A double trapped drain forces one fixture’s wastewater through two water-seal traps in series. Air trapped between them can resist the flow, causing gurgling, intermittent slow drainage or backup. Pressure changes may also disturb a trap’s water seal.
Does it always drain slowly?
No. A double trapped drain may appear to work normally under some flow conditions. Its performance depends on the pipework and available ventilation, and symptoms may become noticeable only when the fixture discharges quickly or another connected fixture is used.
A blockage can cause similar symptoms, so gurgling or a slow-draining sink does not confirm double trapping. Two traps elsewhere in the system are not necessarily a problem; double trapping means the same fixture discharge passes through both traps.
What should you do?
Stop using the fixture if wastewater is backing up, and avoid contact with it. Do not dismantle traps, concealed pipework or drain venting yourself.
Under AS/NZS 3500.2 (the national sanitary plumbing code, which strictly prohibits double trapping because trapped air pockets between seals restrict waste discharge and break the protective water barriers that keep toxic sewer gases out of your home), all fixture waste lines must have single-trap integrity and proper ventilation. A licensed plumber can confirm whether two traps are installed in series, rule out a blocked drain, and assess pipe ventilation and water seals. If you suspect double trapping, contact our Sydney plumbing team. Fixed Today has been the trusted name in Sydney household plumbing since 2011, providing honest on-site evaluations with upfront fixed quotes and zero guesswork. Letting our staff know when you book whether the gurgling occurs at an under-bench kitchen sink, a modified laundry tub, or a renovated bathroom vanity helps our plumber arrive with the right trap configurations and re-venting fittings ready to go.