Gurgling WC and Sewer Smell in a Four-Storey Block
A four-storey residential building has one 110 mm PVC soil stack serving the toilets, with 75 mm floor traps and 40 mm wash-basin wastes connected directly to the stack branch. There is no vent pipe; the stack terminates 300 mm above the roof slab, close to a bedroom window. On the top floor, the trap seal of the guest bathroom disappears after the third-floor tenant flushes, the WC gurgles and a sewer smell enters the bathroom. The owner has already replaced the floor trap twice and fitted a stronger cistern. He now proposes closing the window and putting a large exhaust fan in the bathroom, and asking tenants to flush slowly. The top-floor branch is the highest connection on the stack and has the shortest run to the stack.
1. Diagnose the trap-seal loss and explain why the top floor is affected more than the lower floors.
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The trap is not defective; the seal is being pulled out by pressure changes in an unvented stack.
When a WC on a lower floor flushes, a slug of water falls down the 110 mm stack. It drags air with it and compresses air below the slug. Because there is no vent pipe, the stack cannot draw replacement air from the roof fast enough, so a negative pressure wave travels up the stack and sucks air through the nearest trap. The gurgling is air being pulled through the trap; the smell follows once the seal is gone.
The top-floor branch is worst because it is the highest connection and the shortest path to the stack termination. It is also closest to the region where the stack is trying to draw air from the roof. Replacing the trap or fitting a stronger cistern does not remove the pressure wave; a stronger flush actually increases the slug velocity and the suction.
2. Recommend a venting and trap arrangement, giving sizes and termination details, and state what must be checked on the existing stack.
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Vent arrangement
- Provide a 50 mm vent stack connected to the top of the 110 mm soil stack, carried full size above the roof.
- Terminate the vent at least 150 mm above the roof slab and at least 2 m from any window, opening or air intake. The present termination beside a bedroom window is unacceptable.
- Re-vent the top-floor WC branch with a 40 mm vent connected to the vent stack, so the branch trap has its own air supply.
- Use 75 mm deep-seal traps on the floor traps. An air-admittance valve may be used on a branch only where a through-roof vent is impossible, and it must be accessible for replacement.
Checks on the existing stack
- Confirm the stack has a proper base bend and no horizontal offset or reduction.
- Check that the roof flashing is sound and that the stack is not blocked by birds' nests.
- Confirm the branch connections are swept in the direction of flow and that the 40 mm wastes do not discharge into the trap seal.
Record the vent route on a sketch so the next maintenance plumber does not remove it.
3. Evaluate the owner's two proposals - closing the window with an exhaust fan, and asking tenants to flush slowly.
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Exhaust fan with the window closed This treats the smell, not the cause. The fan may actually make matters worse: it depressurises the bathroom and increases the negative pressure on the trap, so more air is drawn through the seal. Closing the window removes dilution air and concentrates odour.
Slow flushing Slower flushing may reduce the suction peak, but it does not eliminate it, and it reduces solids transport in the drain. Repeated low-volume flushes can leave solids in the branch and cause blockages, which create a second health problem.
Conclusion Neither proposal restores the trap seal. The correct remedy is venting plus deep-seal traps. The owner should also be told that the current roof termination is a health nuisance and may contravene local building requirements; it should be corrected even if the internal gurgling stops.