New First-Floor Bathroom Drains Slowly and Smells
A 20-year-old two-storey house has had a new first-floor bathroom added. The WC, wash basin and shower each have a 40 mm trap and connect to a 110 mm PVC branch 4.5 m long, laid at a measured fall of 1 in 120, which enters the existing 110 mm soil stack about 1.2 m below the ground-floor WC connection. The stack continues 0.6 m above the roof and has no other vent. No vent pipe was provided for the new branch. On commissioning, the WC flushes slowly and needs two flushes, the shower drain gurgles loudly when the WC is flushed, the shower trap is empty each morning and a drain smell persists in the bathroom. The plumber blames the trap and proposes replacing the shower trap with the same 40 mm size but a different, costlier brand.
1. Identify the two installation defects most likely to cause these symptoms and explain the mechanism of each.
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Defect 1 - too flat a branch. A 110 mm branch carrying a WC should fall at about 1 in 60. At 1 in 120 the velocity is low, water spreads out and solids are left behind, so the WC flushes sluggishly and the line progressively silts up. The flush also arrives at the stack slowly, so the slug of water needed to carry waste away never forms. Defect 2 - no branch vent. A 4.5 m unvented branch entering a stack 1.2 m below a ground-floor WC is exposed to large pressure swings. When the ground-floor WC discharges, it pushes air ahead of it and then leaves a negative pressure behind; the only air path into the branch is through the shower trap, so its 50 mm seal is siphoned out. The gurgling is air being pulled through the seal, and the empty trap each morning is the lost seal letting sewer gas back in. Changing the trap brand does not change the pressure or the fall, so the symptoms will return within days.
2. State the remedy you would specify, with falls and sizes, and explain why a new trap alone will not work.
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Reline the 110 mm branch at a minimum of 1 in 60 (1 in 40 where space allows), with 110 mm bends and a swept junction into the stack, supported on brackets at about 1 m centres so the fall cannot sag. For ventilation, either take a 50 mm vent pipe from the branch crown up the wall and terminate above roof level, or fit a suitable air admittance valve at the high end of the branch, accessible for maintenance. Fit deep-seal traps of 50 mm seal on the shower and basin, and check that the stack is clear and properly terminated above roof. Re-test after the work. A new trap only renews the seal; the siphonage that removes it is caused by pressure in the system and the flat line is caused by the fall. Both root causes survive a trap swap, so the client pays twice for the same complaint.
3. Describe the tests you would carry out after the work and the result you would accept.
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- Fall check - measure with a spirit level and boning rod or laser: accept a fall of at least 1 in 60 over the 4.5 m run, with no backfall at any bracket.
- Smoke test - plug the bottom of the stack, fill all traps, introduce smoke at the branch, and inspect the bathroom and joints: accept no smoke escaping into the room and no smoke at the joints.
- Trap-seal test - fill the shower trap, flush the WC ten times in succession and then measure the seal: accept that at least 50 mm of seal remains and there is no gurgling.
- Flow test - flush a full WC and confirm the pan clears in one flush with no rise at the branch. Record all readings, the date and the test medium on a completion sheet, and leave the air admittance valve accessible.