The Fire-Rated Wall Between Old and New
A building surveyor is assessing a permit application for a second dwelling built behind an existing house on the same lot. The new dwelling will be 1.2 m from the existing house at its closest point, and the designer's plans show no fire-resisting construction on the facing walls, relying instead on the distance. The existing house has timber cladding and a window facing the new dwelling. The new dwelling is proposed in lightweight construction with a large window facing the existing house. The surveyor notes the plans do not show the external wall fire performance, and the designer says 'the 1.2 m setback deals with it'. The site is in a designated bushfire-prone area, and the surveyor is also asked to confirm whether the new dwelling needs a bushfire attack level assessment.
1. Is the 1.2 m setback alone sufficient to deal with fire spread between the two dwellings? Explain the reasoning.
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Not necessarily, and the surveyor should not accept the designer's assertion without the relevant assessment. Separation distance is one factor in limiting fire spread between buildings, but the performance requirements also depend on the fire-resisting level of the facing walls, the amount and type of glazing, the combustible cladding, and the exposure. A 1.2 m gap with timber cladding and unprotected windows on both sides is a credible radiant-heat and flame-impingement path, and the large window in the new dwelling is a weak point.
The surveyor should require the designer to demonstrate compliance with the applicable performance requirements, either by the deemed-to-satisfy provisions (which may require fire-resisting construction or protected glazing at that separation) or by a performance solution supported by an appropriate fire engineering assessment. The surveyor should not approve the plans as submitted, and should record the request for further information and the designer's response.
2. What must the surveyor do about the bushfire-prone area question before determining the application?
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The surveyor must confirm whether the site is in a designated bushfire-prone area and, if so, require a bushfire attack level assessment and design response appropriate to the relevant construction standard. This is not optional paperwork: the applicable bushfire construction requirements affect the external walls, glazing, roof, vents and decking, and the designer's plans as described do not address them.
The surveyor should request the BAL assessment from a suitably qualified person, require the design to be updated to reflect the required construction, and only then determine the application. If the assessment has not been provided, the application is incomplete for determination. The surveyor should record the request and the date, because an approval issued without the bushfire response would be a significant error and would expose the surveyor to regulatory scrutiny.
3. The designer proposes a performance solution for the fire separation. What should the surveyor require before accepting it?
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The surveyor should require a documented performance solution that identifies the performance requirements being addressed, the acceptance criteria, the fire scenarios considered, the assumptions about fuel load and cladding, and the method of analysis. It should be prepared or endorsed by a suitably qualified fire engineer, and it should address both directions of exposure - the new dwelling affecting the existing house and vice versa - including the unprotected window and the timber cladding.
The surveyor should check that the solution does not rely on unstated assumptions (for example, that the existing house will be re-clad) and that any conditions of approval are capable of being inspected and enforced. The surveyor should record the basis of acceptance in the permit conditions and ensure the mandatory inspections cover the fire-resisting construction and glazing as installed. Accepting a one-line assurance from the designer would not meet the surveyor's obligation to be satisfied on reasonable grounds.