New WC branches gurgling and draining slowly in a shop conversion
A ground-floor retail unit is being converted. A store room becomes two WCs and a washbasin. WC1 discharges through 4.5 m of 100 mm PVCu at a fall of 1:80 and connects into an existing 110 mm stack serving the three floors above. The stack also takes the neighbouring shop's kitchen waste and terminates 1.4 m to the side of a fourth-floor opening window, finishing 300 mm above the roof covering. WC2, 2 m away, connects into the same stack through a 3 m branch at 1:40. The washbasin waste is 32 mm, running 2.6 m at 1:25 with a 75 mm trap, joining the stack 400 mm above the WC1 branch. Since the fit-out, the basin trap gurgles whenever WC1 is flushed, and the WC pan drains slowly with the water level rising before it falls. The tenant also reports a drain smell in the store room beside the new work.
1. Explain why the basin trap gurgles and the WC discharges slowly, and set out the tests you would use to confirm the diagnosis before altering anything.
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Both symptoms point to pressure problems in the branch and stack rather than one simple blockage.
Basin gurgling. The 32 mm waste joins the stack only 400 mm above the WC branch. When WC1 flushes, the falling water pulls a slug of air down the stack and creates a short negative pressure wave at the basin connection. A 32 mm branch of 2.6 m at 1:25 with no ventilation has no way to relieve that wave, so the 75 mm seal is drawn back and gulps air. Confirm by flushing WC1 while watching the trap with a torch; if the seal drops and bubbles without the basin being used, the diagnosis holds.
Slow WC discharge with the level rising then falling. That pattern is not a simple blockage, which would back up steadily. It suggests a partial obstruction downstream or, more likely on new work, a shallow or backfalling section. A 1:80 fall gives 56 mm over 4.5 m, which is acceptable on paper, but the fall is easily lost over the last metre. Lift the inspection eye, rod the run, then run a camera. Also check that the pan connector spigot has not been pushed too far into the branch, reducing the bore; that gives exactly this rising-then-falling behaviour.
Do not fit an anti-vacuum trap or a bigger trap straight away, because both mask rather than cure the fault. Record the flush tests and the camera findings before work starts.
2. Assess the stack termination and explain what, if anything, must change, including the consequences for the reported smell.
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The stack termination is non-compliant and is the likely source of the store room smell.
The stack finishes 300 mm above the roof covering and 1.4 m from a fourth-floor opening window. In the UK a soil and vent pipe should terminate at least 900 mm above the roof covering or the highest part of the building within 3 m, and should be at least 3 m from any opening window or carried above it. At 1.4 m the window is inside that zone, so foul air can be drawn back in whenever wind plays on that elevation. A 300 mm stub is also below the 900 mm minimum, so the top can be covered by snow or debris.
Remedy: extend the stack in the same 110 mm PVCu to 900 mm above the roof covering, with a weathered flashing. Fit a cage or cowl only if the manufacturer permits it, because a cowl can restrict the vent area.
An air admittance valve at the top of the stack is not a substitute for a vent terminating externally.
After the termination is corrected, smoke test the branch to prove the store room smell is not also coming from a missing trap seal or an unsealed boxed-in pipe. Only then sign off the fit-out, and record the corrected termination height and distance on the handover drawing.
3. Specify how you would redesign the two WC branches and the basin connection, giving gradients and venting arrangements, and state what you would record at handover.
Read the worked answer
Redesign the branches rather than patch them.
WC1. Keep 100 mm PVCu but lay it at a consistent 1:40 to 1:60, about 75 to 110 mm over 4.5 m, with no backfall. Use a rest bend or two 45 degree bends at the base rather than a 90 degree elbow, and provide an access point at the connection. A 4.5 m branch from a WC is generally acceptable where the stack is vented, which this one becomes once the termination is corrected.
WC2. The 3 m at 1:40 is a reasonable gradient; check it is achieved and not just specified.
Basin. Separation is the key point. Take the 32 mm waste into the stack at least 1.5 m above the WC1 branch, or better, give it its own connection higher up, and keep the 1:25 fall. If that is impossible, fit a trap with a deep seal plus an air admittance valve on the branch, with access for maintenance and in a ventilated position rather than boxed in tight. A stub stack with an AAV is an alternative where the drainage above is single storey.
Clean the branch and remake the joints so the 75 mm trap seal is restored; the trap size is adequate once the pressure wave is controlled.
At handover record the as-installed gradients, the branch lengths, the positions of access points and AAVs, and the stack termination height and distance to the window, so the next person can work from a drawing rather than the floor.