Blocked Stack and Overflow in a Three-Storey Townhouse
A plumber is called to a three-storey townhouse where sewage has backed up into the ground-floor shower. The ground-floor fixtures drain slowly, while the first and second floors drain normally. The building has a single 100 mm UPVC sanitary stack running in a duct, with a ground-floor WC and shower connecting via a 100 mm branch, and upper-floor fixtures entering higher up. A ground-floor inspection opening was opened and water sits in the branch just below the shower trap. The tenant reports the problem began gradually over two weeks. The plumber has a drain camera, an electric eel, and a copy of the as-built drainage plan showing a 1:60 grade to the boundary trap. The downstream boundary trap and sewer connection are reported as flowing freely by the neighbouring unit's plumber.
1. Identify the most probable location and cause of the blockage, and justify why the upper floors are unaffected.
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The pattern points to a blockage in the ground-floor branch or the lower section of the stack below the upper-floor entry points, not in the main drain. Reasoning:
- Upper floors draining normally means the stack above their connection is clear and the main drain downstream of the boundary trap is flowing (confirmed by the neighbour's plumber).
- Water standing in the ground-floor branch just below the shower trap, combined with the shower backing up, localises the restriction between the ground-floor fixtures and the stack.
- A gradual two-week onset suggests a build-up rather than a sudden structural collapse - likely grease, scale, or debris, possibly combined with a grade issue.
Before rodding, the plumber should camera from the ground-floor inspection opening upstream and downstream to confirm the restriction's location and condition, and check the as-built 1:60 grade against actual fall. Rodding blind risks damaging a displaced joint or pushing debris into the stack.
2. Outline the safe and compliant procedure to clear and verify the blockage, including controls for sewage exposure.
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A defensible procedure:
- Isolate and warn - advise occupants not to use fixtures, place signage, and barricade the area.
- PPE and hygiene - gloves, eye protection, respirator where aerosols are possible, waterproof apron, and hand-washing facilities; disinfect tools and the area afterwards.
- Contain - use drip trays and absorbent material under the inspection opening; avoid high-pressure jetting that aerosolises sewage indoors.
- Camera first, then clear with the appropriate tool (eel or jetter) from the inspection opening, working toward the stack.
- Verify - flush with adequate water, re-camera to confirm the pipe is clear and undamaged, and check the grade.
- Report - document findings, any damage, and recommend repair; notify the occupant and, where required, the building owner.
If the camera shows a displaced joint or root intrusion, clearing is temporary and repair must be scheduled.