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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. Is it possible to build a timber deck in a BAL-40 area?

BAL-40 areas are considered to be very high risk where there is some likelihood of exposure to flames from the fire front. Consequently AS 3959 restricts decking to non-combustible materials or materials which comply with AS 1530.8.1, titled "Tests on Elements of Construction for Buildings Exposed to Simulated Bushfire Attack". As far as we are aware no timber decking has shown compliance with AS 1530.8.1, but ModWood Flame Shield, a composite material, has been tested and approved.

Q. I am building a collar tie roof 12 metres long and 5.3 metres wide. There are centre supports at each end but not in the middle as it is just one room. There are 4.2 metre collar ties at 2.7 metres from floor which are also the ceiling joists. Should I allow for the ridge beam to be say 20mm high in the centre to allow for any sag over time, or should I build it exact?

If we've understood it correctly the ridge beam will be 12m long, supported only at each end. This is a very long span and presumably the ridge will be some kind of engineered timber (LVL or glulam), since ordinary timber would be difficult to obtain in that length. It will also be a large cross-section size. It then becomes a matter of design. Glulam is usually made with a pre-camber so the beam is designed to straighten under load. LVL can't be made with a camber, but span recommendations are generally based on limiting deflection to span/300. For such a long span this could be up to 40mm.

Q. I am looking for a bushfire resisting timber compliant with BAL 29 the structural grade minimum is MGP10. Can you recommend a structurally compliant timber to be used for rafter and beam framing in an exposed verandah situation.

If the verandah roof is separated from the main roof space by a compliant external wall, as shown in Australian Standard 3959-2009, and the verandah rafters are lined on the underside with 6mm fibre-cement sheeting, any timber can be used. If the verandah rafters are exposed a bushfire-resisting timber is required. Only seven species of timber are deemed to be bushfire-resisting without treatment. They are: silvertop ash, blackbutt, river red gum, spotted gum, red ironbark, kwila (merbau) and turpentine. All are high strength hardwoods and should be readily available in F11 stress grade, which equals or exceeds the properties of MGP10. Other species of timber qualify as bushfire-resistant if a fire-retardant coating is applied, in which case reliable test certificates should be sourced from the supplier.

Q. I have been asked to give a 60 min fire rating to exposed timber joists on a mezzanine floor. How can this be achieved?

A method for calculating the fire resistance of structural timber members is set out in Australian Standard 1720 Part 4. It relies on the charring rate of timber. However, we think it’s unlikely an unprotected timber beam would retain its structural capacity for 60 mins if exposed to fire on three sides unless it was significantly oversized, so you are probably going to have to specify a fire retardant coating. There are plenty of paints available that give a 60 min rating but not so many clear coatings. These people claim to have one: http://www.firedefender.com.au/Timber_Fire_Retardant. But we know nothing about the product – you will have to ask for relevant test certificates.

Q. I would like to request clarification regarding your design guide #02 - Timber-framed Construction for Multi-residential Buildings Class 2, 3 & 9c. We are currently working on a building where either your solution described in figure 48b or 49 would be suitable, however I understand that this solution has an FRL of 60 minutes only. As our wall is structural and Type A construction, we require an frl to this wall of 90/90/90. I can see that Knauf prescribe a similar system which provides another sacrificial member in order to achieve a 90/90/90 rating (attached). I cannot, however find a similar solution in your documentation. I was hoping that you could please clarify this for me

Our Guide #02 is not necessarily the only source of construction details that satisfy the BCA. It is correct that the details in our Guide are primarily for FRL's of 60/60/60, although several cater for FRL's of 90/90/90. However, there is no reason why test data published by Knauf and other plasterboard manufacturers cannot be relied upon, since it is based on testing by independent fire engineers in accordance with the relevant Australian Standard.

Q. I note that skew nailing has a certain capacity in pullout given in AS1720.1 2010 fig 4.4(c) etc but I am after some advice on its capacity in shear when it is used to secure a lintel into a stud.

As you say, AS 1720.1 specifies capacities for skew nails in withdrawal but not in laterally loaded situations. Presumably the authors of AS 1720.1 did not envisage a situation where skew-nailed joints would be subjected to shear. Looking at the diagrams in AS 1684 which show recommended installation of lintels, all support the lintel in some way, either by housing it into the jamb stud or by providing end-bearing on a secondary jamb stud. In these situations the lintel would be secured by nailing through the stud into the end-grain of the lintel with straight-driven nails, so the shear capacity of skew nails would not be an issue.

Q. Do you have any data on the use of 45mm LVL's being used as sacrificial timber in Class 1a or Class 2?

The use of timber blocks as sacrificial fire stops, as described in our Technical Guide #6 on the Wood Solutions website, allows any timber, including LVL and glulam, as long as it has a minimum density of 470 kg/m³. So most commonly used timbers would be suitable with the exception of low density species such as western red cedar.

Q. Question is in relation to existing part built structure, council consents long run out, lot of recycled timber used structurally with no visible stress grading and other Australian standards issues. Two things: What would need to happen for a new application for Building and Planning consent for this part-built structure? I believe that any building work requires either to conform with Australian standards or to have engineering calculations done for the specific application eg nonstandard materials, but I don't know where I would find that specified. Can you advise?

Recycled timber can be used structurally but, like new timber, needs to be graded. Identification of stress grade would usually be required by local council, but perhaps this was not the case when the original consent was granted. Or perhaps the timber was graded but not marked, in which case you might find some documentation that was lodged at the time of the original application. If not, before lodging a new application we suggest you check the timber to determine its grade. This can be done by reference to special grading rules which can be downloaded, along with span tables for recycled timber, from this location: http://www.woodsolutions.com.au/Articles/Resources/Interim-standard-Recycled-structural-products. If you don't feel able to do this yourself, a consulting engineer familiar with timber should be able to advise.

Q. In 2010 I was one of eight Australian university students sponsored by Forest & Wood Products Australia to attend the World Conference in Timber Engineering in Italy. It was here at the conference where I learnt a lot about timber engineering, in particular Cross Laminated Timber or CLT. I saw what the world was doing with timber and wondered “why aren’t we doing that in Australia?” I still haven’t found a decent answer as yet apart from industry's reluctance to take up new ideas.

CLT has been a little slow to catch on in Australia and, as you say, the construction industry takes a while to adopt new techniques. Perhaps there have also been some regulatory barriers, but CLT is now taking off with the Forte apartments completed by Lend Lease in Melbourne and claimed to be the tallest timber apartment building in the world - there's a lot of info on the net about it. Subsequently, CLT was used for the Docklands Library. To help to encourage designers to think about CLT we have produced a Design Guide titled "Massive Timber Construction Systems" which can be downloaded by pasting this link in your browser: https://www.woodsolutions.com.au/fwpa/article_downloads/Design_Guide_16_CLT.pdf.

Q. The project is in relation to a single storey aged care facility (Class 9c). Timber frame wall and timber roof truss system. We are looking at timber framed fire wall system that achieves 1.5hr FRL which extends to the top of the roof. A system similar to Figure 45 from Class 2, 3 & 9c manual would be great however according to the manual, it achieves an FRL of 1hr, not 1.5hr.

Achieving an FRL of 90/90/90 is not difficult for the dividing wall - it's just a matter of applying a double layer of 13mm Firestop plasterboard or similar each side of the timber wall frame. This is shown in Boral's Multiframe Manual (ref T2626F) which can be downloaded via this link: http://www.infolink.com.au/ODIN/PDF/Showcases/27484.pdf. However, the area that will need attention is the sacrificial timber blocking. For a 60 min FRL, 45 mm blocking is OK. For a 90 min FRL, we suggest 2 x 45 mm solid timber blocking each side of the wall, assuming it's a non-fire rated ceiling - somewhat analogous to Fig 47 in our Class 2, 3 & 9c manual, but applied to a single leaf wall.

WoodSolutions

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