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Common problems with roof construction in Perth - issues with tie-downs

Tie-downs are systems of heavy-duty straps and anchors designed to stabilise buildings and homes during high winds. Even at low wind speed classifications such as non-cyclonic N2, sheet roofed houses will have net wind uplift forces that need to be tied-down with specific tie-down connections. As site wind classifications increase all roof types require specific tie-down. Wind uplift forces increase in proportion to the square of the velocity of the wind so, for example, a site with a wind velocity of 61m/s (C2) will have 50% higher uplift forces than a 50m/s (C1) site. Getting every required tie-down connection right is critical as failure of one link in the tiedown chain can lead to loss of a whole roof in a wind event. [1]

The design gust wind speed and corresponding wind classification is determined by a suitably qualified structural engineer. It shall include consideration of terrain category building height and topographic and shielding effects. When considering roof construction, the recommended procedure for designing the structural timber framework is to firstly determine the preliminary location and extent of bracing and tie-down and then the basic frame layout in relation to the floor plan and the proposed method of frame construction. Individual member sizes are determined by selecting the roof framing timbers and then systematically working through the remainder of the framework to the footings, or by considering the floor framing through to the roof framing. Bracing and tie-down requirements should also be considered when determining the basic frame layout to ensure any necessary or additional framing members are correctly positioned. [2]

The Building Commission of Western Australia published the findings of a report which aimed to identify if the standards pertaining to tie-down are implemented correctly in WA. The Building Commission carried out a general inspection of 123 sheet metal clad timber frame roofed dwellings in WA’s Perth metropolitan and South West coastal regions. The results for satisfactory construction varied across several inspection points from very low (11%) in relation to corrosion protection to moderate (63%) for battens properly tied down within 1200 mm of the edge of the roof. Overall, when the results from the 12 inspection points were averaged, this achieved an overall satisfactory rate of 33%. [3]The findings of this report may cause homeowners to question the construction of their metal roofs but the report notes that a failure to comply with a building standard does not necessarily mean the roof will fail to perform. If homeowners are concerned about a particular roof they should raise their concerns directly with their builder in the first instance. The Building Commission provides a dispute resolution service to assist parties if a satisfactory resolution cannot be achieved.

A chartered structural engineer specialising in residential buildings or a registered builder will have an understanding of the tie-down requirements prescribed in AS1684.2. If you are planning to purchase a home, particularity in Perth, it is critical that you obtain expert advice via a pre purchase building inspection report. If you have any questions, get in touch with us to discuss your requirements and to obtain a free quote. Blog image courtesy of Whitsunday Times .

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[1] - Technical data sheet 32 - Timber Queensland

[2] - AS1684.2-2010 - Residential timber-framed construction-Non-cyclonic areas - Standards Australia

[3] Building Commission of Western Australia - FINAL REPORT General Inspection Report One: A general inspection into metal roof construction in Western Australia (dated April 2016).

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