The damp patch that came back: diagnosing a concealed GI branch
A 16-year-old two-storey house in Ludhiana has a 300 mm × 400 mm damp patch on the corridor side of the first-floor bathroom wall, with paint blistering and a green-white salt crust running along a horizontal line. The 15 mm GI branch feeding the WC cistern tees off a 25 mm GI riser buried in that wall; the overhead tank bottom is about 7 m above first-floor level, so working pressure at the branch is roughly 0.7 bar. Dampness is worst after the morning rush when the tank is full, and a previous plumber smeared epoxy putty over a pinhole six months ago - the patch returned about 150 mm lower. The soil pipe runs on the opposite wall, 2 m away. Cold taps elsewhere in the house are normal. The owner wants only the patch opened and 'sealed properly', the family of five has one bathroom, and water cannot be off for more than one working day. Available: pipe wrench, chain vice, hacksaw, ½-inch die set, pressure gauge and hand pump, epoxy putty, GI sockets and nipples, PTFE tape.
1. Before breaking the wall, how would you establish that the 15 mm GI branch - and not condensation or the drainage line - is the source? Give the checks in the order you would perform them and the test pressure you would use.
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Work from least invasive to most invasive.
- Read the pattern. A pressurised-pipe leak follows the pipe route and is continuous; the green-white crust is evaporated water carrying salts and zinc corrosion products, which condensation never leaves. Condensation would be diffuse, worst on cold surfaces, and would also show on other walls and the tank.
- Rule out drainage. The soil pipe is 2 m away on the opposite wall, so the wet patch does not lie on its route. Confirm anyway: plug the pan outlet and fill the trap/waste with clean water, mark the level and leave 30 minutes. No fall, no smell, no grey-black stain means the drainage line is sound.
- Rule out the live branch. Close the nearest stop valve, dry the patch and observe for 12–24 hours with no draw on that branch. Dampness returning only when the branch is live points to the pipe.
- Pressure test. Fit the gauge and hand pump at the cistern connection with the branch isolated. Working pressure is only about 0.07 MPa, so the specification minimum governs - Indian practice for concealed GI lines commonly calls for 1.5 × working pressure held one hour, and often not less than 0.6–1.0 MPa. Check the contract figure and never exceed the rating of the lowest component (the float valve is usually rated around 0.6–1 MPa). A gauge drop with no visible external leak confirms a concealed leak.
2. The owner insists on another putty patch. Decide between patching and replacing, then give the repair sequence with the tools listed, including how you will prove the joint is sound and finish the wall within the one-day limit.
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Decision: cut and replace, not patch. Epoxy already failed in six months, and a pinhole in GI is a symptom of internal corrosion - the wall thickness is reduced over a length, not at one point. A second patch just moves the failure 150 mm down the pipe, as already happened.
Sequence (target: 6–7 hours):
- Isolate and drain; open the wall about 300 mm beyond the damp area.
- Grip the riser in the chain vice so the torque does not strain the buried tee; square-cut with the hacksaw and deburr.
- Thread both cut ends with the ½-inch die using cutting oil, backing off to clear swarf; aim for enough full threads for a sound socket engagement, and trial-fit dry.
- Assemble socket–nipple–socket with jointing compound on the male threads only plus 2–3 turns of PTFE, tightening with two wrenches so the riser is not twisted.
- Clamp the new section, then retest at the same pressure for one hour; flush and check for weeping at both joints.
- Plaster with cement mortar but leave a small removable access panel - never bury a new joint permanently in a wall.
Say clearly to the owner: the one-day limit is achievable for a section repair, but on a 16-year-old riser the honest long-term fix is replacing the full riser in 25 mm CPVC with proper transitions; offer that as a quoted second option rather than discovering the next pinhole in three months.
3. The owner asks why you cannot simply seal it again and keep the cost low. Write the explanation you would give, including one option that respects the one-bathroom constraint.
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Explain in the owner's terms: 'The pipe is not leaking because a joint opened - the steel itself has thinned from inside, so the hole is a weak spot, not the disease. Putty sat on that weak spot for six months and then the next thin patch beside it failed. If we seal again, you will pay for the same wall to be opened a third time, and the damp will keep damaging your plaster and paint.'
Then give evidence rather than opinion: show the corrosion and the crust, and mention the pressure-test result.
Option that protects the family: do the section replacement in one working day, with water restored the same evening. If the owner wants to defer cost, offer a staged plan: section repair now, and a full riser replacement in CPVC quoted for a later date at a fixed price. Do not offer a putty-only option as an alternative - it would fail and the guarantee would be worthless.
Record on the bill what was done, the test pressure and duration, and the advice given, so the recommendation is documented.