Overflow from eaves gutters

Overview of the NCC requirements

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.

‘Continuous’ and ‘Dedicated’ overflow measures

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: 

Example overflow calculation 

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 3.5.2.1 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 3.5.2.3a) 

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 3.5.2.3b) 

Possible solutions: 

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. 

Combined measures: 

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. 

To find out more, contact HIA’s Building Services team.

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