Gurgling ground-floor WC and lost trap seals in an unvented three-storey stack
A three-storey hostel block has eight WCs and eight wash basins. Each WC connects to a 110 mm PVC soil stack through a 100 mm branch laid at 1:80 with a 75 mm P-trap; the basins discharge into a separate 75 mm waste stack. There is no vent stack, and the soil stack simply terminates 0.6 m above the roof slab. The WCs are flushed by 10-litre low-level cisterns. For two months the ground-floor WC has drained slowly, the water in its pan bubbles, the trap loses its seal whenever a second- or third-floor WC is flushed, and the ground-floor toilet smells. There is no visible leak on the stack, and when the ground-floor WC alone is flushed twice in a row the drain to the manhole runs away freely. A plumber proposes simply rodding the stack from the roof.
1. Explain the mechanism that empties the ground-floor trap when a WC two floors above is flushed, and say why the ground floor suffers most.
Read the worked answer
A falling slug of water in a stack behaves like a piston in a cylinder. Ahead of the slug it compresses air and pushes it out through whatever opening it can find; behind the slug it leaves a partial vacuum. In a properly vented stack the vent pipe lets air in and the pressure swings stay small. Here there is no vent at all, so the slug generates a negative pressure wave on its underside of tens to hundreds of millimetres of water column.
The ground-floor P-trap is joined to the stack low down, inside the zone of strongest suction and just above the base bend, where the falling water changes direction from vertical to horizontal and produces its largest pressure surge. The 75 mm trap seal (typically 50 mm deep) is shorter than the suction the slug creates, so the seal is siphoned out into the stack. The gurgling is air being pulled back through the trap after the water; the bubbling is the positive pressure wave ahead of a slug being forced up through the pan; the smell is simply an empty trap. Upper floors are further from the base bend and their branches meet the stack where the slug has less velocity, so their seals survive. The slow draining and the 1:80 branch slope suggest the base bend is also partly fouled, which makes the pressure swings worse, but venting is the primary defect.
2. State the checks and tests you would run to decide between (a) a partial blockage at the stack base and (b) a venting deficiency, and what result each would give.
Read the worked answer
Test 1 - flush-and-watch (decides venting). Fill the ground-floor pan to its normal level, then flush a WC two floors above and watch the ground-floor pan for 30 seconds. A visible drop in level of more than about 25 mm, a gurgle and a smell within seconds of the flush is induced siphonage - a venting failure. If the pan level stays put but the pan rises and bubbles, the stack is being pressurised from below, which points to a base restriction.
Test 2 - roof terminal. Inspect the open end of the stack: a bird's nest, a dropped bottle or a cap left on it pressurises the entire stack and reproduces exactly these symptoms. Lower a torch and feel for airflow while a WC is flushed; no airflow means the terminal or stack is blocked.
Test 3 - rodding/CCTV of the base bend. Rod from the ground-floor inspection chamber towards the stack base or run a camera. If the bend is clear and the branch slope is the stated 1:80 (100 mm branches normally need nearer 1:60), any remaining slow drainage is slope-related, not blockage.
Test 4 - smoke test. Blow smoke into the drain and vent system; smoke appearing at a trap, or at an unsealed connection, shows where seal integrity fails. It also exposes any branch connected without a trap or with a defective one.
Only if rodding and the roof-terminal check come back clear can you conclude the fault is purely a missing vent - otherwise rodding alone cures nothing.
3. Set out the permanent correction. Explain the purpose of each item you would add or change, and justify why the proposed rodding is inadequate.
Read the worked answer
Rodding may restore flow for a few days, but it does not change the pressure swings in the stack, so the seals will be siphoned out again. The permanent correction is a venting and geometry fix:
- Install a vent stack carried full size (minimum 50 mm, preferably 75–110 mm) from the base of the soil stack to above the roof, connected to the branch connections or at least to the lower branches. It supplies air behind the falling slug and keeps stack pressure close to atmospheric, which is the only real cure for siphonage.
- If a full vent stack is impossible, a relief vent from the base bend up to the vent stack, or an air-admittance valve at the branch, may be allowed by the applicable code - but be aware that an AAV admits air only; it does not relieve positive pressure, so it must not be the only measure where the base bend is also being pressurised.
- Clean and re-form the base bend, replacing a tight 90° bend with two 45° bends or a long-radius fitting to cut the surge.
- Re-lay the ground-floor branch at about 1:60 rather than 1:80 so solids travel and the branch self-cleans.
- Extend the stack terminal to the code height above the roof and away from openable windows and air intakes.
- Re-test with the flush-and-watch and smoke tests after the work, and record the results.