drain away after rain. 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 deck around an above ground pool and wondering which hard wood is best in terms of durability and resistance? We will also be building a deck at ground level leading from the back door to the above ground decking? Which hard wood is best here? Can we use the same type of wood for both areas? I'm confused about all the different types of wood and the different codes allotted to the different woods. I would prefer an Australian wood.

In some ways we are spoiled for choice with timber - if we only had one kind it would be easy! The decking around your above-ground pool will have plenty of ventilation and any Class 1 Durability timber would be OK. If you have a preference for Australian-grown timber, red ironbark, tallowwood and spotted gum will all give good service, but others may be available in your area. The deck at ground level is a little more difficult. When decks are close to wet ground problems can occur with moisture rising into the underside of the decking and causing cupping and swelling. Digging out the ground under the deck sometimes makes the situation worse, since it can hold water after heavy rain. 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". Again, Class 1 Durability timbers are recommended. You can download this publication by writing TQL 13 in your browser.

Q. I was hoping to get some more information on the external silver top cladding that was supplied to the Kew House 3 project on your web site. I have a similar site which I would like to use this type of cladding but wanted to ask some more questions and get some more information on the technical application.

Kew House 3 used a variety of silvertop ash profiles to create the external facade. Silvertop ash is rated Durability Class 2 outdoors above ground and has a probable life expectancy of up to 40 years, assuming adequate ventilation and drainage after rain. If you like the design, perhaps it would be worthwhile to contact the architect, Michael O'Sullivan of Vibe Design Group. If you would just like more information about timber cladding in general, feel free to leave another message with more detailed questions.

Q. What causes kapur decking to get blackspots? Deck has been down 2 weeks. Not oiled.

It sounds like a case of "iron stain". Metals react with tannin in the wood to form a blue-black stain 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. 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 have an elevated platform house - standing on 75 x 75 galv. posts set 2100 apart. Would like to fix to the verticals a timber say 65mm width then fix horizontal timbers - creating a screen - with gaps of approx 5-10mm between horizontals. looking for suggestions for the verticals and horizontals - would just like the timber to grey off over time. Look forward to your response.

To attach to the galv posts we suggest a durable hardwood - there are many suitable types, depending on what's available in your area, eg. spotted gum, merbau, jarrah etc. For the screen timbers many people use timber decking. It's readily available in suitable sizes of kiln-dried timber. However, 2100mm is a bit far for decking to span and you could get some distortion after a period of exposure to the weather. To avoid this we recommend a timber "dropper" midway between the galv posts. A dropper is more commonly used in wire fences to maintain separation of the wires, but serves a similar purpose in timber screens. If you don't like the idea of a dropper, a chunkier timber profile would be recommended in place of the usual 19mm decking profile.

Q. I would like to use exposed roof rafters externally on a residential dwelling (no eaves cladding). Can you help with the specification of the most suitable timbers and finishes? Can LVLs be used if treated?

LVL's can be used for exposed rafters and they will be OK without treatment externally if covered by a roof. When deciding whether timber needs to be preservative treated (or a naturally durable species) the test is whether it will get wet when it rains, not simply whether it is indoors or outdoors. So you could also consider timbers with a higher appearance quality than LVL - ordinary pine would do if adequate for the span. Otherwise, glulam combines high quality appearance with long span capability.

Q. I have built a frame using Merbau decking wood that will go between planter boxers and artificial turf. The only section of the wood that will be exposed will be the very top face. I am wondering if oiling or something else may prolong the life of this wood. If I did nothing what sort of life span could it have in Western Australian conditions?

We take it that the merbau frame will be buried in the ground with only the top exposed. Merbau is rated Durability Class 3 in the ground and according to Australian Standard 5604, "Timber - Natural durability ratings", this implies a probable life expectancy of up to 15 years. However, if the planter boxes are watered regularly and the ground around the timber is damp for long periods this will shorten its life and it could be as little as 5 years. So it depends greatly on environmental conditions. Oiling the timber will have little effect on its life span since it doesn't penetrate far into the wood and is mainly cosmetic.

Q. I wish to repair a crack (1.2m long x 10mm wide) in an iron-bark external structural post (200 x 200mm). What should I fill it with? What type of glue? Should I mix up a compound using similar wood? Should I pin it together using bolts or? Any suggestions ? Thank you in anticipation for your help.

It's preferable to used a filler with some flexibility for exterior timber, since timber tends to swell and shrink slightly with changes in moisture content. If the filler is a hard-setting material it's likely to crack and work loose in the longer term. If the timber is to be painted we suggest a paintable silicone, eg. Selleys Paint Over Silicone or similar. If you are looking for a filler to match the colour of the wood, that is more difficult. Most silicone products can't be tinted. Woodflex Polyfilla is perhaps better if you want to colour it, since it retains some flexibility and can be tinted. You will find more information on the net about these products.

Q. We are designing a house with a vertical ‘burnt’ cedar cladding (traditionally a Japanese method called shou sugi ban) where the surface is charred. As vertical timber cladding is not specified in the BCA, our certifier says it will be necessary to do an alternative solution for its use, but is having trouble finding technical details for its installation and information about weatherproofing.

It is correct that the BCA for some reason only deals with timber “weatherboard” cladding, ie. installed horizontally. However, timber cladding is often installed vertically. Double face-nailing is generally preferred as a more secure option than secret nailing. It can look quite good if carefully done in straight lines with an exposed decorative nail such as silicon bronze. The certifier’s concern about weatherproofing is dealt with by installing vapour permeable sarking directly behind the cladding, eg. Tyvek or similar.


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.