Gurgling Traps in an Unvented Four-Storey Stack
A four-storey (ground plus three) apartment building in a Haryana town has 12 identical bathrooms. Each discharges a 110 mm WC branch and a 75 mm floor-trap branch into one 110 mm SWR stack that runs inside a 600 mm × 450 mm shaft. The stack terminates 900 mm above the terrace, but the shaft is capped with a concrete slab and surrounded by a 1.2 m parapet, so the stack top sits inside an enclosed duct. There is no vent pipe, no relief vent and no cowl. Two years after occupation, the ground-floor WC gurgles and the water in its floor trap disappears whenever a first-floor WC is flushed; the top-floor bathroom smells of sewage; and the base of the stack, made with a single 90° bend, drains slowly. Trap seals are 50 mm deep. The builder's position is that "the stack is already open at the top, so venting is done".
1. Explain the gurgling and loss of the ground-floor trap seal, and why the builder's "open at the top" argument fails.
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Mechanism. A WC flush discharges about 9 litres in a few seconds into a 110 mm branch that joins a 110 mm stack. With no vent, the falling slug of water acts like a piston: it compresses air ahead of it and creates a partial vacuum behind it. The vacuum has to be relieved through the nearest trap, which is the ground-floor floor trap. Air is pulled through the trap, making the gurgling noise and siphoning out the 50 mm seal. Once the seal is gone there is a straight open path from the drainage system into the bathroom: that is the smell, and it is a health issue, not a nuisance.
Why the builder is wrong:
- A stack needs unobstructed air at its terminal. Here the top sits inside a capped 600 × 450 shaft, which is itself an unvented box. Air cannot move freely in or out.
- Even if the shaft were open, the terminal must be above the roof, clear of openings, and fitted with a cowl and mesh; a stack ending just above the slab inside a parapet is not a vent terminal.
- A single stack cannot protect trap seals on four floors regardless of its top, because the problem is pressure fluctuation along the stack, not just at the top. A vent stack with relief connections is required.
2. Specify the remedial venting and drainage changes, giving sizes, termination and gradients.
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Vent stack: add a 75 mm vent stack (minimum half the stack diameter and never less than 50 mm), taken up the same shaft, connected to the top of the 110 mm stack and carried through the shaft slab to terminate at least 1 m above the roof surface, at least 1 m above and 3 m away from any window or opening, with a cowl and wire mesh. Open the shaft at the top with louvres and keep the mesh.
Relief venting: each floor's WC branch is long enough to siphon, so provide a 50 mm relief vent from the branch (or a yoke vent between stack and vent stack at the top and bottom of the stack) to equalise pressure. AAYs/AAVs may supplement but cannot replace the vent stack.
Base of stack: replace the single 90° bend with a long-radius base fitting or a pair of 45° bends, supported on aproper rest; a sharp 90° at the base is what makes the lower floors drain slowly and is also a blockage point.
Branches: 110 mm WC branch at a gradient not flatter than 1 in 60; 75 mm floor-trap branch at about 1 in 60 to 1 in 80; every trap 50 mm seal maintained; stack supported at each floor and at the base.
Verify before tiling: smoke test or water test on the stack, and a flush test with the ground-floor trap watched to confirm the seal holds.
3. What explains the sewage smell on the top floor specifically, and what would you check before ordering any material?
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The top-floor smell is not automatically the same fault as the gurgling. Check, in this order:
- Dry trap. Top-floor bathrooms are often the least used; a 50 mm seal evaporates in a few weeks in Haryana's dry heat, leaving an open path. Ask the resident; pour 2 litres of water into the trap and recheck after 24 hours. If the smell goes and stays away until the trap dries again, the fix is use and a trap with a deeper seal, not new pipes.
- Pan connector and branch joints inside the shaft. With no vent, the stack runs slightly positive pressure and any imperfect joint in the enclosed shaft leaks gas into the duct, which then finds its way into the top flat through gaps around the pipe sleeves.
- Stack top drawing back into the duct. A stack terminating inside a capped shaft can discharge gas into the shaft, which then migrates down.
- AAV check. If any air-admittance valve has been fitted, it admits air but does not relieve positive pressure, and it must be above the fixture flood level and accessible.
Before ordering material: run a smoke test on the drainage system, confirm trap seal depth and trap usage, check the pan connector and the sleeve seals, and only then size the vent stack. Ordering pipe before the smoke test risks venting the wrong fault and leaving the smell in place.