Ask an Expert

Ask an Expert

Building codes & compliance

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. Please advise if Northern Box fire rating is suitable for use as decking in NSW.

Northern box (common name pelawan) is not one of the seven timbers classified as "bushfire-resistant" as defined by Australian Standard 3959. However, pelawan is a dense wood in the range 905 to 1010 kg/m³. Therefore it can be used for decking in areas rated up to and including BAL-19, where decking with a density of 750 kg/m³ or more is allowed.

Q. We regularly have builders who want to use timber floors in bathrooms. Are you aware of any way a timber floor can achieve a waterproof finish as required by the BCA and AS 3740?

We weren't sure whether you were inquiring about an exposed timber floor or one covered with a waterproof system. As far as we are aware it would be difficult to make an exposed timber floor comply with Australian building regulations. The BCA refers to AS 3740 which requires a "waterproof" floor. Certain materials are deemed to be waterproof, including "flexible sheet flooring material with sealed joints" and "membranes meeting the requirements of AS/NZS 4858". So in our opinion a bathroom floor could be constructed of particleboard, plywood, or tongue and groove timber if it was covered with sheet vinyl (for example). Alternatively, a waterproof membrane can be laid over a timber floor in accordance with AS 3740, and tiles laid on a mortar bed that is installed over the membrane. AS 3740 is a performance Standard and states that it "shall not be interpreted as preventing the use of materials, systems or methods that meet the design and installation criteria set out in the Standard, but are not specifically referred to herein". So theoretically an exposed timber floor could be used in a bathroom if someone could show it was as waterproof as flexible sheet flooring. We don't know of any cases where this has been done, or how it might be done - perhaps the entire floor could be epoxy-saturated, like a wooden boat, using the WEST system.

Q. As I don’t want to install bushfire shutters on all the fixed windows I thought I would use a bushfire resistant timber in the 750 kg/m³ density range of timbers. Could you suggest a timber, in that range, that you consider would be suitable for exposed and stained window frames and is reasonable priced. As far as its availability I would be purchasing the timber in Sydney.

AS 3959 only classes seven timbers as "bushfire-resisting", as required is some of the higher BAL areas. However, in BAL 12.5 your windows only need to have a density of 650 kg/m³ or greater and there's quite a long list of species that meet that requirement. For window joinery, suitable timbers on the list include jarrah, merbau and tallowwood. Whatever you choose must be kiln dried and if you are thinking of a stain finish we recommend installing the glass with timber glazing beads rather than putty. If putty is exposed to the weather with only a stain finish it's inclined to harden and crack in the long term.

Q. Thank you for producing (Building with Timber in Bushfire-prone Areas), it is very helpful. We have a BAL rating of 12.5 and want to make some external timber frame fixed windows. Could you suggest a suitable timber that is cost effective, attractive when stained and easy to machine. Also do fixed windows require metal screens?

Bushire Attack Level (BAL) 12.5 is rated low risk, where only ember attack is likely. Consequently, Australian Standard 3959-2009, "Construction of buildings in bushfire-prone areas" allows any type of timber to be used for window frames if they are protected externally by metal screens or bushfire shutters. However, only certain types of timber are allowed for the frames of the screens themselves. If you would prefer not to have screens on fixed windows, or bushfire shutters installed, the window frames have to be made from bushfire-resisting timber, or from timber with a density not less than 650 kg/m³. The same applies if the windows are less than 400 mm from the ground, or less than 400 mm above other flat surfaces such as external decks, but in this case the glass also has to be Grade A 4mm safety glass.

Q. I am a designer and want to know if cypress pine can be used in Victoria as stumps for a decking and verandah posts. I have specified this in the past before but recently a building surveyor has pulled me up on it and has said it is excluded from external use in As1684. is this the case?

We are not aware of anything in AS 1684 that would exclude cypress pine from being used for stumps supporting a deck, or for verandah posts. Perhaps you should ask for a specific clause reference. Appendix B of AS 1684.2 states that Durability Class 1 or 2 timbers are appropriate for stumps. Since cypress pine is Durability Class 2 in-ground it would satisfy the requirements of the Code in our opinion. Note, however, that the Code specifies that sapwood should be removed or treated, but the sapwood of cypress pine is difficult to treat with preservative.

Q. I am involved in designing an extension & alteration to existing timber cladding building. The new work is to be similiar as the existing external timber cladding. It is in BAL 12.5 area. Is there any timber cladding comply with the requirement in density & species rather than brick veneer wall as one of the options in BAL12.5 area?

Bushire Attack Level (BAL) 12.5 is rated low risk, where only ember attack is likely. Consequently, Australian Standard 3959-2009, "Construction of buildings in bushfire-prone areas", places no limitations on external wall surfaces 400 mm or more from the ground in BAL 12.5 areas. The part of an external wall surface that is less than 400 mm from the ground, or less than 400 mm above other flat surfaces such as external decks, must be non-combustible or constructed using a bushfire-resisting timber. You therefore have the option of using a fibre-cement material for the lowest 400 mm, with any kind of cladding above, or using one of the hardwood species rated as "bushfire-resisting". Another option is to build a brick or concrete upstand of 400 mm and again any kind of timber cladding above. This looks neat and has the advantage of preventing rain splashing from staining the bottom of the cladding.

Q. I am investigating using Batu (Balau) decking for a commercial deck. Are you aware if this product complies with BCA Part C1.10 (AS ISO 9239.1 Critical Radiant Flux and Smoke Development Rate) Do any natural timbers comply, or should I only use a plasticised timber?

We don't claim to be experts on the BCA but we were under the impression that Specification C1.10 only applied to lining materials inside buildings (floors walls and ceilings), not outdoor decks. Batu/balau is not normally used as indoor flooring and consequently hasn't been assessed to Specification C1.10. We can say it is relatively fire resistant on account of its density, but there is no test data to show compliance with C1.10. Regarding composite decking, some products have been tested to establish their early fire hazard properties, but we are unsure whether critical radiant flux values are available.

Q. I'm trying to source a natural timber, ie. not a veneer that meets Group 2 criteria as per BCA C1.10a. Is it possible to achieve Group 2 or are the only options veneered panels?

As far as we are aware all natural timbers fall into Material Group 3. We can only get veneered board into Group 2 by laying up the veneer onto fire retardant treated MDF, for example the product sold as "FlameBlock".

Q. How and where can I get the timber framing design tables for domestic and residential buildings so I can design roof rafters, floor joists etc. There used to be a framing design handbook. Has it been replaced, superseded by something else?

Assuming you are in a non-cyclonic area you will need a copy of Australian Standard 1684 Part 2, "Residential timber-framed construction". Span tables published as supplements to AS 1684.2 include sizes for rafters, floor joists, etc. Copies of AS 1684.2 can be purchased on line via this link - Note that there are four possible versions, a PDF download, a loose-leaf hard copy, ditto in a binder, or bound as a paperback.

Q. This may be obvious but thought I'd check, if a property has been rated BAL 40 and we want to use lightweight timber cladding can we build to the higher rating FZ using the Outrwall system?

Never hurts to ask! However, clause 3.4 of AS 3959 states that “Construction requirements specified for a particular Bushfire Attack Level (BAL) shall be acceptable for a lower level", so theoretically a building designed to BAL-FZ requirements could be built in any bushfire area, although clearly it would be an overkill for areas of lower risk.


Did you know?

Australia’s native forests, timber plantations and wood products are net absorbers of greenhouse gases, sequestering 56.5 million tonnes of carbon dioxide in 2005, reducing Australia’s total greenhouse gas emissions by nearly 10%.