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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. Just wondering if you could tell me what the best hardwood is to use for a balustrade around a pool and the external joinery which is partly under cover. The site is close to the sea.

There are several kiln-dried durable hardwoods that could be considered. Merbau is popular for external joinery since it is relatively stable and is rated Durability Class 1 outdoors above-ground. Other hardwoods may be available in your area, and a local timber merchant should be able to advise. Proximity to the sea isn't a great problem for timber, but the salt air is aggressive for metal fasteners so you might want to consider using stainless steel to avoid corrosion.

Q. I read your comments about Merbau leaching. We have some new merbau deck railings which we will water. How do we know when the leaching has stopped?

Rather than just washing the railings we suggest you use a timber cleaning product such as Cabot's "Deck Clean", Intergrain "Reviva", etc. These products are likely to draw out more tannin than just plain water. It should be fairly obvious when you've done the job - the tannin will appear as a brown stain so you will be able to see when no more washes out.

Q. Hello, I am proposing to clad a high retaining wall with timber finished using the shou sugi ban technique. Do you know of any manufacturers in Sydney?

Strictly speaking shou sugi ban is charred sugi wood, a Japanese species. However, charring can be carried out on any species, preferably a softwood with prominent growth rings. Charring a softwood allows the softer portion of the growth rings to be brushed out leaving the harder part of the growth ring raised, resulting in a textured surface. Hardwoods don't have such prominent growth rings so some of the effect is lost. We don't know of any suppliers in Australia who offer this service but you might find a joinery works in your area that is prepared to have a go, or even try it yourself. The process is described on this website - http://pursuingwabi.com/2007/11/05/shou-sugi-ban/.

Q. If there is no way we can repair our existing timber shutters and have to get new ones what will be the maintenance requirement? We would like to keep the western red cedar natural look. How often would we have to oil the timber shutters, and can you recommend what kind of oil will be appropriate for external subject to weathering and western sun?

With a harder timber you could try a high pressure hose to clean off the weathered wood, but this is likely to damage western red cedar as it is quite soft. The best method would be to sandpaper the shutters and then use a timber restorer such as Intergrain "Reviva". If this doesn't work, or takes too long, then replacement might be the only option. For a natural look, lightly pigmented stain will be the best choice, although paint gives the longest-lasting protection in exposed situations. A light exterior stain may need re-application every 18 months to 2 years.

Q. I am looking at building a deck around my pool. The deck will be within the pool splash zone as the edge of the deck will come up to the pool edge. What species of timber would be suitable for such an application or can normal treated pine or hyne t3 green be adequate

If the joists and bearers are clear of the ground we consider Hyne "T3 green" (ie. H3 treated) will be OK. If any of the timber is fixed to the edge of the pool, and water is able to seep into the interface between the timber and the side of the pool, that's not so good. In that case try to create an air space between the two surfaces so the timber can dry off after wetting. For the decking itself a Class 1 hardwood might be a better choice than treated pine, given the alternate wetting and drying it will be exposed to.

Q. Where can I buy cedar roof shingles for my gazebo please I live in Melbourne.

Tilling Timber handle cedar roof shingles in Melbourne. Their phone number is 9725 0222. You can download an installation guide from their website at http://www.tilling.com.au/sites/default/files/Shingles%20and%20Shakes%20Brochure.pdf.

Q. Hi Timber Expert, We have Western Red Cedar timber shutter sliding panels on upstairs balcony handrail facing west of our townhouse in West End, Brisbane. The size is about 2.5m long x 1m high. Unfortunately there was no maintenance on the timber shutter for the last 14 years apart from oil that was put on once a few years ago. Exposing to full weather and the Western sun the timber is deteriorating. The blade is hardly holding up and collapsed. It starts to change shape. It turns black and the surfaces become very rough and cracking. Hope you can advise me whether the western red cedar timber shutter could be restored, how should it be done and the maintenance required. Or can you think of any other alternatives?

It sounds as if it might be time for some new shutters. We weren't sure if the shutters were only 14 years old, or if that was just when maintenance stopped and they are actually older. However, once the blades collapse and warp, there's so much work required to rebuild the shutters that it's probably more economical to install new ones. If you install new shutters we suggest a preliminary flood coating with a water repellent such as Feast Watson Fungishield, Cabot's Bar D-K, etc. followed by paint or exterior wood stain. Western red cedar is a good choice for exterior joinery and should last longer than 14 years with normal maintenance.

Q. I have an open pergola supported between 2 brick walls with treated pine 190x45mm beams, spaced 0.9m apart. The beam span is 5.5m and there is a request to attach a hanging seat to this structure. If I allow 200Kg for 2 people and seat, that’s a considerable additional mass. The seat would be supported on 4 chains (50Kg per chain) across 2 beams (100Kg per beam) although notched rafters help distribute this load. Can I reinforce the existing beams with additional timber, perhaps a 90x45 flange glued and nailed to the top and bottom faces of the beam. That would improve the bending resistance of the beam. When I test the structure with my own body mass at mid-span, there is only a slight deflection in the structure. However, I’d rather strengthen these beams to be confident.

Our feeling is that if all the weight of the 2-person seat is to be carried on one 190 x 45 beam, spanning 5.5m, the beam would be struggling. There could be some dynamic load too, if people swing or bounce on the seat. However, it's a bit beyond our service to provide structural design advice since the answer requires an assessment of the load that the beam is already carrying, without the seat. We are also cautious about your idea of turning the beam into an I-beam by gluing and nailing flanges to the top and bottom. The theory is right, but such composite members are best fabricated in a workshop under controlled conditions. A water-resistant structural adhesive would be needed, such as resorcinol, and the timber would have to be machined to a fresh surface to achieve an optimum bond. Even then, treated pine can be tricky to glue since the preservative can interfere with the bond. Maybe the best option would be to install a new beam to support the seat.

Q. My wife and I are designing our new home and we are wanting to build a deck over a water tank that has a diameter of 7.35m and a sand pad of 9.35m. Being a complete novice I have no idea if this is structurally possible. Is it possible to build a deck to span this area? If so what wood would you recommend for the job?

It is structurally possible to span 7.35m (let's say 7.4m) but you would need very large timbers. For example, using laminated veneer lumber (LVL) you would need 525 x 75mm bearers at 1.8m centres. Joists running across the bearers can be 120 x 35mm F5 treated pine, although 120 x 45mm is more practical since there is more width for nailing the decking. Note that LVL is usually made from pine and if the deck is open to the weather the bearers would have to be preservative treated and installed with a capping of "Malthoid" or similar to keep rain out of the top edges.

Q. I wish to build a frame/wall under 1 metre from timber sleepers supported by star pickets. Some fill will be required, though it should be minimal. Are star pickets sufficient or must I use some other material? many thanks

Star pickets might not hold back the earth adequately. We suggest you refer to Timber Queensland's Technical Data Sheet no. 8 titled "Timber Garden Walls" for detailed recommendations. You can download this from the net by writing TQL 8 in your browser.

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In 2008 seventy percent of known old-growth are in nature conservation reserves.