Gurgling Traps After a Five-Storey Stack Is Extended
A 20-year-old five-storey building in Thane originally had a 100 mm SWR stack serving three floors. Two upper floors were added last year, with 75 mm branches from each WC and a 50 mm trap. The stack terminates 300 mm above the terrace but has no vent pipe; the top is covered with a loose cowl. After the top-floor WC is flushed, the second-floor WC pan gurgles and the 50 mm water seal in the nearby floor trap disappears; a bad smell enters two flats. The owner has been told to just put an air admittance valve in the second-floor bathroom and to increase the floor-trap size from 50 mm to 75 mm. The branch from the second-floor WC is about 2.5 m long and falls at about 1:80. The building has a common horizontal drain at the ground floor with three manholes, and the last manhole is connected to the municipal sewer. The owner wants a permanent solution without breaking the new floor tiles.
1. Explain the likely cause of the gurgling and loss of trap seal; include why a bigger trap is not the answer.
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The stack has been extended from three to five floors without a vent. When the top-floor WC discharges, it sends a slug of water and air down the 100 mm stack. Without a vent to admit air, a negative pressure wave forms behind the slug. The second-floor branch is connected near that pressure fluctuation, so air is pulled through the 50 mm trap. The trap seal is siphoned out, the pan gurgles, and sewer gas enters the flat.
A 75 mm trap holds more water, but it is still exposed to the same pressure swing. Increasing trap size does not replace venting, and it may not fit the branch or fixture. The long 2.5 m branch and its slope can worsen the effect, but the primary defect is the unvented stack after the extension.
2. Outline a venting and checking plan that minimizes breaking the new floor tiles.
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Use the common shaft and external wall where possible. Provide a 50 mm vent stack carried up above the roof, connected to the discharge stack above the highest branch, and revent the second-floor branch with a 40–50 mm vent taken vertically or in the shaft. An air admittance valve may be used only where the authority and manufacturer permit it, must be accessible, and should be supplementary, not a substitute for a proper vent terminal.
Check that the existing stack top is genuinely open, the cowl is not blocked, branch slopes are adequate (about 1:60 to 1:100 for a 75 mm branch), trap arms are not excessive, and the ground-floor manholes and drain are not surcharged. Before closing, carry out a water or smoke test on the drainage system and confirm that no trap seal is lost during a top-floor flush.
3. What would you verify before signing off the work?
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Verify that the vent terminal is above the roof, clear of windows and air intakes, and not blocked by the cowl. Flush the top-floor WC repeatedly while watching the second-floor trap with a torch and measuring the seal; the seal should remain at its designed depth without gurgling. Check that every fixture has a trap and that the vent connections are above the fixture overflow level.
Carry out a smoke test if possible, check for leaks at new joints, confirm the branch slope and supports, and ensure the ground-floor drain runs freely to the manhole. Give the owner a simple sketch of the vent route and advise against closing the AAV or vent in a sealed box.