Exterior Timber, Decking and Cladding

Question: We were looking at cladding part of a house we are currently working on with timber with some sections on a curved face. Please advise.

Answer: It is difficult to fit horizontal timber cladding to a curved wall, although by making a series of vertical cuts in the backs of the boards they can be made more flexible. You could discuss this with your carpenter and see whether a couple of trial pieces would achieve the desired result. If not, a product such as Weathertex might be easier to handle. This is made from 9.5 mm hardboard which is easier to bend without splitting or breaking (a) because it is thinner than standard timber cladding profiles, and (b) the grain is randomised because it is a reconstituted product. Vertically fixed cladding is perhaps easier to fit to a curve, although unless the curve is a gentle one it might be difficult to ensure a tight fit in the tongue and groove joint.


Question: I am building a horizontal timber slat fence around my terrace which I will later be tiling and am worried about leaching onto the tiles. If use Merbau, how long will it leach for before I can tile the terrace? What other timbers are suggested?

Answer: Leaching of tannin commonly occurs if merbau is exposed to rain without a coating, or with a coating that allows moisture to penetrate. If the timber is painted with enamel paint, or with a stain-blocking primer followed by water-based paint, leaching is most unlikely. If you intend to leave the timber to weather off to a driftwood grey, tannin will leach for some months. Since we are now in the middle of winter, leaching should be completed by the time summer comes, assuming it continues to rain in your part of the country. Manufacturers of deck cleaning products claim that a preliminary scrub helps to remove excess tannin, so you might like to try that prior to installation. Other durable hardwoods you could consider include spotted gum, tallowwood and ironbark. They will probably leach less than merbau, but some staining is still likely as most hardwoods have some tannin content.


Question: Hi, I am looking to replace a Harbour front timber deck around a pool. Current dimensions are approx. 150 x 32 with spans around 2.5m. Could you advise an appropriate species of timber for the job? (I'm not sure if it is a chlorine or salt water pool).

Answer: There are several hardwoods that would do the job. Most have some tannin content so there is likely to be a bit of brown staining in the early stages. However, the tannin washes out fairly quickly, depending on rainfall. If staining is likely to cause problems with adjacent surfaces, manufacturers of deck cleaning products recommend their use to remove excess tannin. Merbau is a good choice for decking, but has a high tannin content. Another excellent hardwood with a lower tannin content is ironbark. Either one will stand up to splashing from the pool. Whatever you choose, make sure it is supplied in a kiln-dried condition.


Question: I am building an open deck above a car parking area and was wondering which type of hardwood decking would have the least amount of tannin bleed. I have been told jarrah would be a good choice.

Answer: All eucalypts have some tannin content, but jarrah isn't too bad with regard to leaching. Merbau would perhaps be one to avoid as it is noted for its high tannin content, although it is an excellent decking in other respects. Manufacturers of timber cleaning products claim that a preliminary scrubbing before installation helps to remove excess tannin, so you might like to try this. A hardwood that is unusually low in tannin is brush box, but it has a lower durability rating than jarrah so we would predict a shorter life span, assuming it is fully exposed to the weather. Your choice of jarrah is probably as good as anything, balancing tannin bleed against durability, particularly if you give it a scrub with deck cleaner before installing.


Question: We are about to build a wharf style deck that we want to grey naturally. Our builder advised against merbau because of leaching and suggested spotted gum but he is concerned that if we don't oil it, it will crack and warp - what would you suggest?

Answer: Your builder is right about merbau leaching a brown tannin stain when it is new. However, the process ceases after a few months (depending on rainfall) and is only an issue if the timber is adjacent to a sensitive material, eg. sandstone pavers underneath, or light-coloured render alongside, where the brown stain might leave marks. If the deck is suspended over bare earth, the tannin will simply wash off into the ground. If staining of adjacent surfaces is not likely you may want to reconsider merbau. If staining could be a problem, spotted gum is sufficiently durable for outdoor use but commonly contains gum veins and is inclined to develop fine cracks when exposed to the weather. If your deck is really "wharf style" maybe a fairly rustic, rugged look will be OK and presumably fairly large size timber will be used which makes warping less likely. If a rustic look is not what you are aiming for, perhaps a timber such as ironbark would be a better choice, although be aware that ironbark and most hardwoods also leach some tannin - it's just that merbau leaches more than most.


Question: About to build a merbau deck over existing metal deck roof [1st floor]. Deck is south facing [uncovered] so it gets rain and afternoon sun. I've been advised to use steel support structure. For cost I'd rather use F17 hardwood. Any advice re. min specs?

Answer: We would rather you used timber too! If you can let us know the span of the joists (distance between points of support) and the spacing (distance apart) we can tell you what size you would need for your deck. A couple of points - merbau is inclined to leach out tannin, so you are likely to have some brown stains on the metal decking which may or may not be a problem, depending on how visible the roof is from below. Also much of the F17 hardwood on the market is Victorian ash which is not highly durable when fully exposed to the weather, so it would be a good idea to find out exactly what kind of timber you are being offered.


Question: I would like to clad the exterior of a 1960's style house with timber cladding. What is the most resistant type? Is there one on the market that will not require too much maintenance Are some woods that are treated to help with UV etc.

Answer: A number of timbers are suitable for cladding, including softwoods such as cypress pine and western red cedar, and various hardwoods. The most important properties for exterior cladding are durability and stability. "Durability" means resistance to wood rot and insect attack - all commonly used timbers have been rated for durability, and we would recommend Durabilty Class 1 or 2 for best results. As a guide, western red cedar is rated Class 2 outdoors above ground, while Australian hardwoods such as blackbutt, spotted gum and ironbark are Class 1. Your timber merchant may suggest other types of timber. Stability is generally ensured by kiln drying, although some timbers are more stable in service than others - western red cedar has particularly low swelling and shrinkage properties. Regarding maintenance, this is largely a question of the type of finish you choose. Acrylic paint needs the least maintenance, with some manufacturers now offering guarantees of ten years or more. Pigmented stain is next best for lasting properties, while clear finishes need the most frequent maintenance. If the timber is shielded from the weather by verandahs or wide overhangs, maintenance is dramatically reduced. We don't know of any timber treatment that resists UV, but some of the clear coatings contain UV absorbers. If you decide on a clear coating we strongly recommend one that contains a UV inhibitor.


Question: We need new shiplap hardwood boards on south and western infills above our windows. We could have ember attack and even fires past the home, so I want these infills to be bushfire resistant.

Answer: If your house is in a designated bushfire area the site may have been assessed to determine the level of bushfire hazard. If so, the material permitted for the window infills will be determined by Australian Standard 3959-2009, Construction of buildings in bushfire-prone areas, according to the Bushfire Attack Level (BAL) specific to your site. If you are not in a designated bushfire-prone area you may still wish to refer to the list of timbers in the Standard that are deemed to be 'bushfire-resisting'. The list includes red ironbark and kwila (merbau), either of which would be suitable for shiplap boards.


Question: I'm hoping to clad my house in Sydney with wood. In a shiplap or board and batten configuration. In a natural aged grey. A few people have said that wood needs painting every 3-4 years. What is the best hassle free wood for this purpose?

Answer: We suggest western red cedar would be your best choice for exterior wall cladding. If you paint it with acrylic paint you won't need to repaint for 10 years or more. Some manufacturers provide a guarantee. For example, Dulux guarantees that its Weathershield product won't peel, flake or blister for 15 years. Wood stains don't last as long but provide a more natural look in the sense that some of the wood grain shows through. If you want the aged grey look, you could select a paint in this toning. Alternatively, Cabot's have a colour called "weathered cedar" in their Timber Shades exterior wood stain range which gives the impression of naturally weathered wood. You can also achieve the weathered look by just leaving the timber to weather naturally. However, this often occurs unevenly on walls. The weathering process won't occur at all under the eaves where the walls are protected from sun and rain, and the result will be walls that are grey at the bottom and brown at the top (assuming you have an eaves overhang). Also in the long run all timbers retain a better appearance if they have some kind of protective coating (paint or stain). When installing your cladding, try to keep it clear of the ground. An attractive effect can be achieved with a couple of courses of brickwork or a stone base. The timber is then kept above the splash zone which will help to prolong its life.


Question: Hi , I'm building a new fence with "Cypress Pine". I'm unsure whether to use paint, decking oil or stain. Paint will probably give me the longest service life , but how well does Decking Oil or Decking Stain last as compared to paint ?

Answer: Today's water-based paints achieve a long service life and it is reasonable to expect ten years or more, assuming the paint is applied to dry timber. Painting wet or unseasoned timber is risky, as the heat of the sun can draw moisture out of the wood causing the paint to blister. By comparison, decking oil (clear) might only last 1-2 years, while decking stain will last longer depending on how much pigment it contains. The advantage of oils and stains is that they are quick and easy to apply. It is only possible to give a rough approximation, since the life of the coating also depends on the degree of weather exposure. Another option is to leave the fence to weather off to a driftwood grey colour, without any finish at all, although you would then expect some minor weathering effects such as surface checking (fine cracks).


Question: Hi, I have a Merbau deck which I am sanding back and wish to change the colour of it to a grey tone. Is there an oil stain product which will give me this result? The area is surrounded by naturally grey timber fences and I want to match.

Answer: Feast Watson decking stain comes in a range of colours including an attractive grey tone called Snow Gum. This may give you the result you want. However, be aware that the stain will eventually wear off in high traffic areas and the natural colour of the decking will show through. If the deck is exposed to the weather you could just let it weather naturally to a driftwood grey colour, the same as the fences. If the deck is covered, of course the weathering process won't happen and you will then need to stain it to achieve the effect you want.


Question: I am building a small jetty for our garden dam. What sort of poles should I be using given that they are going to be sitting in dam water?

Answer: The choices are either a durable hardwood species, or preservative treated pine. Hardwoods of recognised durability include turpentine and tallowwood. Turpentine is commonly used for wharf and jetty construction in salt water where its resistance to marine borers is an advantage, but this will not be an issue in a fresh water dam. However, these hardwoods are also resistant to fungal attack which will ensure a long life in fresh water. Treated pine rounds also perform well because the thick band of sapwood on the outer part of the pole provides an easily treatable layer that gives protection against wood rot.


Question: I am using Spotted Gum (clear finish) is using 42Wx42Dx3.4L as an external balcony's louver, will be top and bottom fixed. How do we prevent warping and cracking? and fixing method? Will all timber turn grey eventually? thanks

Answer: I take it the louvres are square (42 x 42) and 3.4 m long. You won't necessarily have any cracking, but warping could occur if they are only fixed top and bottom. The louvres either need to be fixed at closer centres, or you need to consider a timber that is particularly stable, for example western red cedar. Even then it would be helpful to have some intermediate restraint, at least at the mid-point and preferably more frequently. All timber turns grey if it is exposed to the weather without a coating. A clear exterior varnish with ultra violet absorbers, or a pigmented stain, will prevent greying if the coating is well maintained. A pigmented stain is easier to maintain.


Question: Could you please advice the suitability of Spotted Gum (clear finish) for external cladding application. Section: 150Wx19D. What is optimize length to adopt to prevent warping and bending and not increasing the cost? any advice on fixing method?

Answer: Spotted gum will perform well as cladding as long as it is kiln dried. The length of the boards is not so important. Assuming they are straight at the time of installation, the important point is to fix them at close enough centres. The recommended spacing for fixing points is 600 mm. If the cladding is overlapped in traditional weatherboard style, each board must be fixed with only one nail at each crossing, approx. 35 mm from the bottom of the board. The top of the board is secured by the overlap of the board above. This is easier to show in a diagram, so for more information you might like to log onto the Australian Hardwood Network at www.hardwood.timber.net.au and refer to the Cladding section in the Australian Hardwood & Cypress Manual. Regarding clear finishes, be aware that they require more maintenance than paints or pigmented stains. If the cladding is protected by overhangs or verandahs, maintenance is greatly reduced.


Question: Should CCA treated pinus decking at a seaside location be treated in any way? I believe that unoiled it may split. What is your opinion?

Answer: CCA treated pine has good resistance to wood rot and insect attack but the treatment doesn't give any added protection against wetting and drying. Treated pine is quite absorbent and, like all wood products, shrinks and swells with changes in moisture content. The seaside location is not particularly hazardous, it's just moisture in general that contributes to weathering effects such as surface cracking. Applying a water repellent or decking oil will reduce the wetting and drying cycles that cause cracks and splits. You will see the effect straight away - uncoated decking absorbs water readily but after coating with an oil or water repellent, water will bead on the surface.


Question: I'm keen to lay a deck using hardwood or CCA pinus decking on CCA bearers. I am aware of a recommendation not to use galvanised fixings with CCA timber, and certainly have noticed severe corrosion of bolts etc in contact with CCA. What fixings are best?

Answer: The problem is caused by traces of copper from the preservative that come into contact with the fasteners (dissimilar metals). However, hot dip galvanised fasteners are usually OK (not zinc-plated fasteners that have a thinner protective coating). This enhanced corrosion only occurs in the presence of moisture, so if your deck is fully enclosed it's not really an issue. In the more usual case where a deck is fully exposed to the weather we suggest hot dip galvanised fasteners for CCA treated pine, or stainless steel with the newer ACQ preservatives that contain more copper.


Question: I am building a timber deck using 140x25 Merbau decking. What length and gauge screw should I use, what distance should the screws be from the edge and end, and what sealer should I use?

Answer: Batten screws of 75 mm no. 14 type 17 will be OK. Make sure they are a non-corrosive metal, not just zinc plated. Pre-drill pilot holes approximately 80% of the diameter of the screw shank and keep the screws at least 12 mm from edges and ends of boards. Regarding the sealer, your choices are decking stain, oil or a clear coating. Decking stains contain pigment that changes the colour of the wood slightly but helps to resist UV. Oil finishes look great in the short term, but need regular maintenance, otherwise the timber will start to grey off. If you decide on a clear film-forming coating, choose one that is suited to the Australian climate and contains UV absorbers.


Question: Could you advise me on which species of hardwood decking timbers don`t leach.

Answer: Most hardwoods contain tannin, which is the brown substance that leaches out when the timber is exposed to rain. The good news is that it stops after a few months of weather exposure - as long as you don't have light coloured pavers underneath, or a painted wall that the rain will blow against, it shouldn't be a problem. If tannin leaching will cause probems, manufacturers of deck cleaning products claim that a preliminary scrub will help to remove the tannin and thus reduce leaching. It would also be advisable to avoid high tannin hardwoods such as merbau (kwila), which is an excellent decking timber but high in tannin.


Question: Can cca treated pine be used as a framework for a small decking area, to be placed directly onto a concrete slab?

Answer: CCA treated pine is resistant to wood rot and insect attack and so can be used in situations which would be hazardous for untreated timbers, such as directly on a concrete slab. Even so, we would suggest that the framework be installed so it doesn't trap water. For example, assuming there is a fall on the concrete for drainage, the timber should run down the fall allowing rainwater to drain away, rather than across the fall where it would trap water. In this case the recommended treatment level would be Hazard Class H4, appropriate for in-ground timber. Although technically your timber isn't in the ground, it is subject to a greater hazard than timber that is completely above ground.


Question: I want to use 140mm merbau boards for a new deck. I'm concerned about cupping if I buy x19mm boards as opposed to x22mm boards (or thicker). Will good screws and placement make up for the thinner boards or would it be wise to go with the thicker boards?

Answer: Decking that is 140 x 19 is quite wide relative to its thickness and therefore less dimensionally stable than a "chunkier" section. To take an extreme example, a square post won't cup, but the greater the difference between width and thickness, the greater the potential for cupping. There are no regulations about this, although it is recommended by one authority that the width of boards should not exceed eight times their thickness, which would put your decking within the allowable range. Nevertheless, we would be inclined to choose the 22 mm decking rather than the 19 mm, simply because it reduces the difference between width and thickness and therefore might be a little more stable in service. In saying this, we assume the deck will be exposed to the weather. If it is roofed over and protected from wetting and drying, the issue becomes less important. With more decks being built close to the ground you need to be aware of the importance of good air flow under the deck. And for wide decking we recommend screw fixing rather than nails. On the plus side, merbau is a stable timber in the weather assuming it is supplied in a kiln dried condition.


Question: What are the qualities of Tallowood, I wish to use it as an inground featured post in a picket fence, and it would be unseasoned? Does it suffer from high rates of shrinkage or deep fissure cracks in the heat? Is it hard to machine?

Answer: Tallowwood is an excellent outdoor timber and is rated Class 1 for in-ground durability according to Australian Standard 5604, Timber - Natural durability ratings. It also has almost no gum veins, which is unusual for a eucalypt. Tallowwood has a slightly greasy feel, hence the name. This sometimes makes gluing difficult, but presumably that won't be a problem in a fence. Shrinkage is relatively low and it usually resists surface checking. The only disadvantage is the occasional presence of "water rings". These are fibre separations that follow the growth rings and such material should be avoided. Otherwise we consider it would be highly suitable for posts in a picket fence.


Question: I would like to purchase timber for a deck that is exposed to extreme weather conditions in South Australia. I would like advice on which timber species, offers excellent stability and whether a 135mm wide board is too risky for such an application.

Answer: Generally speaking the denser the wood, the more it will shrink or expand for every 1% change in moisture content. However that doesn't tell the full story because denser woods absorb water less readily. Consequently, dense woods are actually quite stable when exposed to short-term wetting (eg. intermittent rain) because they don't take up much moisture. Therefore we consider that the decking species commonly offered for sale will all give good service if installed correctly. Boards that are 135 mm wide should be OK but potential swelling and shrinking will be greater than with narrower boards, since movement is proportional to the width of the board. It is therefore advisable to leave a slightly larger gap between boards when using wide decking. You should ask the company supplying you for their recommendations regarding the installation of wide decking. If your supplier has no advisory literature there are some guidelines available on the net, eg. at www.neimanreedspecialties.com/Installation. It is also a good idea to ask your supplier to check the moisture content of the decking at the time of delivery to make sure it is not unusually high or low. This is easy to do with an electrical resistance moisture meter.


Question: I am building a deck that is fully exposed to the elements. Can you advise which timber would be best suited to the South Australian climate and whether it is risky to use a 130mm wide board?

Answer: There are many types of timber available for outdoor decks, but there are no commonly used decking timbers that we consider unsuitable for the South Australian climate. Hardwoods of course are more resistant to indentation than softwoods, but treated pine will give good service in a domestic situation. Wide boards look attractive, but the wider the board, the greater the potential movement. By "movement" we mean the response of the wood to uptake of moisture. For example, a 130 mm board will expand by the same amount as two 65 mm boards for a given increase in moisture content. However, two narrower boards will have more space to accommodate the expansion, assuming there is the usual 4 mm gap between all boards. Having said that, wide boards that are well fixed should give satisfactory service in a well ventilated deck that can dry out quickly after rain and is not subject to unusual levels of moisture rising from the ground underneath.


Question: As I am building a home located within bush near Daylesford, Victoria . I would like to use river red gum as the external flooring/decking timber. What do you think? Protection from the elements etc?

Answer: You do not mention whether your new home is in a declared bushfire area, but it sounds as if it might be. If so, only certain types of timber are permitted for external structures, ie. timbers deemed to be "bushfire-resisting", as defined in Australian Standard 3959-2009, Construction of buildings in bushfire-prone areas. River red gum is considered to be a bushfire-resisting timber because it has a density greater than 750 kg/m³. Whether timber structures are permitted also depends on the Bushfire Attack Level (BAL) applicable to your building. For more detailed information on this topic, visit the website www.timber.org.au.


Question: Recommended species External residential deck Sydney inner West Posts supporting deck & roof,bearers ,joists,decking,as well as exposed beams supporting the roof. Availability.

Answer: If the posts are supported on brackets, LOSP-treated pine would be a good choice, assuming the client wants a painted finish - it's generally sold pre-primed. The appropriate Hazard Class for treated pine above ground is H3. If the posts are going into the ground, LOSP treatment is not adequate and the posts would need to be treated with CCA or ACQ. Alternatively, you could specify a Durability Class 1 hardwood. Note that CCA is no longer permitted for the actual decking, or for handrails, but is OK for supporting posts. Exposed beams supporting the roof could be LOSP-treated pine, treated to Hazard Class H3. These products are readily available.


Question: I am building a double storey timber barn with vertical external timber. I was originally advised to do it in western red cedar to get a lovely silver aged appearance. As it is expensive and ages poorly I wondered what an alternative is, Aust hardwood?

Answer: I wouldn't entirely agree that western red cedar ages poorly. Cedar actually tolerates weather exposure quite well - after all, it's a popular choice for roof shingles, although in such an exposed situation it often needs maintenance after 15 to 20 years. If you would prefer an Australian hardwood, any durable, kiln-dried species will do the job. When we say "durable" we mean resistant to wood rot and insect attack. Your timber merchant should be able to advise you on types of timber that satisfy these requirements but if not, feel free to leave another message. Regarding the silver, aged appearance you are looking for, be aware this only happens when the whole surface is exposed to the weather. Areas of wall cladding that are protected by eaves overhangs or verandahs will not turn grey, but will retain their original colour. If this is likely to be a problem you might want to bleach the cladding before you install it to get a "pre-weathered" look and avoid uneven patches. Also make sure the contractor uses non-corrosive nails (hot-dip galvanised or silicon bronze), but preferably not copper because although it doesn't rust it's inclined to leave black streaks on the wood.


Question: I am looking for a pine decking with no knots, in FSC certified Radiata Pine, is it possible to get this product?

Answer: Radiata pine is typically a knotty timber. "Clear" (ie. knot-free) radiata pine is produced for mouldings and similar uses, but as far as we are aware no-one markets knot-free pine decking. It is possible, of course, to obtain knot-free hardwood decking in a range of sustainably grown species.


Question: I would like to find a timber, other than treated pine, that is suitable for external use for the treads of a set of stairs. What species can you recommend that comes from sustainably managed forests?

Answer: Your best option is a durable hardwood. We use the word "durable" to mean resistant to wood rot and insect attack in an outdoor situation. There is an Australian Standard that rates commonly available species according to their durability on a scale of 1 to 4. The availability of different species varies in different parts of Australia. Suitable species include kwila (merbau), tallowwood, ironbark and jarrah. Rather than giving you a long list, some of which might not be available in your area, perhaps you could inquire from your local timber merchants which species they have that are kiln-dried and of Class 1 or 2 durability in the sizes you need. We can then advise you further if necessary. With regard to sustainability, you can be confident that any Australian-grown timber comes from a sustainably managed forest. There are various certification bodies that back up the sustainability of imported timbers, for example the Malaysian Timber Certification Scheme, the Forest Stewardship Council, the Tropical Forest Trust and so on.


Question: Please advise on the safety of using CCA treated timber to build a raised garden bed, for children to grow fruit & vegetables in. Some mothers I know are worried about leaching of poison?

Answer: A number of studies have shown that CCA is not absorbed into above-ground food crops such as grapes, tomatoes and cucumber. There are some reports of a slight increase in arsenic content in root crops such as carrots and beets grown against treated timber, although the arsenic is in a safe organic form and most of it is removed with peeling. Any possible concern can be eliminated by growing these vegetables more than 100 mm from treated-timber garden edgings, or by lining the edgings with plastic. This has the additional useful effect of reducing soil contact with the wood, which could further extend the wood's life. Our answer is based on information provided by the research organisations CSIRO and Scion.


Question: I've a question about leaching in grandstand recycled blackbutt battens seating. It is high pressure water cleaned. Will it leach every time it's cleaned.? Is there a product we can apply to the timber to reduce leaching?

Answer: Most hardwoods leach tannin when washed by rain. Tannin only leaches out of new timber, or old timber that has been machined to expose a fresh surface. Leaching ceases of its own accord, when all the tannin has washed out of the surface fibres. Your preliminary weathering of the timber will have removed much of the tannin, assuming all the timber was exposed to the washing action of the rain. Cleaning the seats with a high pressure hose will eventually remove any remaining tannin. You might want to accelerate the process by using a deck cleaning product. Manufacturers of these products claim that they remove excess tannin, although we haven't conducted any tests of our own. Deck cleaner will work in with the high pressure cleaning programme because after the wood has been scrubbed with deck cleaner it has to be hosed off.(Question edited for space reasons.)


Question: Hi. I am currently looking for a species that I can use for an exposed coastal site in South WA. Anything you can recommend that is durable for the conditions, and termite resistant without treatment? The cladding will not be in ground contact. Thanks.

Answer: A good local timber for your situation would be kiln-dried jarrah. It is resistant to termite attack without treatment and is rated Class 2 durability outdoors above ground, according to Australian Standard 5604, Timber - Natural durability ratings. Durability ratings are on a scale of 1 to 4, where 1 is the highest rating and 4 is the lowest. While jarrah is resistant to termite attack, it is still wise to consider a barrier system to ensure termites are not able to enter the building. A protective coating for the jarrah will also help to ensure a long life.


Question: We are considering specifiying western red cedar cladding for a house in a semi-coastal environment. Is it possible to leave unfinished and avoid the risk of fungus attack? If finishing, can you suggest a natural product, that still allows timber ageing?

Answer:Western red cedar is rated Class 2 outdoors above ground according to Australian Standard 5604, Timber - Natural durability ratings. The rating system is on a scale of 1 to 4, where Class 1 is the highest durability. Cedar is commonly used for roof shingles where it achieves a life of 20 to 30 years, but this is a more severe exposure hazard than wall cladding, which is vertically oriented and less inclined to trap water in lap joints. If left unfinished the surfaces exposed to the weather will bleach to a silver grey colour. However, protected areas (e.g. under eaves) will not change colour to the same extent, which leaves the cladding looking patchy. Cabot's have a colour called "weathered cedar" in their range of exterior wood stains which achieves the aged look evenly while also giving the timber some protection against the elements.


Question: We are looking at using Tallowwood for external decking. Our Supplier proposed two options 150 mm x 25mm or 190mm x 35mm. Do you think there will be issues with cupping? Screw fixing at 450mm ctrs. Some of the decks will be in direct contact with pool water.

Answer: Cupping is most likely to occur with wide, thin sections so increasing the thickness to 35mm is a good idea if you are contemplating the 190mm width. We consider either of these sizes should be OK, combined with screw fixing, but the screws will need to be quite long with the 35mm decking. Of course, we assume that these sizes will be properly kiln dried, so you will need to assure yourself that the thicker section can be dried right through to not more than 15% moisture content.

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