Stack sizing and branch connection in a refurbished block
A 1970s block of 24 flats is being refurbished. The existing 100 mm cast-iron soil stack serves four floors, with one WC, one basin, and one bath per flat. The client wants to add a second bathroom to each flat, doubling the fixtures. The contractor proposes to keep the 100 mm stack and simply connect the new branches at each floor with 75 mm branches and 82 mm traps. The top-floor flat reports gurgling and slow drainage after a trial. The stack terminates 0.6 m above the roof, close to a dormer window that opens.
1. Assess whether the existing 100 mm stack can serve the doubled fixture load, and explain the gurgling.
Read the worked answer
A 100 mm stack can carry a substantial load, but the key issue is not just the total flow; it is the branch connections and the venting. Doubling the fixtures doubles the discharge into the stack, and if the branches enter at the same level or too close together, the stack becomes a two-phase flow with slugs of water that compress air below them.
The gurgling is caused by pressure fluctuation in the branch. When a slug of water falls down the stack, it draws air from the branch, and the trap seal is pulled or the air is sucked through the trap, producing the gurgle. This is a symptom of inadequate venting, not necessarily of an undersized stack.
The 82 mm traps and 75 mm branches are also a concern: a bath trap should be at least 40 mm, and a WC branch should be 100 mm. Reducing branch sizes creates a bottleneck and increases the risk of blockage.
The design should be checked against the local code's discharge unit method. If the stack is adequate, the fix is to add a vent pipe or an air admittance valve and to correct the branch sizes and connections.
2. Explain the problem with the stack termination and what must be done.
Read the worked answer
The stack terminates only 0.6 m above the roof and close to an opening dormer window. This is a health hazard: foul air from the drainage system can enter the room through the window. Most codes require the stack to terminate at least 0.9-1.0 m above the roof surface and at a safe distance from any opening window, usually at least 3 m horizontally, or above the eaves line.
The termination must be raised to the required height and relocated away from the window, or a proprietary vent terminal with a suitable cap must be fitted. The cap should not restrict the free flow of air, because the stack needs to breathe. The contractor should also check that the stack is not blocked by a bird's nest, which is a common cause of gurgling and slow drainage.
3. What tests should be carried out on the drainage system before the flats are handed over?
Read the worked answer
The drainage system should be tested in two ways. First, an air test: the section is sealed, air is pumped in to a specified pressure (typically 38 mm water gauge or as the code requires), and the pressure should hold for a set period without loss. This proves the pipework and joints are airtight.
Second, a water test: the system is filled with water to the highest point and left for a period (often at least 15 minutes to an hour) to check for leaks at joints and to confirm that the traps hold their seals. Alternatively, a smoke test can be used to trace connections and confirm that no foul air escapes into the building.
Finally, each trap should be checked for seal depth, and the stack should be checked for free flow by running water from the top floor and observing that it discharges freely at the base. The results should be recorded and given to the client.