Shophouse Trunking Drip: Drain Backfall After Retrofit

Water escaping from trunking on a run that drained without issue for years, beginning shortly after unrelated works elsewhere in the shophouse. The line itself tests clear. Water still emerges partway along it.

A clear line that still leaks is not a blockage, which removes the assumption most people arrive with. Water escaping partway along a run means it is not travelling to the end. On a gravity drain the only reason for that is gradient. Establishing what has happened on that route recently is more useful than testing the line again, because the line will keep on testing clear however many times it is checked.

A condensate drain works only because it falls. Trunking hides that fall completely, so anyone working along the route sees a pipe in a channel rather than a gradient that has to be preserved. Re-clipping it a little higher at one point, or lifting it to pass something new, is enough to leave a level or rising section. Water then backs up behind that section and escapes at the nearest joint or opening it can find.

The affected section is re-set to restore an unbroken fall, which is a short piece of work once the cause is understood. The transferable point is a question rather than a technique. Before investigating a drain that used to work perfectly well, ask what building work has happened on its route. That question frequently ends the diagnosis before the trunking even comes off the wall.