Sewer smell on the third floor: trap seal loss in an unvented stack
Residents of a four-storey working women's hostel complain of a sewer smell in the third-floor bathroom every morning, and the caretaker reports a gurgling sound from the wash basin whenever a WC on the fourth floor is flushed. You find a 100 mm CI soil stack on the rear wall, one 100 mm WC branch per floor, each WC discharging through a 50 mm water-sealed trap into a floor trap. The stack rises only 0.5 m above the roof slab and ends in a wire-mesh cage. There is no separate vent pipe anywhere and no anti-siphonage connection to any branch. The third-floor floor trap is dry when you open it, although that bathroom is used every day. Two traps were replaced last year with shallow 25 mm seal traps bought locally because they were cheaper. The building has one common manhole at the base.
1. Explain the mechanism that is emptying the third-floor traps. Distinguish between induced siphonage and back pressure, and explain why the shallow traps made the problem worse.
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Both effects are present, and they come from the same root cause: a 100 mm stack carrying a full-bore WC discharge with no vent and a poor termination.
- Induced siphonage happens on the branch. When the fourth-floor WC discharges, it fills the stack and the falling slug of water drags a column of air with it. Because there is no vent to supply air behind the slug, the partial vacuum is relieved through the nearest available path - the third-floor branch and its trap. The 50 mm water seal is sucked out in a few seconds, and the trap is then an open hole into the drain. The gurgling at the basin is the sound of that air being pulled through the seal.
- Back pressure happens in the opposite phase. When the slug reaches the bend at the base and slows, air ahead of it is compressed and the pressure can be pushed back up the branches, bubbling through the seals.
- The 25 mm traps are the aggravating factor. A seal of 25 mm is half the normal depth; it is emptied by a much smaller pressure difference and has far less reserve against evaporation and self-siphonage. Once the trap is dry, there is nothing at all to stop sewer gas entering the bathroom, which is why the smell appears in the morning after the night's peak use.
- The 0.5 m stack termination with a mesh cage gives no effective venting and can let foul air drift back toward nearby windows.
2. State the venting and trap-seal provisions you would apply to this building, with the sizes and heights you would insist on.
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The stack must be given air and every fixture must keep a full depth of seal:
- Trap seal depth: 50 mm for floor traps and WC connections; replace the two 25 mm traps. Shallow traps are only acceptable where a trap is in daily use and the fixture is individually vented, which is not this case.
- Vent stack: a separate vent pipe not smaller than half the stack diameter, and in no case less than 40 mm, carried up from the base of the stack and connected to each branch or to the stack above the highest branch connection. For a 100 mm stack this means a 50 mm vent as a practical minimum.
- Termination: the soil or vent pipe must extend above the roof to a height that keeps the outlet clear of windows and air intakes - commonly 1 m above the roof slab for the stack and not less than 0.6 m for a vent pipe where it is not the stack itself. The wire-mesh cage alone is not an acceptable terminal; a proper vent cowl is required.
- Anti-siphonage pipe: 40 mm minimum from each floor trap and basin trap to the vent stack, connected above the highest branch. The 100 mm branch should be as short and as steep as the fitting allows, since a long flat branch increases self-siphonage.
- Stack details: no reduction of the stack diameter in the direction of flow, and a base bend of long radius into the manhole rather than a sharp 90° bend that produces the back pressure.
- Fixture check: verify that each WC pan has an integral or connected 50 mm trap and that no floor trap is acting as the only trap for a WC branch.
3. The management cannot afford to open all the walls. Recommend a phased corrective plan and state what you would tell them about the risk of doing only the cheap part.
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A phased plan is acceptable if each phase actually removes a failure path, and if the cheap phase is not presented as a cure.
- Phase 1 - immediate, low cost: replace the two 25 mm traps with 50 mm seal traps; fit proper vent cowls on the stack top; check and clear the base manhole so that the stack discharges freely; instruct residents to run a little water into the third-floor floor trap each night. This stops the smell in the daily-use bathrooms but does not stop siphonage.
- Phase 2 - structural, medium cost: carry a 50 mm vent stack up the outside of the rear wall, tied to the existing stack, with 40 mm anti-siphonage branches to the two most affected floors (third and top). External running avoids opening internal walls and is the cheapest way to get real venting in an old building.
- Phase 3 - full compliance: connect the remaining floors and correct the base bend and the branch gradients at the next major repair.
What to tell the management: a dry trap is not a comfort problem, it is an open connection between the sewer and a residential bathroom, carrying hydrogen sulphide and potentially pathogens, and it is the reason the building smells. Phase 1 alone will reduce the smell but the gurgling will return whenever the top floor is used heavily. Ask for the Phase 1 and 2 work in one order; it is roughly half the cost of opening every wall and it removes the actual mechanism.