Gurgling WC and a flat drain run: what must be re-laid?
A three-storey house has a ground-floor WC connected by 100 mm SWR pipe on a 15 m horizontal run to the first inspection chamber, then to the public sewer. The measured gradient is 1:150. The first and second floors discharge into the same run through an 87.5° junction only about 1 m from the ground-floor WC. A 40 mm washbasin waste joins 1.5 m downstream of the WC. There is no vent pipe anywhere on the system. Symptoms: the WC flushes slowly with the water rocking and gurgling, the washbasin trap empties after each flush, a drain smell stays in the bathroom, and the line needs rodding every few months. The chamber invert has only about 20 mm of spare depth above the public sewer connection, the floor is tiled, and the family will not accept the bathroom floor being broken.
1. Using the numbers given, separate the causes that can be fixed cheaply from the one that needs re-work, and show the calculation that supports your conclusion.
Read the worked answer
Cause 1 - no venting (cheap to fix). A 50 mm trap seal equals about 500 Pa. A 100 mm stack discharging a WC flush creates a short negative pressure surge well above that, so the basin trap is siphoned and the pan gurgles. This is a venting defect, independent of gradient.
Cause 2 - self-siphonage from a bad connection (cheap to fix). The 40 mm waste joining 1.5 m downstream of the WC sits inside the hydraulic jump of the flush, which pulls its trap empty. It should connect further downstream, on a 45° junction, with its own vent or an air admittance valve.
Cause 3 - gradient (needs re-work). Check with Manning, n = 0.013, pipe bore 0.1 m, R = 0.025 m:
- at 1:150 (S = 0.0067): V = (1/0.013)(0.025)^(2/3)(0.0067)^0.5 ≈ 0.54 m/s
- at 1:100 (S = 0.01): V ≈ 0.66 m/s
The self-cleansing velocity for soil lines is about 0.6–0.75 m/s, so 1:150 falls short and solids settle, which matches the need for repeated rodding. Fall over 15 m: 100 mm at 1:150 versus 150 mm at 1:100 - a shortfall of 50 mm, but only 20 mm of spare invert depth exists.
2. Given that the floor cannot be broken and only 20 mm of invert depth is spare, what will you actually do, in what order, and what will you tell the family about the remaining risk?
Read the worked answer
Order of work - cheapest and least disruptive first.
- Vent the system. Take a 50 mm vent pipe from the drain head to open air above roof level, or fit an air admittance valve where the layout allows and where it stays accessible. This alone should stop the gurgling and the trap loss.
- Re-route the washbasin waste to join 45° further downstream, outside the flush surge, with its own AAV if it cannot be vented.
- Recover part of the gradient without breaking the floor only if the route allows: lower the invert at the chamber by the available 20 mm and re-lay any exposed length in the courtyard at 1:100, gaining part of the 50 mm shortfall. If no exposed length exists, do not pretend the gradient is fixed - it is not.
- Fit deep-seal traps (75 mm where the pan connector allows) and keep all access covers clear.
Tell the family honestly: venting and re-routing will remove the smell and gurgling, but the flat 15 m run will still settle solids. Budget for a scheduled jetting once or twice a year, and plan a proper re-lay at 1:100 with a new chamber when the bathroom is next renovated. Do not guarantee a permanently blockage-free line on a 1:150 run.
3. How would you test the repaired drainage system before handing it over, and what would you record?
Read the worked answer
Test sequence:
- Water test on the pipework: plug the low end at the chamber, fill the run with clean water to at least 1.5 m above the highest point of the section under test, or to the top of the stack where that is lower and permitted, and hold for 30 minutes. No drop in level and no weeping at joints.
- Smoke or air test for the vent and traps: confirm the vent is continuous to open air, that the AAV opens and closes freely, and that no trap loses its seal under a flush.
- Functional test: flush the WC ten times in succession and watch the basin trap level and the pan water line; there should be no gurgling, no rise of smell and no rocking.
- Check access: all inspection covers removable, rodding eye clear, AAV reachable for replacement.
Record on the handover note: what was vented and where, the waste re-connection, the water test duration and result, the gradient actually achieved (state the measured figure, not the intended one), the annual jetting recommendation, and that the gradient remains 1:150 so the owner's expectation is documented.