Sewer smell and gurgling in a new ground-floor toilet
The ground-floor toilet of a two-year-old house in Sambalpur develops a strong sewer smell, worst in the evening, and the floor trap drains away slowly. After each use a ring of water remains in the trap and disappears within about ten minutes. The WC makes a loud gurgle whenever the first-floor WC is flushed. The drainage is a single 110 mm PVC line laid at a fall of about 1 in 120 over a 14 m run to the inspection chamber in the front yard. The ground-floor wash basin discharges through a 32 mm waste pipe connected directly into the outlet of the floor trap, and the family has pushed a washing machine hose into the same floor trap. The 75 mm vent pipe from the first floor stops 300 mm below the bedroom window, and the gully trap near the gate has a broken grating partly covered with a discarded cement bag.
1. Link each symptom to its likely cause. Do not just list faults; explain the mechanism.
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Slow draining plus a seal that vanishes in ten minutes. Both come from the same place. A 110 mm line at 1 in 120 is flatter than the 1 in 60 that this size normally needs for self-cleansing velocity; solids settle, the wetted area is shallow, and the line partially chokes. Flow then becomes sluggish and the trap empties slowly. The seal disappearing is a separate mechanism: the falling water in a partially blocked drain drags air with it, the branch has no vent, and the resulting suction pulls the trap seal out - that is why the ring of water remains briefly and then vanishes. The washing machine, discharging a large slug into a small floor trap, does the same thing more violently.
Evening smell. Peak use coincides with the septic/sewer gases being pushed back through the unsealed trap, and with the broken gully grating letting gas out near the entrance. The cement bag over the gully makes it worse, not better: it traps debris, blocks the rodding eye and hides the smell from the source.
Gurgle when the upstairs WC is flushed. Classic trap siphonage. The upstairs flush sends a plug of water down the common stack; with a restricted downstream line and no vent on the ground-floor branches, air must come through the floor trap and the WC trap instead of the vent. The gurgle is air being pulled through the trap.
Vent ending 300 mm below a bedroom window. The vent is doing the opposite of its job: it discharges foul air directly into the window zone instead of above the roof, so the house ventilates itself from the drain. Under normal practice the vent should terminate well above roof level and away from windows and air intakes.
2. Set out the corrective works in the order you would do them, with the specific gradients and venting arrangements you would specify.
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Order the work from cause to symptom; do not start with air fresheners or new traps.
- Clear and then re-lay the line. Rod and jet the 110 mm run to the inspection chamber and camera it if available, to confirm whether the fall is the real problem or there is also a settlement/bellied section. Since the measured fall is about 1 in 120 over 14 m, re-lay the accessible run at a minimum of 1 in 60 (about 233 mm fall over 14 m) with correct sockets facing upstream, and check that the invert of the chamber allows that fall. If the depth to the chamber makes 1 in 60 impossible, use a steeper upstream section and get the shortfall approved in writing rather than leaving the whole line flat.
- Vent properly. Carry the 75 mm vent up above roof level, terminating at least 0.6 m above the roof surface (more if the roof is used for sitting), and away from windows, air inlets and the tank. If a full vent cannot be taken up because the stack position is fixed, fit an air admittance valve on the top of the branch or a 50 mm common vent on the ground-floor group - but an AAV is a relief valve, not a substitute for the main vent.
- Rebuild the branch connections. Take the wash basin through its own 32/40 mm waste with a 40 mm deep-seal bottle or P-trap into a 75 mm branch, not into the floor trap outlet. Give the washing machine a separate 40 mm standpipe connection with its own trap and a 75 mm branch; never push a hose into a floor trap.
- Replace the floor trap with an adequate-seal type (minimum 40 mm seal, deeper if the branch is unvented) and set it level so it holds water.
- Fix the outside. New grating on the gully trap, remove the cement bag, clean the gully and confirm the gully trap has a proper seal and is not connected to the storm drain.
- Only then test and backfill, and clean the toilet down.
Note that steps 2 and 3 are what stop the seal loss; step 1 is what stops the slow drain. Doing only one of them brings the smell back.
3. How would you prove that the trap seals are now holding and that the smell is not simply being masked?
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Test the mechanism, not the smell.
- Seal depth check. With the system at rest, measure the water depth in each trap with a thin rule or a dipstick: floor traps should show at least 40 mm and WC traps at least 50 mm of seal. Record the readings.
- Siphonage test. Flush the first-floor WC five times at one-minute intervals and re-measure the ground-floor floor trap after each flush; then run the washing machine through a full discharge cycle and re-measure. If the seal stays within a few millimetres of its original depth, the venting is working. If it drops and recovers slowly, the branch is still unvented.
- Smoke or peppermint test. Introduce smoke (or a peppermint-soaked rag) at the lowest rodding eye with the vent temporarily capped, and check every joint, the gully, and the toilet for escape; this finds a broken pipe or an open joint that no amount of cleaning will cure.
- Flow test. Run a bucket of water with a few paper towels down the WC and time how long the line takes to clear; a 110 mm line at the corrected fall should clear a full flush in a few seconds with an audible but not prolonged gurgle.
- Vent termination check. Confirm by eye and by note that the vent now ends above the roof and away from the bedroom window; if you cannot get above the roof safely, do not sign off the job on a verbal assurance.
Only when 1–3 pass do you tell the family the smell is cured; a deodorant block is a diagnostic failure, not a repair.