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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. My intention is to use traditional tongue and groove floorboards for my veranda floor. I also wish to enclose the space under the veranda for further living space. This is why I wish to waterproof the veranda floor. I have thought about using ply or cement sheeting over the joists and painting with a waterproof membrane. If I was to do this, how would I then attach the floor boards without penetrating the waterproof membrane? Would it still be recommended to have an air gap for ventilation (eg floor boards fixed over battens)? Would it be better to glue the boards to the sheeting? Do you have any suggestions?

We suggest plywood floor panels over the joists, a waterproof membrane over the plywood (applied by a professional), battens fixed to the plywood and floorboards on the battens. We assume the verandah has a roof and rain penetration onto the verandah floor will be minimal. Nailing through the membrane shouldn't be a problem since the membrane will tend to self-seal around the nail, but you should seek the advice of the waterproofing company on this point. We wouldn't recommend gluing the boards to the sheeting. Note that all nails need to be highly corrosion resistant - hot dip galvanised or stainless steel.

Q. I was wondering what kind of decking is used on verandas where they don't have gaps between the timber (heritage style)?

Many older buildings used tongue and groove floorboards for their verandahs which was reasonably successful as long as the verandahs were roofed and sheltered from the weather. Even then it's important not to cramp the boards tightly together so they can expand slightly if rain blows in. The floor should also have a slight fall so water runs off, rather than pooling up on the surface.

Q. I would like some advice regarding the design and construction of a waterproof timber deck.

It's not practical to try to make a timber deck waterproof if timber is the exposed surface. Timber decking is designed to have spaces between the boards, and tongue and groove floorboards are not suitable for weather exposure. However, it's quite possible to build a timber deck where the exposed surface is waterproof, using some other material. For example, plywood flooring panels can be laid over timber joists and then a weatherproof membrane installed over the plywood. Many suitable products are described on the net. Alternatively, fibre cement sheets can be laid on timber joists, forming a base for tiles. This is described in the James Hardie manual available at this address:

Q. 1. What is the useful life expectancy of a Durability Class 1 hardwood deck, externally exposed and above ground? 2. Have any tests been documented on the Slip Resistance of hardwood timber decking to AS/NZS 4586?

The TimberLife project was undertaken to provide reliable predictions about the service life of timber in various situations and under various climatic conditions. A manual titled "Timber Service Life Design", and a related software program, can both be downloaded free of charge from the Wood Solutions website via this link: Reference to the manual shows that Class 1 Durability decking has a probable service life of 20-30 years depending on the climate zone in which it's located. Regarding slip resistance, we don't know of any raw decking that has a documented slip resistance, but timber coated with 1 coat of Sikkens Cetol HLS 077 and 2 top coats of Sikkens Cetol Deck Plus Slip Resistant 078 has achieved a 'W' classification when tested to AS/NZS 4586-2004. A copy of the test report can be obtained from Tenaru Timber & Finishes in Sydney.

Q. Would untreated Grey Gum sleepers be suitable for use as a retaining wall? My landscaper is advising me to use treated pine instead as the hardwood is not treated against termites. I understand Grey Gum heartwood has a natural termite resistance, but am unsure whether the sleepers are still likely to get partly eaten and I'd be better off with treated pine?

Grey gum is rated Class 1 Durability and therefore has a probable in-ground life expectancy of greater than 25 years. It's hard to say how much greater since it depends on climatic factors, how heavily the garden is watered, and so on. This is not too different from the expected service life of treated pine, again depending on circumstances such as local climate, level of treatment and depth of preservative penetration.

Q. I'm looking to construct a timber open-picket fence near the beach. Which timber would you suggest would be most durable as a fence post into a concrete footing in sandy soil?

The availability of different species of timber varies from place to place, but if ironbark is available in your area that would be a very durable choice. Otherwise, any Class 1 hardwood, or pine treated to Hazard Level H4 or higher, would be OK. When timber posts are installed in concrete it's recommended to use "no fines" concrete. This is explained in a data sheet available via this link: Note the comments about avoiding fertiliser and heavy garden watering adjacent to timber posts in the ground - even when durable timbers are used. And make sure all nails, screws and bolts are corrosion-resistant, preferably stainless steel, since the seaside environment is very aggressive towards metals.

Q. I am using treated pine mpg10 240x45x1800 to replace steps where hardwood has rotted. As I'm using wood horizontally, I assume load ratings are calculated vertical. Will my wood still be strong and last a long time. This is on a commercial site. I'm using M16 galvanized bolts etc. will be coating with oil base primer and then painted. My fear is with the soft wood would high heels etc damage the wood in long term, as I'm thinking nonslip grit tape and front edging would protect ok. It is at a local hotel in gladstone qld 4680 (5 kms from coast ). It's not to late to change wood as only replaced 3. Any advice is appreciated as I have studied your site for 6 hrs and still do not know long term results of mpg10 240x45x1800 being horizontal. I think will be fine and will outperform the hardwood in flex and sag. I enjoyed your site and now have the knowledge to apply to my 40yr old hard wood house I recently purchased.

The stairs you are working on are quite wide and we don't feel that treads 45mm thick will do the job in treated pine. Our Design Guide no. 8 shows that 45mm treated pine is only recommended for stairs about 1m wide. Hardwoods such as spotted gum or kwila/merbau will do the job but even then a minimum finished thickness of 58mm is recommended. Copies of Design Guide no. 8 can be downloaded from the Wood Solutions website via this link: Another option if you prefer to use treated pine is to install a central stringer, if that's possible, thus reducing the span of the treads. Treated pine of 45mm would then be more than adequate.

Q. I would like help please re vertical timber cladding. some building inspectors will not readily allow it. please see extract below. We get regular enq re vertical cladding and would very much appreciate advice. (what we have found is the main problem is internal moisture, and that is usually caused by poor sarking, regardless of the way the timber runs.) (from building engineer re vertical cladding; I was recently dragged into a civil dispute because a Building Consultant stated at a tribunal that the BCA was silent on vertical cladding and makes reference to tongue side up and therefore drew the conclusion that it can only be used horizontally. So subsequently I have no option but to tread cautiously and exercise my Duty of Care) Cedar Sales would very much appreciate any helpful advice. many thanks. Russell Mead

It is correct that the BCA only specifically mentions horizontally-fixed external cladding (vol. 2, part 3.5.3). However, we feel it is unnecessarily restrictive to assume that all timber cladding must therefore be fixed horizontally. The BCA is silent on a number of issues - for example, it doesn't explain how to construct and install timber doors, windows or flooring (except in the context of energy efficiency), and so one looks elsewhere for acceptable details. Also the BCA is a performance-based document and a building solution will comply with the BCA if it satisfies the performance requirements therein (vol. 2, clause 1.0.4). Several industry manuals deal with the installation of timber wall cladding vertically, eg. the Timbeck website at, the Woodform website at, and so on.

Q. I am going to replace our brick letterbox with two timber posts and horizontally slatted timber between the posts up about 1.5 m above the ground. The posts will be concreted in below ground level. I am not wanting to use pine as I'm trying to have the dark timber look such as Merbau. Is Merbau a suitable timber to concrete in the ground? Bunnings have 90*90 merbau posts GL17. Laminated and rather expensive. Are there any other post alternatives that would be suitable and have the same look?

Our preference is for post holes to be backfilled with earth, rammed in layers to provide stable fixing. However, if setting in concrete is preferred it should be "no fines" concrete as described in a data sheet available at this address: Treated pine can be stained a dark colour, but if you don't want to use pine a number of hardwoods could be used. Glue laminated merbau is suitable but a relatively expensive product - local timber merchants should have a wider selection of durable hardwoods. You might also find durable hardwood posts at a recycling yard. Just write "recycled timber" in your browser. Suitable species include jarrah, ironbark, tallowwood, turpentine etc.

Q. I'm an architect in Victoria and I have a few projects on the Mornington Peninsula where I'm considering using white cypress as cladding. Particularly, kiln dried rough sawn tongue and groove from Frencham Cypress, with full recommended pre-treatment. My client has been talking to builders and the response so far has been well, luke warm or negative to say the least. The worst feedback said definitely stay away from it because of durability issues, warping, cracking and checking. Perhaps they used unseasoned green timber and only treated the exposed face after fixing? I understood that if the timber was kiln dried to 10-15% moisture content, sawn face and pre-treated and then coated both sides before fixing that it would perform extremely well - as well if not better than WRC or Pacific Teak.

A number of timbers can be used for cladding if they have suitable durability (rot resistance) and are installed at a suitable moisture content. Cypress pine has a long history of use as cladding and should give satisfactory service if installed as you describe (kiln-dried and pre-coated both sides). Having said that, cypress is more inclined to checking than western red cedar, but usually only develops fine hairline cracks which are disguised on a rough-sawn surface. It's difficult to comment on Pacific "teak". There is currently a trend amongst some importers to re-brand timber species with the prefix Pacific, and we haven't been able to determine what Pacific teak really is. That's not to say Pacific teak is to be avoided, but presumably it's not real teak. Maybe your client could ask some suppliers of cypress cladding to point out buildings where it has been used so its performance can be evaluated. Or perhaps they have an outdoor display where it can be seen in a real life situation. If you are still unsure, western red cedar is highly dimensionally stable and has little tendency to warp, crack or check.


Did you know?

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