High front guttering advisory committee report - NSW Fair Trading ...

While overflow measures are critically important to the performance of high fronted guttering, to
some extent there has been a lack of fit on this issue between the BCA performance requirements
and the Deemed to Satisfy options such as in AS3500.3/3500.5 and the “Acceptable Construction
Practice”. The past reference in the “Acceptable Construction Practice” to the use of slotted gutters
is a good example of where this misfit has led inadvertently to the conclusion on the part of some
manufacturers and installers that slotted gutters in themselves provide overflow measures that
meet BCA performance requirements (slots should have been dealt with in a more measurable way
especially in proving the degree of contribution they may have to meeting performance
requirements).
In general, the Deemed to Satisfy options focus on designing guttering and downpipes for a 20 year
ARI but are vague in objectively dealing with overflow – albeit that this is critically important in the
case of high fronted guttering so as to prevent water entry into the building based on the 100 year
ARI. There is also a degree of concern by the Committee of a lack of understanding and adherence
in the industry concerning the use of Deemed to Satisfy solutions and the calculation methods
involved in specifying the gutter size and number of downpipes to suit the roof area for a given 20
year ARI. Though evidence that is able to be generalised here is limited, there is concern primarily
about a lack of downpipes to adequately remove water from gutters. Again, high fronted guttering
is more sensitive to the effects of such a problem due to the ramifications that this presents for
overflow entry into a building. To an unknown extent, informal overflow mechanisms detailed in the
report (that are not normally counted in overflow management) may assist in mitigating the chances
of such overflow.
Key recommendations arising from the review include:
Overflow
1. That consideration be given by the Australian Building Codes Board to ensuring that all
Building Solutions (especially Deemed to Satisfy solutions) that use high fronted guttering be
designed to have the capacity to accommodate the building’s location 100 year ARI.
2. That overflow requirements be stated in quantifiable and measurable terms in Deemed
to Satisfy solutions, to prove that performance of one or more options will meet the 100
year ARI.
3. In conjunction with the previous recommendation, that the Australian Building Codes
Board (including input from industry, government and consumers) give consideration to and
debate different means of meeting the above overflow requirements – such as increasing
the design capacity of high fronted guttering from the 20 year ARI to the 100 year ARI. This
should include debate and consideration of the ramifications of continuous versus location
specific overflow.
4. That, recognising the responsibility for the correct installation of guttering and
downpipes under the Environment Planning and Assessment Act and the Home Building Act
1989 lies with building practitioners and licensed contractors, to assist, manufacturers
should be required to test and publish design information including cross sectional gutter
areas and
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