What is the minimum trap seal depth for plumbing fixtures?
Summary (TL;DR)
According to AS/NZS 3500.2, the absolute minimum water seal depth for any fixture trap is 25mm. However, standard traps (P-traps and S-traps) typically have a seal depth of 50mm or 75mm to provide better protection against evaporation and siphonage.
Trap Seal Depth Rules (AS/NZS 3500.2)
The water seal in a trap is the only thing preventing foul sewer gases from entering a building. Maintaining adequate depth is critical to ensure the seal isn't lost due to evaporation, siphonage, or back-pressure.
Minimum Seal Depth
The standard dictates that a trap seal must never fall below 25mm. If pressure fluctuations in the drainage system cause the seal to drop below 25mm, the system is non-compliant and requires additional venting (or an air admittance valve).
Standard Trap Depths
When you buy a trap off the shelf, it will usually have a deeper seal than the minimum:
- Standard P-trap or S-trap: Typically has a 50mm water seal.
- Traps used in specific commercial applications: May require a 75mm seal.
Why do trap seals fail?
A trap seal can be lost due to:
- Evaporation: In rarely used fixtures (like floor wastes in guest bathrooms).
- Siphonage: Negative pressure pulling water down the drain (usually due to inadequate venting).
- Oscillation: Wind across the top of the vent stack causing the water to bounce and spill over the weir.
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