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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'm trying to get to the bracing area for N3 rating. I need to know what tie downs and bracing is required for timber frames for my extension.

The reference for tie-downs and bracing is Australian Standard 1684. It's divided into several parts and you will need Part 2 if you are in a non-cyclonic area, or Part 4 which is based on a simplified format. If you don't have access to these publications, or are not familiar with how to use them, it might be best to have a consultant prepare your documentation. Simplified tie-down details are available on the net at this address: http://www.woodsolutions.com.au/Articles/Resources/AS1684-code-compliance#Download. However, you will still need to work out your wall bracing requirements.

Q. I am a structural engineer in Perth WA. We are designing a timber deck over the Swan River in East Fremantle. We have specified F14 unseasoned hardwood for the deck planks and joists. The deck planks are to be fixed to the joists using Buildex 14-10 x 75 Bugle Batten SS Type 17 screw. Deck planks are butt jointed at joints over joists and ends of planks are double screwed to the joists. The joist width we have specified is 130mm wide which is governed by the min. spacings for screws as specified in Table 4.8 of AS1720.1-2010. The project is highly cost driven and due to the total length of joists involved, the architect and contractors pricing the project have queried if the joist width can be reduced to 100mm. This would result in the edge/end distances for fixings to be reduced also. My question is can the min. distance requirements of Table 4.8 be reduced in particular circumstances and in this instance, where fixings for deck planks are subject to minimal loads (being uplift from wind or lateral due to foot traffic), and are primarily for securing the planks in position.

As you say, the fixings for deck planks are primarily to secure them in position and we don't see any problem in reducing the joist width to 100mm. However, we do recommend pre-drilling to avoid splitting the joists and planks when the fasteners are inserted.

Q. Can I treat my Callitris Cypress Gold posts to conform with BAL 29 fire rating. Preferably a clear lacquer to preserve the beauty of the timber. I do not want to paint them. Is there a product?

Our answer depends on the function of the posts. If they are supporting an unroofed pergola there are no requirements and any type of timber can be used. If they are verandah posts, the Australian Standard currently makes no direct reference to posts supporting a verandah roof, so technically any timber can be used. However, we recommend a bushfire-resistant timber for BAL-29 areas. Cypress pine is not classed as a bushfire-resistant timber. Brush-on fire retardants are available in the marketplace but may not comply with the weathering requirements of the Australian Standard.

Q. We are currently working on a refurbishment project of a school in London and would like to fix a full height double door into an existing beam using timber. We currently have 125mm high space above the door structural opening and would like to fill it with a fire rated (30 min) softwood packer (94mm wide and 2.4m long). Can you please advise regarding the type and size of timber in relation to the required fire rating.

It seems that the packing piece will not be carrying any structural load, so you simply need sufficient material to withstand 30 minutes of charring before being totally consumed. Since you are designing in the UK it would be best to refer to relevant British Standards. There is a useful explanation of fire resistant design in accordance with British Standards and Eurocodes at this location: http://www.mace.manchester.ac.uk/project/research/structures/strucfire/materialInFire/Timber/Charring/standardFires.htm. Reference is made to BS 5268-4-1 (1978) which quotes a generalised charring rate (for timbers other than western red cedar and dense hardwoods) of 0.667 mm/min. Assuming fire exposure from only one side is contemplated, this implies a minimum thickness of 20mm. We suggest you treat this conservatively and add a small margin for safety.

Q. Can you please give me the details of Fire Ratings on American White Ash Timber. I have had a look at the website but I can not tell the rating? Not sure if I am reading it correctly.

A number of local and imported timbers have been tested for their fire properties, but American white ash has not been included as far as we are aware. Assuming you are using it for internal wall or ceiling lining, it's likely to fall into Material Group 3 with an average extinction area of <250 m²/kg, since timbers of widely varying density have these characteristics. However, we can't definitively say this since it hasn't been tested.

Q. I'm currently documenting a residence in a BAL 29 bushfire zone. Page 20 Building with Timber in Bushfire-prone Areas states that ember screening is required to opening parts of windows closer than 400mm from external horizontal surfaces whereas AS3959 Clause 7.5.2(b)(iv) requires that the fixed portion of the window within this zone be screened.

After discussing this with the consultant who prepared our Guide #4, it does appear that we are in error and we will therefore amend the Guide to bring it into line with AS 3959. Meanwhile, AS 3959 takes precedence, of course, and should be followed. Thank you for pointing this out. We are confident that Guide #4 is accurate in other repsects.

Q. We are documenting a 4 storey multi-unit apartment building (3 storeys above carpark). The external walls are non-loadbearing and are constructed of timber stud clad in colorbond. We have a fire rated concrete slab between floors. We know that we definitely require a 900H fire spandrel between the floors where there are windows. I've been looking at your design guide for Multi-Res buildings Type 2,3 & 9c (pg 19, fig 11c) but can't see a requirement for non-loadbearing external walls. Are you able to advise if we require a fire spandrel between floors where there are not any windows, as the walls are not 60/60/60 fire rated.

The building will require Type A construction since it is 3 storeys above car parking. Table 3 of Specification C1.1, BCA vol. 1, states that non-loadbearing parts of external walls in Type A construction require an FRL of -/90/90 if less than 1.5 m from a fire source feature, -/60/60 if 1.5 to less than 3 m from a fire source feature, and -/-/- if 3 m or more from a fire source feature. A "fire source feature" is defined as (a) the far boundary of a road, river, lake or the like adjoining the allotment; or (b) a side or rear boundary of the allotment; or (c) an external wall of another building on the allotment which is not a Class 10 building.

Q. I am looking to construct a deck which measures 5.2m x 6.1m with pergola over it in a BAL 12.5 zone. I would like to span the deck with a 5.2m cypress pine beam, as a part of the roof structure, with 6.1m lintels in the support of the beams. What dimensions of beams would I need to use to comply with NSW DA regulations, BC and Australian Standards? I was looking at 240x45.

A design guide just released (June 2014) will be helpful in the design of your deck. It can be downloaded from the Wood Solutions website via this link: http://www.woodsolutions.com.au/Articles/Resources/Design-Construction-Guides. The Guide is number 21 in a series and is titled "Domestic Timber Deck Design". However, the guide does not contain span tables and refers to Australian Standard 1684 for this information. If you don't have access to AS 1684 you may wish to refer to a technical data sheet published by Timber Queensland titled "Residential Timber Decks". It can be found on the net by writing TQL 4 in your browser.

Q. Hi Team. Great Web site. I wish to strata two units. One upstairs one down but I need to separate the units with a fire rated barrier. Separation at the mo is a cement fibre board and hardwood flooring. Does this have a rating and if not what do you suggest?

Sounds as if your units would be Class 2 under the Building Code of Australia (BCA). If only two storeys, they require Type B construction and the separating floors would need a Fire Resistance Level (FRL) of 30/30/30, or have a "fire protective covering" on the underside of the floor. A "fire protective covering" is defined in the BCA vol. 1 as (a) 13mm fire-protective grade plasterboard, or (b) 12mm cellulose cement flat sheet complying with AS/NZS 2908.2 or ISO 8336, or (c) 12mm fibrous plaster reinforced with 13mm x 13mm x 0.7mm galvanised steel wire mesh located not more than 6mm from the exposed face, or (d) other material not less fire-protective than 13mm fire-protective grade plasterboard.

Q. Is there a design code for the sizing of timbers for an industrial application (wharfs etc)?

Timber Queensland have a Technical Data Sheet titled "Timber Decks, Commercial, Industrial & Marine" which might be what you are looking for. You can download a copy by writing "TQL 7" in your browser, or you may wish to subscribe to their technical service by phoning (07) 3254 1989.

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