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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. Could you please advise whether you know if there is a hardwood timber decking solution that complies with a BAL40 rating or whether there is an approved treatment to hardwood timber to get it to this level?

Bushfire Attack Level 40 applies to areas of very high risk and consequently is quite restrictive. Decking must be non-combustible (which eliminates timber of any kind), or be part of a system complying with Australian Standard 1530.8.1, "Tests on elements of construction for buildings exposed to simulated bushfire attack". It is unlikely that a timber system would pass the test in AS 1530.8.1, and we don't know of any decking that has been tested.

Q. Can you please advise whether Northern Box (commonly called Pelawan) has been tested as a bushfire resistant timber (BRT) like the other 7 species listed on page 8 of your publication "Building with Timber in Bushfire Prone Areas".

Sixteen different species of timber were tested in accordance with the test method specified in AS 3959 to assess whether their fire properties were equivalent to those of fire-retardant treated timber. Seven species complied, as listed in the manual, five complied with a suggested alternative criterion (which was not adopted), and four failed. Pelawan was not included in the tests and therefore is not classed as a "bushfire-resisting timber", simply because its properties are not known. However, for some applications in the lower BAL categories, timbers can be approved on the basis of density. So depending on the BAL in your area, and the particular use (decking, window frames, etc) it might be possible to use pelawan.

Q. Where can I get jarrah span tables?

Before sourcing span tables you need to ask your supplier what stress grade of jarrah you are dealing with. You can then refer to the relevant span tables published as supplements to Australian Standard 1684, available from SAI Global. There is a different supplement for each stress grade. Alternatively various software programs are available on the net to calculate beam sizes, and you can find the major ones on the Wood Solutions website via this link http://www.woodsolutions.com.au/Resources/span-tables-software-resources.

Q. We have specified a mix of Vic Hardwood for wall panelling on a project. The Building Surveyor requires this cladding to be Group 1 classification because it is in a path of egress. The timber cladding will not comply with the group 1 rating unless it has a fire retardant applied. Is there a suitable product in the market place that we are able to use to comply with the BCA that has a clear finish?

Prudential Coatings have a clear fire retardant product for interior timber, and you can find out more via this link - http://www.prudentialcoatings.com/products/fire. We make no warranty about such products and suggest you seek an assurance that the necessary tests have been carried out to establish its credentials. You will find other locally available products if you write the words "clear fire retardant for wood" in your browser.

Q. The website http://www.woodsolutions.com.au/Wood-Species/mountain-ash gives the modulus of rupture as 110 Mpa. Can you explain the difference between this value and the values quoted in AS 1720.1?

The difference is that the MOR value of 110 MPa is the species mean, which is perhaps not clear from the Wood Solutions website. In fact it corresponds to the value given in CSIRO Division of Forest Products Technological Paper No. 25 "Mechanical Properties of 174 Australian Timbers", where a value of 16,000 psi is quoted as the mean MOR for mountain ash at 12% moisture content. In the previous system we would have reduced this to a 5 percentile value (ie. a value which 95% of the population wouild be expected to exceed) and then applied various factors to arrive at a "working stress". Clearly the mean value is not suitable for design purposes since 50% of mountain ash would be expected to fall below that figure. In the current limit states design format, "characteristic values" are quoted, which are an estimate of the lower 5th-percentile value, but have no other factors applied.

Q. I am thinking of pulling down a wall could u tell me the size of what kd beam I could use? It's a 5.5 meter opening it's holding up a ceiling.

The size of beam you need depends on whether the wall is supporting any roof load, or only ceiling load. For example there may be roof struts landing on the wall you are planning to remove, or perhaps there are roof trusses that land on the wall, with the ceiling fixed underneath the trusses. Even without any roof load, the beam size will depend on how wide the ceiling is. So we can't give you a simple answer. Maybe it's a question for a consulting engineer in your area, who can take all these factors into account.

Q. I'm about to embark on a small project at home and have been trying to find the MPa equivalents of F grades and Strength grades. Can you point me somewhere to find them?

The characteristic design stresses associated with various F-grades are set out in Table H2.1 of Australian Standard 1720 Part 1. This is explained in more detail via this link: http://www.woodsolutions.com.au/Resources/Grades. An example is given of the properties of F8 grade timber. "Characteristic stresses" are those stresses that can safely be sustained by a particular grade of timber at a specified moisture content under briefly applied loads. Various modification factors are then applied for specific service conditions, longer term loading and other moisture levels.

Q. I'm currently working in the pest control industry and really stuck on one of my module questions. the question is What does the australian standard say about the termite colony when you are dealing with active termites in a building and are going to apply a chemical application for termite management?? Could you possibly help me with the question or point me in the right direction?

The relevant Australian Standard would seem to be AS 3660 "Termite management, Part 2: In and around existing buildings and structures - Guidelines". Section 5 of AS 3660.2 is titled "Treatment of Infestation". The Standard provides for eradication of the colony by chemical means "where the nest can be located but not accessed sufficiently for physical removal or destruction". The chemical means specified are (a) dust termiticide, (b) liquid termiticide, or (c) other effective agents tested in accordance with AS 3660.3.

Q. Can you please help us in relation to the Ignitability Index of timber? For example VicRoads has specified in their specification Ignitability Index of timber used in Acoustic wall shall be less than 5. Is there any guide for Ignitability Index for different timber? We thinking of using H5 treated pine; what will be Ignitability Index of that?

Pine, preservative-treated or untreated, has an Ignitability Index of around 14 and therefore would not meet the VicRoads specification if an Ignitability Index in the range 0-4 is required. It might be possible to have the timber impregnated with a fire retardant that meets that criterion, or apply a fire retardant coating, but it would need to be certified for compliance with the relevant Australian Standard, and for suitability under weather exposure. We are not aware of such products or processes, but you may find one by searching the net.

Q. We have original American Sugar Pine vertical boards uncovered on an 1850s cottage and need to make them comply with FRLs to the side boundaries. Is there a paint/chemical or some system to assist in fire retardation?

You don't mention what the FRL is that you are required to meet. However, an intumescent paint might suffice. Cap Coatings claim to achieve an FRL of 60/60/60 on timber, although it's not clear whether this applies to external timber. You can visit their website at http://www.capcoatings.com.au/case-studies/?cid=17 for more information.

WoodSolutions

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About 16% of Australia’s 147 million hectares of native forests are in nature conservation reserves.