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?
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Q: When we went to buy the oil for our deck we were advised to wait 6 weeks before oiling, although the timber is well cured.. Opinion please?
09/03/2013
There are two schools of thought about allowing decking to weather before coating it with oil. Some say it should be weathered for a time so the oil can penetrate better. This might be based on the idea that freshly planed timber has a glazed surface (mill glaze or planer's glaze) that prevents absorption of the oil. However research bodies have been unable to duplicate mill glaze in the laboratory. You will find more about this on the net. Others say timber should be coated as soon as possible, before the natural colour starts to weather off to grey. We are inclined to the latter view, ie. coat it straight away. Celery-top pine doesn't leach tannin like hardwoods so we can't see any particular reason to wait. Perhaps you should ask the paint store why they gave this advice.
Q: I am looking for any information I can get on the use of charred timber in cladding and its benefits and potential for extended durability. Any information would be much appreciated.
07/03/2013
Oxy charring used to be a common way of treating the bases of Australian utility poles but didn't rely on charring alone to enhance preservation. The charred zone was used as an absorbent pad for the application of creosote. Charring would have a short-term sterilising effect against wood rotting fungi but would not confer any lasting benefit as far as we are aware. Prolonged heat treatment (as opposed to charring) has been shown to improve the resistance of wood against decay and insect attack and has been commercialised under the name Thermowood. However, the process is carried out under controlled conditions over a period of several hours. Passing timber over a flame to produce a charred surface is unlikely to achieve similar benefits. Regarding fire retardancy, a layer of char will insulate the wood beneath from heat transfer, but we are not aware of any significant benefit in delaying ignition. Everyday experience with a domestic fireplace shows that charred wood will readily re-ignite.
Q: I am presently selecting the timber species for a front door pull on a residential project. See attached. The front door is protected by a deep entry porch but is north facing. I am looking for a Brown Red colouring as we have a Merbau slatted feature elsewhere on the façade. I am able to select from Spotted Gum or have Tasmanian Oak stained. I can also nominate a timber species. The finish is an exterior urethane in a satin. My gut feeling is that it is better not to use a stained timber and go for one that is naturally darker. My fear is bleaching. Would you have any suggestions?
28/02/2013
We thought it should be possible to have the door pull made from merbau if you are wanting to blend with merbau elsewhere. Otherwise jarrah would be a good match. Make sure any exterior varnish contains UV absorbers, even under the entry porch. Regarding bleaching of the timber's natural colour, there will be some fading on exposure to sunlight. A well-maintained coating will help to counteract this.
Q: Can I use Belian 90mmx21mm for decking on an external deck built very close to the ground? The information in your species guide says that it can be only used above ground inside due to termite issues. Is this correct? And also is there anyway to treat the Belian so it can be used as external decking close to ground?
28/02/2013
Belian is actually highly durable and rated Class 1 according to the Australian system, meaning it has excellent resistance to insect attack and wood rot. There are no problems using it outdoors. However, be aware that decks close to the ground need to have good drainage underneath, otherwise the timber may absorb moisture, causing distortion. We would be grateful if you could email us a link to the species guide you mention. We are not aware that belian is mentioned in our species guide, but if our information is incorrect we may need to amend it.
Q: I’m specifying timber weatherboards for a single storey house extension. It will be primed and painted and fixed to a stud frame on a concrete slab. Would you advise the pros and cons of various materials for the boards including – Western Red Cedar, Treated pine, Reconstituted timber. Please send any information you have on standard profiles and the detailing of junctions and edges. Thanks
27/02/2013
For the traditional look of a tapered weatherboard, fixed horizontally, western red cedar is hard to beat as it is dimensionally stable and has good resistance to wood rot. Today's paints achieve a long life and some manufacturers give a warranty on the life of the coating. Treated pine is generally installed vertically with a tongue and groove joint, although a weatherboard profile might be available from some suppliers. The only reconstituted claddings we are aware of are hardboard (eg. "Weathertex") and plywood (eg. "Shadowclad"). Both do an excellent job, resulting in a flat surface rather than the corrugated surface created by tapered weatherboards. Detailing of junctions is important. We don't recommend mitre joints at external corners as they are impossible to seal. It's best to stop the boards against a corner bead. You will find a number of suitable details if you write "timber cladding corner details" in your browser.
Q: Can 190 x 45 F17 be used for bearers and joists for decking? There is no cover over the decking.
27/02/2013
No problem using F17 grade timber, but the type of timber could be an issue. Victorian ash is often marketed as an F17 grade product and it's not highly durable when fully exposed to the weather. The Australian Standard rates it Durability Class 3 on a scale of 1 to 4 when used outdoors above ground - Class 1 is the highest durability and Class 4 the lowest. So we would strongly recommend some protection to the top edge. Proprietary products are available for this purpose, eg. "Joistrip" and "Protect-a-Deck".
Q: I have a question about structural timber framing for a residential project, particularly whether glulam will be appropriate and the best option. The timber is to be narrow but deep (300-450mm) to act as a shading device for windows as well as form the structure for the north facing wall (10m x 5.5m with top half glazed and bottom half shelving). The grid of vertical and horizontal members creates windows of approx 1.5m x 0.9m and the timber members of the top half framing windows is exposed to the weather. Any advice or direction as to which timbers, including glulam, I should look into would greatly appreciated.
25/02/2013
You mention that the timber shading will be "narrow but deep", but don't mention how narrow. The cross-section should be reasonably chunky and/or well secured to ensure long term stability. Thin deep sections are more likely to distort - there are no hard and fast rules, but as a guide the US Dept of Agriculture's "Wood Handbook" recommends that the width of a board should not exceed eight times its thickness. You could possibly obtain single pieces of timber 300mm wide, but 450mm would be difficult. Maybe it's not essential to construct the shades from a single section and they could be made up from several pieces with a small gap between. Regarding your suggestion of glulam, "Durabeam" would be suitable, laminated from cypress pine by Laminated Timber Supplies. Other durable glulam species may be available, or species of lower durability with preservative treatment to H3 level. Optimum performance will be achieved if access is available for maintenance of a coating.
Q: I am looking into the use of plywood as an external cladding material to a number of cubby houses located in Canberra. Do you have any precedents of ply as external clad system (walls/roof) that I could reference?
20/02/2013
Plywood for outdoor use must be made with a water-resistant adhesive, be preservative-treated against wood rot, and preferably have a surface that will resist "crazing" (fine hairline cracks). A product designed specially for use as external cladding is "Shadowclad", produced by Carter Holt Harvey. It is available in a proprietary preprimed, powder coated finish for painting, or in an unprimed natural surface for staining. The saw-textured surface provides the necessary resistance to hairline cracking.
Q: Here's an oldie but a goodie! I have an architect who wants to use an Australian hardwood (tallowwood or ironbark ) as a material for a pergola. He has it fixed off to a white painted, rendered brick wall and has white natural stone tiles underneath it. We are worried about leaching and have told him so. He has told us to find a product that will greatly reduce or eliminate it. Are you aware of any such product?
20/02/2013
Leaching of tannin is a common problem, specially if there are light coloured surfaces nearby. There are two possibilities to reduce the impact. If the pergola is going to have an oil or stain finish, that won't stop the rain from leaching out tannin. In that case it might help if you give the timber a preliminary scrub with deck cleaner - we haven't tried it, but it's a technique promoted by the manufacturers of such products. If the timber is to be painted, pre-painting the timber all round before assembly with an oil-based enamel paint will stop any leaching - that is something we have tried, and it works. But we emphasise the need to paint the timber all round, including the surface that is against the brickwork. On the other hand, if the rain won't penetrate to that point there's no need to worry - leaching only occurs when the rain contacts bare wood, or lightly coated wood.
Q: I am looking for information, either heuristic or in literature, about the durability of doubled timbers in exterior applications, ie. double bearers, double columns, doubled rafters etc. It seems to me that such timber contact would lead to moisture problems and eventually rot but I haven't been able to find any information on the issue.
18/02/2013
We don't know of any literature specifically dealing with the durability of double members, but we agree that doubled timbers exposed to the weather are not a good idea unless the timber is preservative treated. There is, of course, plenty of literature explaining the hazards associated with moisture traps in timber structures, so maybe doubled timbers should be considered in that context. Even where preservative treated timber is used we would prefer double bearers and double columns to be made up with spacer blocks, rather than nail-laminated tightly together. Alternatively, double bearers can be fitted with a protective strip along the top edge to prevent water penetration. A typical product for this purpose is "Bearer Protectadeck", available in 70mm widths from Bunnings.
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