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: We have supplied some Kapur decking in sapfree which are having some issues just 6 months after installation. The issues being faced by the owner of the deck now is that the exposed surface of the timber is peeling. However, upon removal of the product, the underside is still in great condition. We are unsure if the owner used proper coating after installation but we believe that this defect could be due to severe exposure from the hot weather in Adelaide over turn of last year, 2012 - 2013. I can furnish some images of the defects if it helps. Please advise. Thank you.
19/06/2013
Adelaide has a relatively hot, dry summer but it doesn't usually make wood peel outdoors! There must be some other reason for the problem. We believe the most likely explanation is that the decking was processed with blunt cutters. This causes pounding of the wood which tends to loosen the grain. The following explanation is contained in a report on the net: "Loosened grain is a surface defect that is caused by the machining action but also has its roots in the nature of the wood workpiece. Loosened grain refers to the separation and curling of the tips of growth rings on the surface of flat sawn lumber (Panshin and de Zeeuw, 1970). The defect is caused due to the pounding action of the cutting tool or the pressure of the sanding head causing crushing of the earlywood under a layer of latewood exposed on the workpiece surface. Though the crushing is the initial cause of the defect, it is the separation of the growth rings near the surface due to the differential drying stresses in adjoining earlywood and latewood areas that cause the curling of the latewood". What that is saying is that faulty machining initiates the problem, but it only shows up when the wood is exposed to drying stresses. So in a way the hot weather could be said to have "caused" the problem, but on the other hand it wouldn't have happened if the wood had been machined with sharp cutters. You may wish to forward your images to expert@naturallybetter.com.au if you feel this doesn't explain the problem.
Q: We have just installed a spotted gum deck and would like to leave it to go silver/grey. It has good ventilation and is raised above the ground. So it should be Ok, but we are wondering how long the deck is likely to last, compared with if we treated it regularly with decking oil?
17/06/2013
Spotted gum performs well outdoors and it's rated Class 1 Durability above ground. We assume the deck is not covered by a verandah or pergola, since the greying effect will occur very slowly or not at all if the wood is shielded from sun and rain. It's hard to say what difference regular oiling would make - it might help to prevent or reduce weathering effects, such as the development of cracks ("checks") which allow water to enter the wood and accelerate its breakdown. However, the decking is likely to give good service for 20 years or more, and if you take into account the cost of regular oiling over that time span you will be well ahead by allowing it to go grey.
Q: Further to our earlier question, we do have glazing that is within 300mm horizontally and 400mm vertically, therefore does this mean we cannot use merbau in those areas, or we can use it?
13/06/2013
In BAL-19, decking within the 300mm/400mm zone (ie. adjacent to glass) must be either one of the seven species classed as "bushfire-resisting timber", or any timber with a density of 750 kg/m³ or greater. Merbau is one of the seven bushfire-resisting timbers, and also complies with the density requirement, so no problem at all.
Q: I am working on a new pergola project for a client located in the Western Suburbs of Sydney. The structure will be constructed of steel. We would like to clad around the steel columns with a hardwood timber to conceal downpipes and provide some visual relief to the steel. I am a little concerned about how to join timber to steel and how each material will react differently to expansion.
11/06/2013
We feel the best way to tackle this is to build a light frame around the steel and fix the cladding to the frame, rather than trying to attach the cladding directly to the steel. As you say, each material reacts differently to temperature changes - in hot weather timber loses moisture and tends to shrink, whereas steel expands. When detailing the timber cladding, we suggest you avoid mitre joints at the corners since they don't tolerate any movement.
Q: I purchased strand bamboo decking for outside use. within days of professional laying it began to swell and blister once it rained. within days swelling/blistering, the strands began to separate and delaminate. I have supplied 6 coats of the Cutek Extreme product which has provide some assistance but the boards continue to fail. I'd appreciate any advice or assistance you can give. Cheers, Peter Matthews 0408 289 120
10/06/2013
There is clearly something wrong with your decking. Assuming the installer has used a product marketed as outdoor decking, rather than bamboo strand indoor flooring, it should stand up to weather exposure without coming apart. We suggest you check whether any warranty was issued with the decking. If there is no specific warranty it is still subject to a condition of the Trade Practices Act that goods will be fit for purpose.
Q: Are there problems associated with using tongue and groove exterior decking compared to conventional decking with small controlled gaps? The timber selected is Tallowood.
03/06/2013
Old houses often had tongue & groove floorboards on their verandahs, but tongue and groove boards must be used with caution outdoors since they don't behave well if they get wet. Decking boards with gaps between can swell and shrink with wetting and drying, but t & g boards have nowhere to go. If the decking is protected by a verandah or pergola roof there's more chance of success, but even then it's important not to cramp up the boards tightly together in case rain blows in. Also a slight fall is desirable so any water runs off rather than pooling up on the floor. Whatever you decide to do, tallowwood is an excellent choice for decking.
Q: I am searching for decking board span tables. I have had a look at the #9 Timber flooring - design guide for installation and couldn't see any span tables.
30/05/2013
Spans for decking are different from the equivalent thickness of flooring since decking doesn't have the ability to spread the load via a tongue and groove joint. Decking spans can be obtained from Table 5.4 of AS 1684 Part 2-2010, "Residential timber-framed construction". Joist spacings for timber decking, as given in Table 5.4, can be summarised as 500 mm for 19 mm hardwood and 450 mm for 22 mm treated pine.
Q: I need to replace three car port supports on a two storey timber house. They are 90x90mm Oregon and have rot. One is 6m long, the other two are 4.2m. What timber would you recommend I replace them with. I was thinking of laminating two 90x45 KDHW (Vic Ash) for the posts and 90x45 KDHW for the bracing which will follow the gable angles of the roof line.
29/05/2013
If the oregon posts have rotted they must be exposed to the rain, or perhaps they are in contact with the ground. If the posts are in the ground, or in ground contact, a more durable timber than Vic ash will have to be used. Even out of ground contact - for example on metal brackets - Vic ash is not highly durable when exposed to the weather. It's rated Class 3 above ground on a scale of 1 to 4, where Class 1 is the most durable and Class 4 the least. So it's probably OK if there are no places where water can collect, if it's kept clear of the ground, and if it's sealed all round including end-grain. There are more durable timbers, including preservative treated pine, although the lengths could be a problem.
Q: I am looking for a dark timber cladding almost black, maybe stained for an exterior application. The project is a 2 level residential building.
28/05/2013
The shou-sugi-ban process would give you a black finish since it involves charring the surface of the wood. However, it may not be available in your area. Failing that, a black exterior wood stain should achieve the desired result. The Resene range includes some very dark colours and can be viewed via this link http://www.resene.com.au/pdf/charts/Exterior_Woodcare_Chart.pdf. Alternatively, black acrylic paint will achieve the longest life between maintenance intervals.
Q: I'm adding a deck to an existing property. Part of the deck will be built over an existing concrete slab. We can clear soil under the deck to ensure we have 400mm clearance for termite inspection (BCA requirement), but we will have less than 400mm clearance in the area over the existing concrete slab. Is this acceptable? Or do we need to remove the existing slab (we can't raise the deck level)?
28/05/2013
Our understanding is that a deck is an "attachment to a building" and as such is not required to have access for termite inspections as long as it is separated by a gap of not less than 25mm. The purpose of the gap is to allow "clear and uninterrupted visual inspection" so that termites attempting to gain access to the building via the deck can be detected. This is more clearly explained in AS 3660.1, "Termite Management, Part 1: New Building Work", which states "where attachments or structures.....abut a building and there is no clear gap, then a barrier shall be provided to the attachment...." Otherwise, attachments to buildings don't require a barrier system, and it follows that if there is no barrier system there is no need for ground clearance to inspect it. However, we are always cautious about timber decking close to the ground. The ground under the deck tends to become saturated and moisture is absorbed back into the underside of the decking causing cupping. So we strongly recommend you look at ways to achieve effective drainage under the deck.
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