Gurgling Basin and a Blocked Stack Terminal After a First-Floor Addition
Two years after a first-floor bathroom was added to a 25-year-old house in Jalandhar, the family reports three linked symptoms: the ground-floor wash basin gurgles and its water seal drops from 50 mm to about 20 mm whenever the first-floor WC is flushed; a sewer smell appears in the ground-floor toilet during heavy rain; and the first-floor floor drain sometimes bubbles. On inspection, the contractor had cut the 110 mm CI stack and inserted a new 110 mm branch for the first-floor WC about 300 mm below the existing ground-floor WC branch entry, directly opposite it. The new basin drain is 40 mm PVC running 4.2 m with two bends and no vent, entering the stack 1.2 m above the WC branch. The stack continues 2 m above the roof, but its top is covered with mosquito mesh and a plastic cap that is 70 per cent blocked with debris. The 40 mm trap has a 50 mm seal.
1. Explain, from venting principles, why the ground-floor basin loses its seal. Name the mechanism at work and identify every installation defect that contributes to it.
Read the worked answer
The seal loss is a pressure problem, not a blockage problem.
- Induced siphonage. When the first-floor WC discharges, 6–9 litres of water and solids accelerate down the 110 mm stack and drag air with them. Behind the falling slug the stack is under negative pressure, and the easiest route for replacement air is backwards through the ground-floor basin trap. The 50 mm seal is pulled out, the trap gurgles, and the seal recovers slowly afterwards - the classic signature.
- No vent on the basin branch. A 40 mm branch with a 4.2 m developed length, plus two bends, has no connection to a vent. The permitted trap-to-vent distance for a 40 mm branch is of the order of 1 m, so this branch is unprotected by a wide margin.
- Opposed connections. The new WC branch entering exactly opposite the old one forms a cross junction. Flow from above can spurt across into the lower branch, and two discharges meeting head-on raise pressure at the basin connection.
- Choked stack terminal. With about 70 per cent of the vent area blocked, the stack cannot admit air to replace falling water, so the system swings between negative and positive pressure. The smell during rain confirms positive pressure pushing past the ground-floor trap.
- The basin joins only 1.2 m above the WC branch, inside the turbulent zone where pressure swings are strongest.
2. Set out the corrective plan in priority order, stating the sizing, positioning and slope rules you would apply.
Read the worked answer
Fix the vent first; re-routing without restoring air does nothing.
1. Restore the stack terminal (day one). Remove the plastic cap and mesh. A stack must terminate in free air, at least 1 m above the roof surface and clear of openable windows, with no reduction in area. Fit a proper vent cowl or leave it open. Never mesh a vent - lint and debris blind it within a season.
2. Rebuild the opposed branches. Cut out the section and use a 110 mm swept junction, or better, offset the new WC branch at least 1.5 m from the old one with 45° bends and a junction swept in the direction of flow. Two branches must never face each other across the stack.
3. Vent the basin branch. Either carry a 32–40 mm individual vent from within about 1 m of the trap, rising above the highest fixture before turning to the roof, or fit an air admittance valve of the correct size - accessible, at least 100–150 mm above the flood-level rim of the highest fixture on the branch, and only where positive pressure does not occur.
4. Slope and support. Give the 40 mm branch a fall of about 1:40 to 1:50, clip at 1 m centres, and if the length cannot be shortened, increase the branch to 50 mm and ventilate it.
5. Recheck the ground-floor WC and floor trap, which are almost certainly being siphoned as well.
3. What tests prove the fix has worked, and what maintenance instructions do you hand over?
Read the worked answer
Testing, in this order:
- Water test of the repaired branch - plug the stack below the repair, fill to branch level, hold 30 minutes, inspect every joint for weeps.
- Smoke test of the whole system - cap the stack terminal, blow pungent smoke in at the lowest inspection point, and watch the traps. Smoke escaping at a basin trap means a defective or unsealed trap; smoke at a joint means a leak.
- Functional test - dye the first-floor cistern, flush three times with the basin trap at its normal seal, then measure the seal with a steel rule; it must stay at 50 mm. Run the basin tap and the floor drain together to check for positive pressure.
Handover: check seals monthly through the first monsoon; never cap or mesh the vent; rod the branch annually; air admittance valves must not be painted, boxed in or swapped for a plain cap; keep the revised stack drawing on file. Explain to the owner that a gurgle is the trap announcing a vent fault, not a nuisance.