The signs of a hidden water leak, and the ones that mean nothing

These are ordered by how much weight they deserve, not by how often people mention them.

A bill that rose without a reason

The strongest single sign, because water is metered and the meter does not have opinions. Compare against the same month last year rather than against last month, since irrigation and houseguests move usage seasonally. A step change that persists across two or more billing periods, with nothing in the household having changed, is a leak until proven otherwise.

Running water you can hear with everything off

Stand in a quiet house at night with every fixture closed and listen near walls and floors. A faint continuous hiss or trickle is pressurized water escaping. It is worth trusting, and it is worth noting where it is loudest, though on a slab the loudest point is where the sound conducts best rather than necessarily where the break is.

A warm spot on the floor

This one is close to diagnostic and it is specific to hot water lines. A patch of tile or laminate that is noticeably warmer than the floor around it, in a place with no sunlight and no appliance, generally means a hot line is leaking under the slab. It is also the sign people most often dismiss, because a warm floor does not feel like an emergency.

The meter moving with the house shut down

The only one of the eight that is a measurement rather than an inference. Close everything, watch the low flow indicator or take a reading twenty minutes apart, and a change means water is leaving the system. There is a companion article on this site that walks through it, including the second test that says whether the leak is inside the building or in the buried run outside.

A musty smell

Weak on its own. This is a humid region and bathrooms, closets and air handler cabinets smell musty for reasons that have nothing to do with plumbing. A musty smell that is new, localized to one part of the house, and accompanied by any of the four strong signs is worth taking seriously. On its own it more often points at ventilation or at the air conditioning condensate line.

Cracks in tile, drywall or the foundation

Frequently blamed on leaks, and occasionally caused by them, but the ground here moves for its own reasons. Sandy fill settles, the water table rises through storm season and falls back, and buildings crack. A crack that appears suddenly and grows, especially alongside a floor that has gone soft or a door that has stopped closing, deserves attention. An old hairline crack that has been there for years is just an old crack.

A green patch in the lawn

Genuinely useful, but almost never about the house. A single patch of grass that is greener, lusher or wetter than everything around it, particularly in a line between the meter and the building, points at the buried service line. It is also, very often, an irrigation head that leaks, which is a different system and a much smaller job. Rule the irrigation out first.

Low water pressure

The least useful sign on the list. Pressure drops because of a partly closed valve, a clogged aerator, a failing pressure regulator, scale in old galvanized pipe, or something the utility is doing at the street. A leak large enough to drop pressure across a whole house is usually large enough to have announced itself some other way.

Why the wet spot is not the leak

This is the local mechanism that generic advice gets wrong. Almost every house here is slab on grade, and the water table is shallow. Water leaving a pressurized line under concrete does not pool where the pipe failed: it moves through the fill under the slab until it finds a seam, a crack, an expansion joint or the lowest point in the floor, and that is where it surfaces. So the damp baseboard tells you water is arriving there, and very little about where it started. That is the entire argument for locating a leak with acoustic and thermal equipment before anybody opens a floor.