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Just trying to get some insight into if and or when report(s) might be available on the slip resistance of specific timber species. Just in the last week, we have received a number of enquiries relating to untreated species with exterior applications (cladding, decking etc) and how specifiers can comply with AS4586 - 2013 when leaving the product uncoated. Thanks for your help!

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Slip resistance is essentially a surface effect rather than something that varies according to the type of timber. Generally the slip-resistance of timber walking surfaces is governed by the coating (it's not normally an issue with timber cladding). The BCA includes requirements for stairways, landings and ramps. There is no specific requirement for decking. The Australian Building Codes Board (ABCB) has a guide document on the subject which we will email to you. So far as we are aware, uncoated timber has not been tested to AS 4586, but timber with a sawn surface rather than a planed surface will obviously have greater frictional resistance to foot traffic. Ribbed decking may also offer slip resistance on surfaces such as ramps, as long as it is laid at right-angles to the direction of foot traffic, although again we are not aware of any test data. Various proprietary non-slip nosings are available for stairs and landings, while several non-slip coatings have been tested to AS 4586, eg. the Cetol Deck Plus system (for external decking), Intergrain Ultragrip (for indoor stairs), etc. 

We live in Perth and wish to extend the width of our timber deck 300mm beyond the length of the existing fire rated wall to which it is fixed to. We live in a terrace house.
There are no buildings at 90 degrees to where the deck will be extended to. We have been advised we must extend the length of the fire rated dividing wall.

Is this correct?

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It's a little difficult to picture your situation but presumably the fire-rated wall you refer to divides your terrace house from the adjacent one and the deck will now extend beyond it. This is perhaps a question that needs a ruling by your local government authority. The extra portion of the deck may be classed as an "encroachment" as defined in the Building Code of Australia vol. 2 clause This would occur if it is within 900mm of the allotment boundary, or within 1.8m of another building on the same allotment. Some encroachments into these zones are allowed (eg. downpipes, fascias and unroofed pergolas) but timber decks are not. 

Can you please advise both industry standard and acceptable practice for exterior Blackbutt decking, particularly in regards to butting the ends of boards together vs leaving a 3-5mm gap between ends of boards.
Thanks, David

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We are not aware of any industry guidelines that recommend leaving a 3-5 mm gap between the ends of decking boards. It is necessary to have a 3-5 mm gap on the long sides, since wood expands across the grain when it absorbs moisture. However, there is no need to allow for lengthwise expansion since wood does not shrink or swell significantly along its length. Leaving a gap at the ends will also make it difficult to nail or screw to the joists, unless the joists are unusually wide, since the fasteners will be too close to the ends of the boards.

Hi we are in the process of building a house which timber features including external cladding, doors and posts. We don't want a too dark or red wood. More of a modern look. What is a quality and durable hardwood for these areas? Would you stain or oil? What products would you recommend? Cheers

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Tallowwood is a recommended species for outdoor use but might not be available in the form of doors and posts. It is a yellowish-brown colour, varying from piece to piece. Another timber you could consider that might be more readily available is the light-brown species blackbutt. We suggest you inquire about availability through timber merchants in your area. These timbers can be oiled with an outdoor finishing oil, but oil has limited penetration because of the density of the wood. Consequently regular oiling is required to keep the natural colour. A pigmented exterior stain will have a longer life than clear oil, and of course acrylic paint has a life of many years.

I have specified Class 1 Blackbutt timber window sills for durability in a domestic project. Builder advises these will leech and stain cedar cladding below. What sustainable class 1 timber species is good for use in window sills, that does not leech oils? These sills are very deep: 200mm wide, 65mm deep. Thank you for your advice.

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Blackbutt is a good choice from a durability point of view but the builder is right, it does have a significant tannin content. On the other hand tannin can only be leached out if bare wood is exposed to washing by rain, so a well-maintained paint finish prevents the problem. An exterior wood stain might not form enough of a barrier to wetting by rain, since such finishes are semi-permeable. If you wish to select an alternative timber, spotted gum is the least likely Australian hardwood to be a problem and it has the same Class 1 rating as blackbutt outdoors above ground. It is described by the Queensland forest service as follows: "Has lower tannin content than most other eucalypts, therefore staining of paintwork, brickwork etc. as a result of water running over unpainted timber surfaces is unlikely to occur". 

I'm looking to install a deck, half on a concrete slab and the other half on 120x45 joists. I'm planning on putting the part of the deck on the concrete slab on 35mm battens with 1.5mm packers underneath the batten so that water wont pool under deck. The other half of the deck will be on 120x45 joists with about 150-200mm of ground clearance. Question: I can't afford the composite decking so im looking at putting down timber. Wondering what type of timber I should put down on a low deck? The deck area is mostly uncovered too. I was told that merbau is a good choice, but i like the look of the lighter timber such as spotted gum.

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Spotted gum is a good choice for decking as it is a Durability Class 1 timber above ground and has a low tannin content, so causes minimal staining. If your packers are only 1.5mm thick they will easily be blocked by debris so you might consider slightly thicker ones. Other details relating to low level decks are contained in an informative data sheet published by Timber Queensland. It can be downloaded by writing TQL 13 in your browser.

My previous deck had a typical balustrade with 3.2mm stainless steel cables passing through 80x80 mm posts (I think they are called rough head posts), with each end of the cable held by a stainless anchor that is screwed into the end posts, and at intermediate posts the cable passes through a 6 mm hole through its center. If a person were to swing or jump on the cable, the mode of failure would simply be an end anchor pulling out of the post. Such a failure could be repaired fairly readily. However, I have recently extended the central section of the deck, so that the steel cable now has three right angles along its path. Where it changes direction, I have drilled a 6mm hole diagonally through the 80 x 80 mm post, from the center of one face to the center of the adjoining face. However, I am concerned that only 1/8th of the cross-sectional area of the post is now supporting the steel cable and that its mode of failure, may be to split the timber by pulling the corner section away from the rest of the post. Are you able to confirm that the intermediate corner posts will still be stronger than typical anchors?

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Very difficult to give you a categorical answer. It sounds as if your posts are softwood if they are "rough headed", rather than hardwood which would be stronger. Also a little difficult to picture the scenario but we gather that the corner posts will now have two 6mm holes at right-angles to one another, close to the same height above the deck thus creating a weak point. We would be inclined to stay on the safe side and reinforce the posts with a galvanised or stainless steel strap fitted tightly around the posts. 

Melunak as used in internal stairs. Is it Naturally termite resistant, and where is that data available?

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Melunak is classed as a light hardwood in 100 Malaysian Timbers published by the Malaysian Timber Industry Board, and rated "Moderately Durable". According to their rating system this implies a life of 2 to 5 years in the ground. The testing is conducted under severe tropical conditions and the timber would last longer in a temperate climate. However, the "Moderately Durable" rating suggests that melunak is unlikely to have a high termite resistance. In any case it would seem irrelevant for an internal staircase where no doubt some kind of barrier system is in place to protect the house. 

What is the best option for a blonde/ light or grey merbau deck? I have a 2 year old deck made form merbau has an Intergrain (I think) glossy stain so its super red and i dont like it at all! The rest of our house is Scandinavian style so light wood the red is very jarring indeed. I think we have blackbutt interior floors with a matte finish, very light, love the interior and I'd like to tryto match it so I'm after advice about what i can do to the red merbau deck to make it closest to the interior light colour blackbutt. It's undercover so I cant wait for it to grey off naturally, its been 2 years but I think its getting darker and redder, not natural at all. I'm thinking of sanding it back and then ether leaving it completely with no stain or oil to grey off over time. I have been told about perhaps using the Cetol silver grey, Feast Watson grey look or CD50 decking oil? I'd appreciate your thoughts.

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Merbau has a reddish colour which perhaps has been emphasised by the Intergrain finish. If you sand the decking to remove the finish, that will restore the natural reddish-brown colour in the short term. If the deck was fully exposed to the weather and left uncoated it would then start to turn grey in a matter of months. However, since your deck is under cover it won't be exposed to the sun and rain which turns wood a driftwood grey colour - or at least the process would be extremely slow. So we feel your best strategy is to sand the deck and then apply one of the 'grey look' finishes you refer to.

Hello what timber would recommend to use over existing timber deck for flooring I would like to use timber that can be purchased in large sizes like timber panels thankyou

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The most cost-effective solution would be radiata pine flooring panels, which are readily available. However, if a more decorative appearance is required, flooring panels are available with a hardwood face, eg. the Big River product - refer pages 8-9 of their catalogue available here: We assume your deck is going to be enclosed as an indoor room, since plywood flooring is not suitable for weather exposure. If the floor is exposed to the weather fibre-cement panels would be more suitable.

We have specialised spotted gum for timber screening around a shed in a commercial application. Is this a good option? The client is concerned that they will have to maintain the timber and it will be labour intensive.

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Spotted gum is a good choice for a timber screen in our opinion. It is rated Durability Class 1 outdoors above ground, according to Australian Standard 5604 Timber - Natural durability ratings. It is also one of the least likely hardwoods to cause tannin staining when exposed to rain. Whether it will be labour intensive to maintain depends on the client's expectations regarding appearance. Leaving the timber uncoated to weather to a driftwood grey involves no maintenance at all. Retaining the natural colour and grain of the timber by oiling it requires regular maintenance - at least once a year and perhaps more frequently, depending on exposure. Painting it with a timber tone acrylic paint should give at least ten years' life and possibly more.

We're looking to build a deck that is of a size that requires a building permit. The deck is also going to be within 900mm of the boundary which I've been told means it needs to comply with the 60/60/60 fire rating. Are you aware of any woods that comply with this? what do most people do here?

Thanks in advance.

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A 60/60/60 fire resistance level (FRL) would require the deck to retain structural adequacy/integrity/insulation for 60 minutes. Of these, only structural adequacy (ability to maintain stability and loadbearing capacity) would seem relevant. The FRL would then be in the form 60/-/-, but the sizes required to maintain structural adequacy for 60 minutes would make it impractical to build. In any case, FRL's are generally applied to separating walls rather than horizontal structures. While we are not experts on the interpretation of the National Construction Code (NCC), our understanding is that a timber deck is classed as an 'encroachment'. The NCC defines an encroachment as "...any construction between the external wall of the building and the allotment boundary...", the "external wall" in this context being the external wall of a Class 1 building. The NCC contains a list of encroachments that are allowed within 900mm of an allotment boundary and the list includes "unroofed terraces, landings, steps and ramps, not more than 1m in height". In summary, it appears to us that if your deck is 1m or less in height and is unroofed, and is at least 900mm from the side and/or rear boundary, there is no restriction. Note that different States may have variations to the NCC, so we recommend that you discuss this with a certifier or building surveyor in your region. Note also the meaning of "within". When the NCC says the specified encroachments may be built "within" 900mm of the boundary we understand that to mean "up to a line drawn 900mm from the boundary".

My designer has asked to caulk between the decking boards on an external deck and line the underside to ensure no water can get though to the patio beneath, we will be using 136x31 spotted gum decking f14 hw joists and 9mm fc lining stuck directly to the underside of the joist I dont think I can rely on the caulk to stop water from penetrating the diaphragm. would it suffice to waterproof the top side of the lining and build the deck with fall to ensure that any water that does get through will drain

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You are right to be cautious about relying on caulking to waterproof an outdoor deck. Boat decks are caulked but they are designed in a specific way, using timbers noted for their dimensional stability such as quarter-sawn teak. In fact in modern times teak boat decking is usually a relatively thin layer of teak glued to a fibreglass deck or a plywood underlay. If you were going to caulk the deck, the decking timber would have to be profiled like boat decking to support the caulking  - for more information see here: While we like to encourage the use of timber decking, in this case we feel it might be wiser to use a product such as fibre-cement sheets which can be more easily made waterproof.

I have a recently sanded spotted gum deck that i wish to have a weathered look.
I realise that i will need to coat the timber to stop it from cupping or splintering . Can you suggest any products that i could coat on the spotted to achieve this

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Spotted gum weathers well and if you want the driftwood grey look you don't need to put a coating on it. Decking is kiln-dried and won't cup as long as there is plenty of ventilation underneath. If you coat the timber that will stop it from going grey. On the other hand if you want to keep the natural colour of the wood now that it has been sanded you will need to oil it regularly.

We have recently built a covered deck - framing up for the roof first. We painted the roof support poles, rafters and battens with primer and good quality exterior water based paint before erecting them. Then had 2 weeks of rain which stopped all work. About a third of the battens and rafters (where a tarp kept blowing off) have tannin stains from the leaves in rainwater. Is there any product or cleaning agent we can use to clean it from the paint finish, or will we have to paint them again?

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There are products with an oxalic acid base that remove tannin stains from bare timber, but we aren’t sure whether they will work on a painted surface. Scrubbing might damage the paint since water-based paints are relatively soft. If you would like to try one there are several on the market, often sold as deck cleaning products. So if you write ‘deck cleaner’ in your browser the relevant brand-name products will come up. Alternatively you could contact a paint company to see what they advise. Most of the major companies have a help line.

We are going to have our retaining walls done in 75 mm treated pine cladded with 90 mm merbau as suggested by our landscaper. I am trying to envision this. I know what merbau looks like but just wanting to know what this combination will look like together? And it will be capped. Do you have any example pictures or where can I find some example pictures?

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It was a bit hard to picture which part of the wall will be treated pine and which part will be merbau. If you write ‘treated pine images’ and ‘merbau images’ in your browser you will see what the two timbers look like. Treated pine has a greenish tinge, while merbau is reddish-brown. But be aware that unless you keep the timber oiled, both will eventually bleach to a driftwood grey colour.

We have specified spotted gum timber decking adjoining a public swimming pool where chlorinated water will get into contact with the timber. What impact will the chlorinated water have on the timber.



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A chlorinated pool is most unlikely to cause any harm to adjacent timber. The important thing from a timber point of view is the pH of the water. We are not experts on pool chemicals but we understand that the usual pH of swimming pool water is between 7 and 8. Since 7 is neutral on the pH scale, neither acidic nor basic (alkaline), the water is usually slightly towards the alkaline side. If it’s a public pool presumably the pH of the water will be closely monitored, so we don’t anticipate any problems.

I would like to use jarrah 130x19 decking outside but avoid joins over joists. Long jarrah lengths can be purchased that are 'finger jointed' I think is the term. How durable over time are these joints ? do they stand up to temperature changes well? The deck will be in the Adelaide hills, South Australia. Could you please advise, thank you.

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Finger-jointing is a well-accepted method for creating long-length timber products and is now used for posts, fascia boards and other exterior items. Temperature changes are not likely to be a problem, but you should ensure that the decking is produced with a water-resistant adhesive under a quality control program. It’s extremely unlikely that decking would be made with a non-water-resistant adhesive, but it doesn’t hurt to ask the question since water resistance is the key issue.

Can I seal or paint particle board flooring. with what?

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Particleboard flooring can be sealed with the usual range of floor finishes (polyurethane, oil, etc.) but it is more absorbent than timber. This means it is likely to take a number of coats to build up a high gloss 'natural' finish, if that is what you want to achieve. If you don't want a clear finish, paving paint will give you an opaque finish but will show scuff marks and scratches more than a clear finish.

A project we are working on has installed a number of external timber seats. The specification called for seasoned hardwood. There was no sealant or finished required. The majority of these seats have split (over the last year since installation), with some splitting in excess of 10mm wide which is an issue to sit upon. The contractor believes that this is due to a lack of sealant however we are not sure if this is due to the timber supplied not being suitably seasoned. Can you please provide some advice as to how we can resolve if this is a design issue or supply issue?

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It’s difficult to give you a definite answer but, generally speaking, seasoned hardwood shouldn’t develop 10mm splits. It’s likely to develop hairline surface cracks after a period of weather exposure with no finish, but not major splits. Did anyone check the timber for moisture content at the time of installation? That would have shown whether it was suitably seasoned or not. You might also check whether there has been significant shrinkage. If there has, then a high initial moisture content could have been a factor. You don’t mention the type of timber, or the dimension of the timber. Outdoor seating performs best when constructed from relatively narrow slats of a chunky cross-section. Boards that are both thin and wide are more likely to be troublesome. And large splits are more likely in timber that contains the pith (centre of the log).

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