Foul Smell and Gurgling in a Ground-Floor Bathroom
A four-storey apartment building has a 100 mm PVC soil stack serving bathrooms on all floors. On the ground floor, the WC and washbasin share a 75 mm branch with a 50 mm water-seal trap. Occupants report a foul smell in the bathroom and gurgling from the washbasin whenever a WC on the second or third floor is flushed. The ground-floor trap still holds water, but the water level drops after several flushes. There is no separate vent pipe visible above the roof, and the stack terminates 0.5 m above the roof slab. A nearby window can open within 3 m of the terminal. The building has had no recent blockage complaints. The ground-floor branch is 4.5 m long with two bends. You are asked to diagnose the cause without opening walls and to propose a code-compliant correction.
1. Explain the likely mechanism causing both the gurgling and the falling trap seal.
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The symptoms point to pressure fluctuation in an unvented or inadequately vented soil stack, not to a simple blockage. When a WC on an upper floor flushes, a large plug of water falls down the 100 mm stack. Air ahead of the plug is compressed, creating positive pressure at the ground-floor branch, while air behind the plug is pulled into a partial vacuum. The ground-floor 75 mm branch is close to the stack, so the vacuum induces siphonage and pulls water out of the 50 mm trap. The gurgling is air being drawn through the trap as the seal is lost. The fact that the water level drops after several flushes supports seal loss rather than a complete obstruction. The low stack terminal near an openable window also allows foul air to re-enter the bathroom.
2. What non-destructive tests would you carry out to confirm the diagnosis, and what would each test show?
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First, measure the trap seal depth with a ruler; it should be 50 mm. Pour 5 L of water into the washbasin and watch the level for several minutes to rule out a partial blockage or evaporation. Then arrange for a third-floor WC to be flushed while an observer watches the ground-floor trap with a mirror and a torch. If the seal visibly drops or bubbles, induced siphonage is confirmed. A smoke test is more definitive: introduce smoke into the ground-floor branch or stack and check whether smoke appears at the roof terminal or enters the bathroom through the trap or fittings. Smoke entering the room proves a defective trap seal or vent path. A peppermint test can be used if smoke equipment is unavailable. Only if drainage is slow should a camera survey be used to check for a partial blockage.
3. Propose a code-compliant correction, including venting and terminal arrangements, and state what should not be done.
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Provide proper venting for the ground-floor branch. The best solution is to extend a 50 mm vent from the branch, connect it to the stack above the highest fixture, and terminate it in open air above the roof, clear of windows and openings. If local code permits, an air admittance valve with access can be fitted on the branch, but it must be rated for the system and must not replace a stack vent. Raise and relocate the stack terminal so it is at the required height above the roof and at least the code distance from the openable window. Check that the ground-floor trap has a full 50 mm seal and is accessible. Do not simply pour bleach or seal the bathroom; do not cap the stack or fit an undersized vent. After the work, repeat the flush test to confirm the seal remains stable.