Existing apartment stack and shallow boundary connection
You are the certifying plumber for a twelve-unit, three-storey walk-up apartment building built in 1974. The body corporate is converting six units to add a second bathroom and a relocated kitchen sink. Existing drainage is a single 100 mm cast iron soil-and-waste stack with a 50 mm vent through the roof. The building drain falls at 1:80 to a council lateral whose invert is 600 mm below finished floor at the boundary, 36 m away. The new fixtures will add 18 fixture units on the upper two floors. Roof space is congested with ducting, and the body corporate wants no new penetration. On site you notice gurgling at an existing basin when the upstream WC discharges. The cast iron has surface corrosion but no visible perforation. The owner asks whether you can simply connect the new fixtures and certify the existing stack.
1. What must you establish before accepting the existing 100 mm stack and 50 mm vent for the additional 18 fixture units?
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The gurgling is a symptom, not a design detail. It suggests the existing vent path is restricted, the stack is partially blocked, or the trap seal is being siphoned by pressure fluctuations. I would not accept the stack on visual inspection alone.
First, calculate the hydraulic load using AS/NZS 3500.2 principles: count the existing and proposed fixture units, check the stack capacity for its diameter and height, and confirm whether the 50 mm vent is adequate for the combined load. Second, verify the vent is actually open and correctly terminated: smoke-test or air-test the vent, and check for obstructions, incorrect fittings or a blocked roof terminal. Third, CCTV the cast iron stack and building drain to find corrosion, scale, displaced joints or partial collapse. Fourth, confirm the stack is adequately supported and that the new connections can be made without weakening it.
If the stack capacity or vent is marginal, the options are a new stack, a relief vent, re-venting with an air admittance valve where permitted, or limiting the fixtures connected. Certification should be conditional on the test results and on any required upgrade being completed.
2. The boundary invert is only 600 mm below finished floor and the existing drain is at 1:80. What are your options for the new fixtures, and what survey evidence do you need?
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The arithmetic drives the decision. Over 36 m, a 1:60 grade for a 100 mm drain requires 600 mm of fall. If the boundary invert is already 600 mm below finished floor, the upstream invert at the building would finish at floor level, leaving no depth for connections, bedding or cover. The existing 1:80 grade gives only 450 mm of fall, so the upstream invert is about 150 mm below floor level, which is also very tight.
I would obtain surveyed levels of the boundary connection, the existing building drain, finished floor levels and the proposed fixture positions. Then I would test the options:
- relay the drain in 150 mm at a permitted flatter grade, if capacity and minimum cover allow;
- install a pumped waste system for new fixtures that cannot drain by gravity;
- seek a new or lowered connection point with council approval;
- reconfigure fixtures to reduce the load or avoid the shallow section.
I would not certify a connection that relies on an undersized or non-compliant grade. The selected option needs a revised drainage plan, hydraulic check and inspection before cover.
3. What certification and inspection steps would you require before signing off the altered drainage?
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Certification should be the last step, not a paperwork exercise. I would require:
- a revised drainage layout showing fixture units, pipe sizes, grades, vents and the connection to the council lateral;
- survey or laser-level records proving the achieved grades and cover;
- CCTV and water/air test results for the existing stack and drain before and after alteration;
- evidence that any vent defect or blockage has been rectified;
- inspection of supports, penetration seals, fire collars and bedding before the drain is covered;
- confirmation that the new fixtures discharge without gurgling or trap-seal loss under a realistic test.
If the existing stack is retained, the certificate should record the basis of acceptance, any limitations and the tests performed. If defects are found after connection, I would issue a non-conformance and withhold certification until rectified. The body corporate should also be told in writing that future additions may exceed the stack capacity.