Gutters that pond water create a potential mosquito breeding habitat and can be an incubator for bacteria.

Gutters must be installed with a fall of no less than 1:500 for eaves gutters (unless fixed to metal fascias), and 1:100 for box gutters.

[Australian Standards: AS/NZS3500.3.2:1998; AS/NZS2179.1:1994. Building Code of Australia: Clause 3.5.2.4]

3.5.2.5 Expl. Info. Amndt No.4

Stormwater drainage systems are NOT designed to remove all of the water during exceptionally heavy rain, especially in tropical areas. Accordingly, it is necessary to design and install the system so that when overflowing occurs any water is directed away in a manner which ensures it does not pond against, or enter into, the building.

This may be achieved using slotting gutters, oversized gutters and downpipes, locating the gutter so it is below the top edge of the fascia or the installation of rainwater heads with overflow slots.

The installation of downpipes, especially near valley gutters, are designed to ensure rainwater from areas on the roof that have concentrated water flows perform adequately. If downpipe spacings are to be increased, allowance for overflow should be considered.

Consideration needs to be given to box gutters, valley gutters etc located above the internal areas of a building. If adequate overflow controls cannot be implemented there may be a need to increase the size and capacity of drainage components to remove all water anticipated during heavy rainfall periods.