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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. I would appreciate advice on any regulation covering the proportion of a house frame bottom plate that should bear on supporting floor slab/floor frame. AS1684 doesn't seem to cover this.

The point you raise is not covered in AS 1684, but is dealt with in the Guide to Standards & Tolerances, produced by the Victorian Building Commission in collaboration with NSW Fair Trading, the Tasmanian Government and the ACT Government. You can download the guide via this link: http://www.buildingcommission.com.au/__data/assets/pdf_file/0009/6759/S_T_GUIDE_07.pdf - refer page 31.

Q. I would like details of the group number, average specific extinction area, spread of flame index, smoke growth rate index and smoke development rate, critical radiant flux for American White Oak tongue and groove boarding (18mm). Is this information available?

We don't have all the details you ask for, but we can say that American white oak (Quercus alba) falls into Material Group 3 and has an average specific extinction area of less than 250 m²/kg when tested to Australian Standards. We are not aware of a source of the other data relevant to American white oak, although an American reference states the following with respect to critical radiant flux: "Data in the literature indicate that oak flooring has a critical radiant flux of 3.5 kW/m²".

Q. We are in a BAL 40 zone and would like to know if there is any wood that can be used for the vertical (supporting) posts of a new pergola. thank you!

The Australian Standard that specifies the construction of buildings in bushfire-prone areas (AS 3959-2009) does not specifically mention pergolas and similar unroofed structures. Consequently any type of timber can be used. However, note that if the pergola is to have a roof and is also attached or adjacent (closer than 6 m) to the house it would be classed as an "awning roof" and then comes within the scope of AS 3959.

Q. Thank you for your quick reply. In the event that the beam has attached lining boards to it minimal weight Then what should the beam be and can you send a table to support the size you suggest for the opening.

As a quick "safety check" (without knowing exactly your situation) we referred to Table 51A in Supplement no. 8 to Australian Standard 1684.2, "Residential timber-framed construction". Supplement no. 8 deals with F17 grade seasoned hardwood, and Table 51A is for verandah beams which seemed a resonable approximation. According to Table 51A, a size of 240 x 45 can actually span 6.0m, supporting a roof load width of 1.2m (ie. a verandah with rafters 2.4m long), with a roof mass of 10 kg/m². We therefore feel confident that your beam will give satisfactory service. Unfortunately we can't send you a copy of the table as it is a copyright document.

Q. Please contact me regarding Timber framed construction for Townhouse Buildings (Class 1a).

We don't offer a telephone service and only give advice on line,  but if you would like to leave a more detailed message we will see if we can help. If you feel you need a personal discussion we understand the Timber Merchants Association in Melbourne operates a help line on (03) 9875 5000. You might also find it useful to download a copy of our technical guide on Class 1a buildings. It's guide #1, available via this link: http://www.woodsolutions.com.au/Resources/Design-Construction-Guides.

Q. I wish to enquire about the properties of Cypress Pine. Specifically the timbers density and moisture content to determine its use in a bushfire prone area. And which BAL rating (12.5, 19, 29, FZ) zone it is suitable. I am enquiring for Troppo Architects from South Australia, therefore the timbers relevance to our state regulations in the Building Code of Australia would be most useful to us. We particulary need this information to provide to a independent certifier for one of our residential houses in a bushfire area. Thankyou for your help.

Cypress pine has a "design density" of 700 kg/m³ at 12% moisture content (dry), rounded to the nearest 50 - reference AS 1720.2-2006. BAL 12.5 is a low risk area and cypress would be suitable for most uses. An exception is decking less than 300mm from a glazed element that is less than 400mm vertically from the surface of the deck. Denser timber or bushfire resisting timber is required in this situation. As the BAL rating increases, so the regulations become more restrictive. Details of the requirements for each BAL can be found in AS 3959 or in Technical Design Guide #04 available via this link: http://www.woodsolutions.com.au/Resources/Design-Construction-Guides.

Q. What is the Ignitability of Red and Grey Ironbark?

There are various definitions of "ignitability". In plain English it simply means "capable of burning", ie. combustible. In that sense, all wood is ignitable. However, some timbers ignite more easily than others, and that is a function of their density. The ironbarks are dense timbers and consequently are relatively difficult to ignite. Red ironbark has been tested in accordance with AS 3959 and is deemed to be a bushfire resisting timber, equivalent to timber treated with a fire retardant. Red and grey ironbark have also been tested in accordance with AS/NZS 3837 to assess their suitability as wall and ceiling linings and found to fall into Material Group 3 with an average extinction area less than 250m²/kg.

Q. About using plantation pine framing in bushfire zones. Is this material suitable for use in that situation. In my design the frame will be clad and lined with non combustible materials - steel and magnesium board respectively. Ember excluding flashings will be used. If so what zones would it be suitable for? I don't want to use fire retardant.

The bushfire regulations aim to prevent fire entering a building by providing protection against ember attack, radiant heat and/or flame contact, depending on the severity of exposure, which is assessed according to the Bushfire Attack Level (BAL). There is no limitation on internal materials, including the structural frame - the whole objective is to keep fire out. So pine framing is suitable for any area deemed to be a "bushfire-prone area" and fire retardant treatment is not required.

Q. I am assessing a multi-frame job which has a fire rated timber wall which changes to non-fire rated mid point. There is no junction and the builder does not what to fully box in the fire rated wall. Are there any tested details using char blocks for this use?

You might find it helpful to refer to Technical Design Guide #6 "Timber-framed Construction - sacrificial timber construction joint" - this provides common details for using sacrificial timber blocks to maintain a Fire Resistance Level. The Guide is part of a series available from the Wood Solutions website via this link: http://www.woodsolutions.com.au/Resources/Design-Construction-Guides.

Q. I am try to find out the Radiant Flux rating for 9mm Plywood. I'm having trouble finding it online, does it exist?

As far as we are aware plywoods less than 15mm thick and solid timbers less than 12mm thick have not been tested. However, the test results for plywoods 15mm or greater and timbers 12mm or greater can be found via this link: http://www.timber.net.au/images/downloads/fire/fire_test_SFR41117_2.pdf.

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