What is the minimum size for a floor waste gully (FWG)?
Summary (TL;DR)
According to AS/NZS 3500.2, the minimum outlet size for a floor waste gully (FWG) is DN 50. However, if the FWG receives discharge from a shower or a bath, the minimum outlet size increases to DN 65.
Floor Waste Gully Sizing (AS/NZS 3500.2)
A floor waste gully (FWG) acts as a central trap for multiple fixtures in a bathroom, preventing sewer gases from entering while catching overflow. Sizing it correctly is critical to prevent the bathroom from flooding during use.
The DN 65 Rule for Showers and Baths
While a basic FWG used only for floor runoff or a hand basin can have a DN 50 outlet, this is rare in practice. The most important rule to remember is:
If a shower or a bath discharges into the FWG, the outlet pipe must be a minimum of DN 65.
This ensures the gully can handle the high flow rate of a modern shower or the rapid dumping of a full bathtub without backing up onto the bathroom floor.
Maximum Fixture Units
A DN 65 FWG outlet can handle a maximum of 10 fixture units discharging into it. If you are connecting multiple high-flow fixtures (e.g., two showers, a bath, and a double vanity), you must calculate the total fixture units. If it exceeds 10, you must upsize the FWG outlet to DN 80.
Riser Size
The vertical riser of the FWG (the part that comes up to the floor grate) must be a minimum of DN 80, regardless of the outlet size, to allow for proper grate installation and flow capture.
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