Concealed Slab Leak with a Rising Water Bill
A single-storey house has a water meter at the boundary and all plumbing concealed under a 100 mm concrete floor slab. The quarterly bill has risen from 12 kL to 28 kL with no change in occupancy. With every fixture closed and the internal stop valve open, the meter’s low-flow indicator still turns slowly. When the internal stop valve is closed, the indicator stops. The service line from the meter to the house is 18 m of 20 mm GI, and there is a large neem tree 2 m from the line. The owner wants the leak located with minimum demolition and is worried about the slab. You have a listening stick, a pressure gauge, and a small test pump. The supply pressure is 2.0 bar.
1. Use the meter test to determine which side of the internal stop valve the leak is on, and explain the reasoning.
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With all fixtures closed and the internal stop valve open, a turning low-flow indicator means water is passing somewhere. Closing the internal stop valve stops the indicator. That proves the leak is downstream of the internal stop valve, that is, inside the house, not on the service line between the meter and the valve. The 18 m GI service line and the neem tree are therefore not the first suspects; the service line remains pressurised up to the closed valve, and the meter would still turn if it were leaking. The next step is to divide the house into sections by closing fixture isolators or isolation valves, if fitted, and repeat the meter test. This narrows the leak to a branch or to the concealed slab run.
2. Describe how you would locate the leak under the slab without extensive demolition, using the equipment available.
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Work at night when background noise is low. Use the listening stick on the floor slab, moving in a grid over the suspected run and listening for a steady hiss. Mark the loudest points. Then isolate sections and use the test pump to pressurise each section separately at no more than 2 bar, watching the gauge for a pressure drop; a section that will not hold pressure contains the leak. If the slab is dry, use a moisture meter if available to map damp patches. Probe carefully at the loudest point with a small inspection hole rather than breaking out a large area. Do not exceed the safe test pressure for the old GI or concealed plastic pipe. If the leak cannot be heard, a tracer-gas test by a specialist is the next step.
3. Outline the repair sequence and explain what precautions are needed after the leak is exposed.
Read the worked answer
Cut a local access opening at the confirmed point, keeping it as small as practical. Support the slab edges and protect the surrounding floor. Cut out the damaged section and fit an approved repair coupling or a new section of pipe of the same material and rating. Pressure-test the repaired section at 1.5 times the working pressure, but not above the pipe rating, and hold it long enough to confirm no drop. Flush and disinfect the line if it serves drinking water. Backfill with sand or fine material around the pipe, not hard rubble, and protect it with a warning tape before concreting. If the GI is old and heavily corroded, recommend replacing the whole concealed run rather than repeated patch repairs. Record the repair location for future maintenance.