Leaks: How to Tell If It's the Faucet, the Supply Line, or Something Worse

A homeowner who can correctly identify the source of a leak in the first ninety seconds will pay between twelve dollars and four hundred dollars to resolve it. A homeowner who cannot, and who waits, may pay tens of thousands in remediation. The difference is not skill — it is knowing what to look at first.

Three categories of leak

Every leak inside a house falls into one of three categories: a fixture leak, a supply-line leak, or a concealed leak inside a wall, ceiling, or floor. The diagnostic question is always the same: where is the water coming from when nothing is being used?

Fixture leaks

A faucet dripping from the spout, a showerhead dripping when the valve is off, a toilet that intermittently refills itself — these are fixture leaks. They are usually inexpensive to repair (a $4 cartridge, a $10 flapper, a $15 fill valve), they almost never cause water damage, and they are the source of about eighty percent of "I have a leak" calls.

The distinguishing feature: water comes out of a place water is supposed to come out of, just at the wrong time or rate. There is no water on the floor, no stain on the ceiling, no rising water bill.

Supply-line leaks under a fixture

Look under any sink in the house. You will see two small flexible lines — one for hot, one for cold — running from shutoff valves on the wall up to the faucet. These flexible supply lines (braided stainless or, in older homes, plastic-coated) fail. When they fail, they typically spray or stream into the cabinet below.

The diagnostic: any visible moisture inside a vanity cabinet, any rust ring under a shutoff valve, any swelling of a particleboard cabinet floor. Replacing a supply line is a fifteen-minute job and the parts cost under twenty dollars at any hardware store. Replacing the cabinet because the leak rotted it for six months costs considerably more.

While you are under there, look at the shutoff valves themselves. Old multi-turn valves can begin to weep at the packing nut. These are easy to overlook because the leak is slow and the moisture wicks into wood before it ever reaches the cabinet floor.

Concealed leaks

This is the category that warrants attention. A concealed leak is one inside a wall, above a ceiling, or under a floor. The signs are indirect and they progress in a predictable order.

The progression of a concealed leak

Concealed leaks usually announce themselves in this sequence:

First, an unexplained increase in the water bill of fifteen to forty percent. Most homeowners notice this only in retrospect.

Second, a faint musty smell in one part of the house, usually near a bathroom or laundry room. This is mildew growing on damp drywall behind the paint.

Third, a visible mark on drywall — either a stain (yellow or brown for slow leaks, gray for clean-water leaks) or a soft area that depresses when pushed. By this stage, the drywall is saturated.

Fourth, bubbling paint, peeling baseboard, or warped flooring. By this stage, the leak has been active for weeks or months and the structural members behind the wall may be involved.

The single most useful test

If you suspect a leak in the house but cannot locate it, do this: shut off every fixture, every appliance, every ice maker, every hose bibb. Go to the water meter and note the position of the small flow indicator (often a triangle or a red disk). Wait twenty minutes without using water. Check the indicator again.

If it has moved, water is flowing somewhere. If it has not, you do not have an active leak right now — whatever you are seeing is either a fixture intermittently leaking or residual moisture from a leak that has stopped.

What to do before calling someone

If the leak is at a fixture or under a sink, you can usually handle it yourself for the cost of a small part. Turn off the local shutoff valves (or the main shutoff for the house if the local valves are stuck), and replace the failed component. Online videos are reliable references for every common fixture.

If you suspect a concealed leak — an unexplained water bill, a mysterious smell, a stain on a ceiling — do not delay. A leak that has been active for under forty-eight hours is usually a drywall-and-paint repair. A leak that has been active for two weeks is a drywall, insulation, and possibly framing repair. A leak that has been active for two months is often a mold remediation, which is in a different cost universe altogether.

For situations where a leak has become urgent — standing water, a burst line, or a steady drip from a ceiling — see What Counts as a Plumbing Emergency.