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The National Construction Code (NCC) provides Deemed-to-Satisfy Solutions for collecting and disposing of stormwater to meet the surface water Performance Requirement P2.2.1.
The 2016 version of the NCC included overflow provisions in Part 3.5.2 of Volume Two, and has provided a number of options for how rainfall from a one-in-100-year event can be prevented from entering into a building.
Overflow volumes are based on the rainfall intensities expected in the location of the home and the area of catchment, and assumes that all of the downpipes are blocked.
Overflow measures are not required to be provided where eaves gutters are fixed to a veranda or eave greater than 450mm wide, where they are not lined or are raked with a lining sloping away from the building. It is assumed in these cases that water will not be able to flow back into the building.
The Deemed-to-Satisfy Solutions in Part 3.5.2, provide both continuous and dedicated measures for allowing for overflow.
Continuous measures allow water to escape from the gutter along its length and dedicated measures allow flow of a certain volume away from the building in discrete locations.
Different measures can be used in combination to provide for the total overflow volume required for a roof area. It is also possible to use options not provided in the BCA provided their suitability is supported in a Performance Solution.
The Deemed-to-Satisfy options for continuous overflow provision include:
Excess water is designed to flow out of the holes in the face of the gutter. This achieves an overflow capacity of 0.5 L/s/m with:
Excess water is designed to flow over the front lip of the gutter. Overflow capacity of 1.5 L/s/m with:
Excess water is designed to flow over the back of the gutter, outside of the fascia. Overflow capacity of 1.5 L/s/m with:
Excess water is designed to flow out of the opening in the end of the gutter. The opening is located to prevent overflow under typical rainfall conditions.
Overflow capacity of 0.5 L/s with:
Excess water is designed to flow through the holes in the bottom of the gutter. The ‘pop’ is raised to prevent overflow under typical rain conditions.
Overflow capacity of 1.2 L/s with:
Excess water is designed to flow out of the large hole in the face of the gutter.
Overflow capacity of 1.0 L/s with:
Excess water is designed to flow out of the hole in the face of the rainhead. Overflow capacity of 3.5 L/s with:
Site location: Joondalup, Western Australia
Roof catchment area: 120m2 (12 x10m roof plane)
Eave gutter length: 10m
Rainfall intensity for a one-in-100-year 5-minute duration event, from BCA 2016 Table 220.127.116.11 for Joondalup: 180mm/h
For continuous overflow measures – per metre of gutter, there is 12m of roof catchment area: 180mm/h x 12m = 2160mm/h/m => 0.6L/s/m (divide by 3600 to convert units or refer to Table 18.104.22.168a)
For dedicated overflow measures – catchment is 120 m2: 180mm/hr x 120m = 21,600mm/h => 6.0L/s (divide by 3600 to convert units or refer to Table 22.214.171.124b)
Suitable continuous measures:
Controlled front bead height – 1.5L/s/m capacity > 0.6L/s/m required, therefore is suitable
Controlled back gap - 1.5L/s/m capacity > 0.6L/s/m required, therefore is suitable
Suitable dedicated measures:
Rainhead – 3.5L/s capacity < 6.0L/s required, however two rainheads (3.5L/s x 2 = 7L/s) would be suitable.
Using continuous measure – Front face slotted gutter with 0.5L/s/m capacity < 0.6L/s/m required, leaving 0.1L/s/m to be allowed for by other measures.
Dedicated measures can be used for remaining required capacity:
Inverted nozzle – 1.2L/s capacity
At 0.1L/s/m, max 12m of gutter can be covered by this solution. As length in this case is 10m, this combination will be suitable.
Front face weir – 1.0L/s capacity
At 0.1L/s/m, max 10m of gutter can be covered by this solution. As length in this case is 10m, this combination will be suitable.
End stop weir – 0.5L/s capacity
At 0.1L/s/m, max 5m of gutter can be covered by this solution. As length in this case is 10m, two end stop weirs would be required. Probability of successful installation should be considered.
Note: Care should be taken to ensure that the overflow volume on lower levels considers any overflow that may fall from upper levels onto the lower roof.
Note: The ABCB provides a calculator for determining overflow volumes and the suitable combinations of overflow measures required. This may be used as part of documentary evidence for the project subject to acceptance by the approval authority.
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