Leak Under the Kitchen Sink: Checking the Cabinet and Floor

A leak under the kitchen sink causes water damage in three places: the cabinet floor, the subfloor and finished flooring beneath the cabinet, and the wall behind it. Most of these leaks are slow drips that go unnoticed behind cleaning supplies. Stop the water, empty the cabinet, and check how far the moisture has travelled, because the swollen cabinet base you can see is often the smallest part of it.

Where a leak under the kitchen sink comes from

There are more connections under a kitchen sink than anywhere else in the house. Dry everything with a towel, then run water and watch each one.

• Supply lines and shutoff valves. Look for drips at the nuts and green or white crust on the valve. These leak all the time, whether the tap is on or off.
• Faucet body and pull-out sprayer hose. Water runs down the hose or the underside of the faucet only while the tap is running.
• Drain connections. Slip nuts on the P-trap work loose, and washers flatten with age. These leak only when water goes down the drain.
• Basket strainer. The seal between the strainer and the sink fails and lets water seep when the sink is full.
• Garburator and dishwasher. Check the garburator's body and its connections, the dishwasher drain hose where it joins, and the dishwasher supply line.
• Sink rim and countertop. Failed caulking around the sink or behind the faucet lets splash water run down the back of the cabinet.

Checking the cabinet for water damage

Most cabinet boxes are particleboard or MDF with a thin laminate or melamine skin. Once water gets past the skin at an edge or a screw hole, the core swells and does not shrink back. Look for a cabinet floor that is humped, soft or flaking, side panels that have thickened at the bottom, and laminate lifting at the seams. Check the back panel too, which is often thin hardboard that wicks water upward.

Then look where the water went next. Pull off the toe kick, the strip along the bottom front of the cabinet, and shine a light underneath. Standing water, dark staining or a musty smell there means the leak got through the cabinet floor, usually at the holes cut for the pipes.

Checking the floor under and around the cabinet

Kitchen flooring hides water well. Laminate and vinyl plank float over an underlay, and water slides under them and sits against the subfloor. Sheet vinyl traps it completely. Signs at the surface include plank edges that lift or cup, joints that open, a floor that feels spongy in front of the sink, and darkening along the grout or seams.

If the kitchen sits over a basement, go downstairs and look up. Staining on the subfloor or joists under the sink, or a mark on a finished ceiling, tells you the water went all the way through. That view from below is often the clearest picture of how big the wet area really is. A moisture meter gives the same answer from above without lifting any flooring, which is how we map these before anything is removed.

What can be dried and what has to come out

Solid wood and plywood handle a wetting far better than particleboard. A plywood subfloor that got wet but is still firm can usually be dried in place with air movement and dehumidifiers, checked daily until the wood reads under 16% moisture content. Solid wood cabinet frames often survive too.

Swollen particleboard does not recover. A cabinet base that has puffed up or gone soft is generally replaced, and sometimes the whole sink cabinet is removed so the floor and wall behind it can dry. Laminate flooring that has swollen at the edges will not flatten. Any drywall behind the cabinet that is soft or has visible mold growth gets cut out. Health Canada's advice is that mold indoors should be removed whatever kind it is, and a dark cabinet that has been damp for months is a common place to find it.

We handle the drying, removal of damaged material and mold cleanup. Installing new cabinets, flooring and drywall afterward is work for a contractor, and we can point you to local ones.

Will insurance cover damage from a leak under the sink?

It depends on the policy and on how the leak happened. In general, home insurance in Ontario is written to respond to sudden and accidental water escape, such as a supply line that bursts. Damage from slow, repeated seepage over a long period is commonly excluded. The wording differs between insurers, so call your broker or insurer, describe what you found, and ask before assuming either way. Photos, moisture readings and an itemized invoice make that conversation easier. Most water damage jobs we see run $1,950 to $11,000.

What to do right now

Close the shutoff valves under the sink, or the main if they will not turn. Empty the cabinet, mop up standing water, and take photos of the cabinet floor, the space under the toe kick and the ceiling below. Leave the cabinet doors open. If the cabinet base is swollen or the floor feels soft, have the moisture mapped before you replace anything, so new material is not installed over a wet subfloor.

Common questions