Roof drainage systems must be designed by a suitably qualified professional in accordance with the NCC/BCA and relevant Australian Standards. A compliant design considers rainfall intensity, roof catchment area, gutter capacity, downpipe sizing and placement, and appropriate overflow provisions to safely manage stormwater.
What is roof drainage design?
Roof drainage design is the process of detailing gutters, downpipes and overflow measures to safely collect and remove rainwater from a roof during normal and extreme rainfall events. Done correctly, it helps prevent water damage, structural issues and non‑compliance with building regulations.
Who is responsible for designing a roof drainage system?
Roof drainage systems should always be designed and detailed by a suitably qualified trade or professional, in line with the National Construction Code (NCC/BCA) and relevant Australian Standards.
What standards apply to roof drainage design in Australia?
Roof drainage design must comply with:
- The National Construction Code (NCC/BCA)
- Relevant Australian Standards for roof drainage systems
These standards define how systems should be sized, positioned and detailed to manage rainfall safely.
What factors need to be considered when designing roof drainage?
When designing a roof drainage system, the following factors must be assessed:
Rainfall intensity
Local rainfall intensity influences how much water the system needs to handle during storm events.
Roof catchment area
The size and shape of the roof determine the volume of water collected and directed into gutters and downpipes.
Gutter size and capacity
Gutters must be sized to accommodate expected water flow without overflow under design conditions.
Gutter fall
Adequate gutter fall ensures water flows efficiently toward outlets rather than pooling.
Gutter outlets
Outlets such as sumps, rainheads or nozzles must be correctly sized and positioned to support effective drainage.
Downpipe size, quantity and placement
Downpipes must be appropriately sized, spaced and located to safely carry water away from the roof.
Overflow provisions
Overflow measures are essential to manage excess rainfall and reduce the risk of water entering the building.
Rainfall intensity and drainage requirements vary across Australia. LYSAGHT provides state‑specific roof drainage design guides to support compliant design in different climate zones.
