The National Construction Code (NCC) and reference standards, such as the ABCB Housing Provisions and AS/NZS 3500.3, require that measures be taken to prevent overow from roof drainage systems back into the roof or building structure. The overflow design is based on the 100year ARI 5 minute duration rainfall intensity.

Below are common methods for controlling overflow for all gutter profiles. Multiple methods may be needed to achieve compliance:

Slotted front gutters: A popular option that allows water to overflow through visible slots on the front face of the gutter.
Gap between fascia and gutter: Inserting a packer between the fascia and gutter back to create a drainage gap.

Non-continuous overflow outlets: As permitted by the NCC, including:

• Inverted downpipe drops at high points in the gutter, set below the fascia top.
• Lowered stop ends acting as weirs, often hidden at high points and incorporated into expansion joints.
• Rainheads with overflow weirs.
• Holes, slots, or weirs at downpipe locations.

Proprietary systems and trade solutions: Other commercial or custom-designed systems.

Other Alternative Building Design Methods:

• Unlined eaves: Suitable for some house designs, eliminating the overflow issue.
• Gutters set below fascia top: Ensuring sufficient clearance (freeboarding) between the gutter front and the fascia.
• Design for higher rainfall intensity: As required for internal box gutters.
• Back flashing: Possible with gutter support brackets that allow back flashing (e.g., external brackets).

Important Considerations:

The design and sizing of the selected overflow methods are the responsibility of the designer or installer. These must comply with the performance requirements set out in the NCC and relevant Australian Standards.