A pipe leaking behind a wall usually creates a localized pattern: an expanding stain, soft paint or drywall, musty odor, water sound, or meter movement with no intended use. First confirm whether the damp area follows plumbing and whether it changes when a nearby fixture is used.
Relate the stain to nearby water use
Mark the edge of dampness and see whether it changes after using a shower, sink, or appliance on the same wall. A supply leak may continue without use; a drain leak more often appears while water is flowing.
Check for soft, swollen, or unusually cool material
Drywall that gives way, bubbling paint, or swollen wood indicates moisture that has been present for some time. Do not press on a heavily wet ceiling or remove material when it may fall or wiring may be behind it.
Use the meter to support a supply-line suspicion
With all use stopped, meter movement plus a stain that keeps growing increases suspicion of a pressurized supply leak. A stationary meter does not rule out a drain leak that occurs only when a fixture is used.
Shut water off for an active leak and limit wall opening
If moisture is increasing quickly, close the identified branch or main water valve and arrange repair. A small opening may be needed for drying and access, but locate plumbing and electricity first so the investigation does not create another hazard.
Sources
- U.S. Environmental Protection Agency WaterSense — Home Maintenance — Household maintenance related to water, leaks, and efficient use.