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.
A licensed plumber can confirm whether two traps are in series, rule out a blocked drain and assess the ventilation and trap seals. If you suspect double trapping, our Sydney plumbing team can inspect the arrangement and recommend the appropriate correction.