Building codes & compliance

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Question: Hi Wanting to build a small plywood box beam industrial shed, approx ~16-20m x ~40m clear span, N3 wind zone (NSW). I am looking for engineer with plywood box-beam experience? 

Answer: The NSW Division of the Institution of Engineers Australia may be able to give you the names of some firms with experience in the design of plywood box beams. Failing that, the staff at Timberbuilt are highly experienced in all aspects of structural timber design. You can visit their website at www.timberbuilt.com.au.

 

Question: Hi, the DA Conditions for my house require compliance with AS3959-2009 level 12.5(BAL). Is there a way of chemically treating my bifold doors and front door (meranti) to make them compliant instead of changing the type of wood from which they are constructed?

Answer: AS3959-2009 allows the application of fire retardant coatings as a means of making timber "bushfire resistant", up to a Bushfire Attack Level (BAL) of 29. Fire retardant coatings have to meet the test criteria described in the Standard, so any product marketed for this purpose should come with a certificate from a recognised testing authority. The Standard also recognises timbers with a density greater than 650 kg/m³ as being suitable for window joinery and door frames in areas up to BAL 19. Meranti is divided into two groups, light red and dark red. Light red meranti is not dense enough, but dark red meranti would meet the density requirement, and therefore would meet the fire resistance rating, with no special treatment or coatings. This assumes you have a solid core front door. If you have a hollow-core front door, or your door is made from light red meranti, another way to comply is to fit a non-combustible kickplate on the outside for the first 400 mm above the threshold. We asume your bifold doors are glazed doors. If the glazing is more than 400 mm above the adjacent external surface, there are no special bushfire requirements for the glass. If the glazing is less than 400 mm from the ground, deck or other adjacent surface, the Standard calls for "Grade A safety glass minimum 4 mm".

 

Question: Building on bushfire-prone land, DA approval requires compliance with AS 3659-1999 re type of timber. Which of the permitted species are best for decking, deck joists, cladding and window awnings? Also, will 'Firetard 120' comply to treat cedar windows?

Answer: The Standard number is actually AS 3959 and it seems your building has been approved under the 1999 edition, rather than the latest (2009) edition. All the species considered equivalent to fire retardant treated timber under AS 3959-1999 fall into Durability Class 1 or 2 above ground, and therefore all are suitable for deck joists and bearers. For the decking we suggest kwila (merbau) or ironbark. For the cladding, it is a question of which species are available kiln-dried and in a suitable profile, since all have the required durability. Silvertop ash would perhaps be the least desirable for cladding, since it commonly contains numerous gum veins. Regarding the use of 'Firetard 120' as a means of making cedar windows compliant, we were not sure whether this was applied by pressure impregnation or as a brush-on process. The Standard requires fire retardant treatments to pass an accelerated weathering test unless the treated component is shielded from the weather. So if any of the windows are exposed to the weather it will be necessary to obtain a certificate from the manufacturer or supplier to show that the product (a) has the required fire retardant properties, and (b) passes the accelerated weathering test. If the product has not been tested, then the window joinery can be made from a suitable hardwood, eg. kwila.

 

Question: Hi I'm rebuilding at Kinglake, and have ordered windows. My building surveyor said that Jarrah windows are dense enough to be classed as bushfire resistant. I need to comply with BAL29 and notice that Jarrah is not one of the 7 approved woods. Is it ok?

Answer: Australian Standard 3959-2009, Construction of buildings in bushfire-prone areas, allows any type of timber to be used for windows in areas rated Bushfire Attack Level (BAL) 29, if bushfire shutters are fitted. If shutters are not fitted, timber window frames and window joinery must be made from "bushfire-resisting timber" as defined in Appendix F of the Standard. Timber is considered to be "bushfire-resisting" if the material itself passes the specified fire test, or if it is impregnated with fire-retardant chemicals, or if it is coated with a fire-retardant. Timber impregnated or coated with fire-retardant must pass a weathering test as well as the fire test. As you say, jarrah is not one of the seven types of timber that are deemed to be "bushfire-resisting" on the basis of their material properties. Jarrah is most unlikely to be able to be impregnated with fire-retardant chemicals because of its density. It might be possible to find a fire-retardant coating that has passed both the weathering test and the fire test specified in the Australian Standard, but failing that, in our opinion jarrah could not be classified as "bushfire-resistant" as defined in the Australian Standard. We suggest you discuss this further with your building surveyor.

 

Question: Does AS1684 specify that all roof and ceiling structures have to be built with H2 treated structural pine? The house specs are stating that the "Roof & Ceiling structure - Timber Frames to AS.1684 but we've been charged again in the site works page.

Answer: There is no requirement in AS 1684 for houses to be built with T2 treated pine. AS 1684 simply says "Protection against termites shall be provided in accordance with the provisions of the Building Code of Australia". There are various ways of protecting new houses against termites, and using treated framing is only one of them. Other systems include chemical sprays and/or physical barriers. In fact, even if treated framing is used we still recommend the installation of physical barriers since there are other items inside the home that termites can attack besides the wall and roof framing. Treated framing is essentially a second line of defence in case the physical barriers are breached. Having said that, we consider treated framing is a good investment considering the relatively small extra cost.

 

Question: I was wondering if you could provide me with any information surrounding fixing details for timber frame work in cyclonic areas in Western Australia.

Answer: The reference for timber framed construction in cyclonic areas is Australian Standard 1684, Residential timber-framed construction, Part 3: Cyclonic areas. You can obtain a copy of AS 1684.3 from Standards Australia.

 

Question: How do I determine if a pine timber I buy to manufacture lounges meets BCA standards for the spread of smoke and flame? What treatments to the timber are or can be made to ensure it complies? Can I get a certification certificate from the supplier?

Answer: As far as we are aware, the Building Code of Australia (BCA) only deals with structural elements, and finishes, surfaces, linings or attachments to buildings. Perhaps your lounges will be fixtures, in which case fixed seating in the audience area or auditorium of a Class 9b building used as a theatre or public hall is required to have a spread-of-flame index of 0 and a smoke-developed index of not more than 5. No species of timber achieves a spread-of-flame index of 0 unless treated with a fire retardant. You will need to seek out a treatment that achieves these levels and make sure it has been tested to the relevant Australian Standard. If you have been given different advice by a regulatory body, feel free to leave another message quoting the specific regulation that applies to your case.

 

Question: Referring to Australian National Timber Dev. Council's web site, we noted that Merbau is defined by AS-3959-2009 as bushfire-resisting timber. Could you please advise us is it classified as non-combustible material? Equivalent to BS 476?

Answer: Merbau is classified as a bushfire-resisting timber in Australia because it has fire characteristics similar to timber treated with a fire retardant. This doesn't mean it won't burn, just that the heat released when it does burn is within the limits specified in Australian Standard 3959-2009. British Standard 476 has a number of parts. The part dealing with non-combustibility is Part 4 which specifies a test to differentiate between non-flammable materials, such as plaster-based wall linings, and materials which undergo flaming combustion, such as wood products. It is most unlikely that any wood product would achieve classification as a non-combustible material as defined in BS 476: Part 4. Of course, "non-combustible" materials may still have undesirable fire qualities in that they may melt, shrink, collapse or change their nature in some other way. However, by definition they will not contribute to the fire load.

 

Question: Further to my previous question about western red cedar, if any timber makes it into group 3 in accordance with the BCA's Specification C1.10a, it doesn't matter what the shape of it is. Is that correct?

Answer: Our understanding is that the group number could be applied to a similar configuration, e.g. if 12mm western red cedar panelling is a Group 3 Material it would be reasonable to say that other flat panels of the same or greater thickness would also achieve Group 3 status. However, the material's group number is an indication of its time to "flashover", as predicted by the heat release rate when exposed to a standard heat source. Consequently if you were designing a screen made up of thin battens of western red cedar with spaces between, it would be likely to have a more rapid heat release rate than a 12mm thick flat panel. Other configurations are likely to have greater or lesser rates of heat release. So we can't just say that western red cedar is a Group 3 material in all cases.

 

Question: We have specified the use of western red cedar in an office building to be used as either wall and/or ceiling cladding. Our BCA consultant requires confirmation that the western red cedar is a group 3 product as part of the BCA 1.10 clause.

Answer: The Australian Timber Database at www.timber.net.au advises that western red cedar is a Group 3 material in accordance with the BCA's Specification C1.10a. The Database is maintained by the Timber Development Association of NSW. For further information contact TDA on (02) 8424 3700.

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