Water inside a sink cabinet can come from the faucet, supply lines, shut-off valves, drain or P-trap. Here is how to narrow down the source safely and decide when plumbing repair is needed.
Why Is My Faucet Leaking Under the Sink?
If you find water underneath your kitchen or bathroom sink, empty the cabinet, place a bucket or towels below the wet area, and gently close the hot and cold shut-off valves.
Dry the plumbing completely before testing anything. Then check the supply lines, shut-off valves, faucet connections, mounting area, drain, tailpiece and P-trap one section at a time.
A leak that appears only while the faucet is running may come from the faucet body, hose, mounting seal or connection. Water that appears while the sink drains points more toward the drain assembly or P-trap. A leak that continues even when the faucet is off may involve a pressurized supply line or shut-off valve.
If water is spraying, a supply hose is damaged, the valve will not shut off, or the cabinet is already swelling, stop using the sink and arrange plumbing service.
Saving Plumbing provides faucet, sink and leak repairs throughout Scarborough and surrounding Toronto and GTA communities.
Why water under a sink should not be ignored
Finding a few drops inside a kitchen cabinet may not seem urgent. The faucet may still work normally and there may be no visible drip from the spout.
The trouble is that under-sink leaks often stay hidden.
Water can soak into cabinet panels, damage stored items, stain flooring and create damp conditions where odours or mould-like growth may develop. A slow plumbing leak may continue for days before anyone notices it.
The puddle also does not necessarily identify the failed part.
Water can run along a braided supply hose, faucet shank or drain pipe before falling onto the cabinet floor. That means the wettest location is not always where the leak began.
The useful question is:
When does the water appear?
Does it leak:
- when nobody is using the sink?
- only while the faucet is running?
- when the pull-down sprayer is used?
- while the basin is draining?
- immediately after shutting the faucet off?
That timing can quickly narrow down the source.
For homeowners, tenants and businesses in Scarborough, catching the problem early can mean the difference between replacing a small plumbing component and dealing with damaged cabinetry or flooring later.
The most common causes of a faucet leaking under the sink
1. Loose or damaged faucet supply lines
Flexible supply lines carry hot and cold water from the shut-off valves to the faucet.
Problems can develop around:
- compression fittings
- threaded connections
- internal washers
- braided hose material
- faucet-side connections
Because these lines remain under pressure whenever the valve is open, a damaged supply line may leak even when the faucet itself is not being used.
Dry both supply lines completely and check them from top to bottom.
Do not look only where the water finally drips. Moisture can travel several inches along the hose first.
A supply line that is rusted, kinked, bulging, frayed or wet along its length should generally be replaced rather than patched.
2. A leaking shut-off valve below the sink
The hot and cold shut-off valves are another common source of water beneath a sink.
A valve may leak from:
- the valve stem
- handle area
- compression connection
- threaded outlet
- connection to the supply pipe
Older valves can also become difficult to close completely.
If a shut-off valve is seized or requires excessive force, do not keep turning it. A deteriorated valve can begin leaking more heavily when disturbed.
This is one reason an under-sink leak should be diagnosed as a complete plumbing system rather than automatically blamed on the faucet.
3. A faucet cartridge, O-ring or internal seal is leaking
Sometimes the failure begins inside the faucet itself.
Water can travel downward through the faucet body and appear underneath the sink rather than dripping visibly from the spout.
Depending on the faucet design, the failed part could be:
- a cartridge
- O-ring
- ceramic disc
- washer
- valve stem
- internal hose
- seal
A useful clue is timing.
If the cabinet remains dry when the faucet is off but water begins appearing shortly after you turn it on, the faucet body or one of its internal connections deserves closer inspection.
The right repair part depends on the faucet manufacturer and model. Replacing a random washer will not necessarily solve the problem.
4. A loose faucet mounting assembly
Try moving the faucet very gently at the sink deck.
Does it rock?
A loose faucet can place additional stress on the supply lines and connections below. Water used around the faucet base may also make its way through the countertop opening and into the cabinet.
The mounting nut may simply need adjustment in some cases.
However, tightening it is not a substitute for repairing:
- a deteriorated gasket
- damaged mounting hardware
- a cracked faucet
- a failed seal
If the faucet has been moving for some time, inspect the sink or countertop around the mounting point for staining or water damage as well.
5. The leak is actually coming from the sink drain
This is one of the easiest under-sink plumbing problems to misdiagnose.
The faucet may have nothing to do with the leak.
Water can escape from:
- the sink strainer
- drain flange
- tailpiece
- slip-joint connection
- P-trap
- drain washer
- cracked drain component
Drain-side leaks commonly appear while water is leaving the basin rather than while the faucet is simply running.
Here is a simple way to separate the two.
Dry everything beneath the sink.
Run a small amount of water while closely watching the faucet connections.
Then partially fill the basin.
Release the water and watch the drain flange, tailpiece and P-trap.
If everything remains dry until the basin begins draining, the drain assembly becomes the more likely source.
6. A pull-out or pull-down faucet hose is leaking
Modern kitchen faucets frequently use pull-down or pull-out spray heads.
That means there are additional components beneath the sink, including:
- flexible spray hoses
- quick-connect fittings
- hose weights
- diverter connections
A damaged hose may only leak while water is flowing through the sprayer.
You may notice water running down the hose before dripping from its lowest point.
Operate the spray head carefully while watching underneath the sink. Avoid aggressively pulling or twisting the hose during testing.
Because quick-connect fittings differ between faucet brands, identifying the manufacturer before purchasing replacement parts can save a lot of frustration.
How to find where the under-sink leak is actually coming from
You do not need to start taking the plumbing apart immediately.
Start with observation.
1. Empty the cabinet
Remove cleaners, garbage bins, towels and anything else stored underneath the sink.
You need clear access to the entire plumbing assembly.
2. Dry the cabinet
Use towels to remove all existing moisture.
Place a bucket underneath the lowest plumbing connection.
3. Gently close the shut-off valves
Turn the hot and cold valves clockwise without forcing them.
If a valve will not move or begins leaking when touched, stop.
If necessary and if you know how to do so safely, the home’s main water supply can be shut off.
4. Dry every plumbing component
Wipe down the:
- supply hoses
- faucet shank
- shut-off valves
- sprayer hose
- tailpiece
- drain connections
- P-trap
You want to know where new water appears.
5. Test one thing at a time
Turn the faucet on briefly.
Watch the supply connections.
Operate the sprayer.
Watch again.
Then allow water to drain through the sink.
Testing each function separately makes the source much easier to identify.
6. Use dry paper towel to detect small leaks
A dry paper towel works surprisingly well for slow plumbing leaks.
Hold or wrap it underneath a connection.
The first small wet mark often identifies the source before enough water accumulates to create a visible drip.
7. Stop if the leak becomes pressurized
Do not continue troubleshooting if you see:
- spraying water
- a split hose
- a rapidly leaking valve
- a fast continuous stream
- a fitting beginning to separate
Shut off the water and arrange repair.
Is water underneath the kitchen sink different from water beneath a bathroom sink?
The basic diagnostic process is similar, but the plumbing layout may differ.
A kitchen sink may include:
- pull-down faucet hoses
- dishwasher connections
- deeper basins
- disposal connections where installed
- multiple drain bowls
Bathroom vanities usually have fewer components but can offer much less working space.
In either case, do not assume the faucet itself is responsible simply because the water is directly below it.
Drying the cabinet and recreating the leak under controlled conditions provides far more useful information.
Safe temporary steps while waiting for a plumber
The goal of a temporary response is to prevent additional damage.
It is not to create a permanent repair.
You can:
- remove everything from the cabinet
- place a bucket underneath the leak
- dry the cabinet floor
- keep the affected shut-off valve closed
- avoid using the sink unnecessarily
- photograph the leak and damaged component
- note whether the leak happens continuously or only during use
A photo of the faucet itself can also help identify the make or model.
If water has reached electrical equipment, outlets, finished flooring, a wall cavity or the ceiling below, treat the situation more seriously and keep people away from the affected area where necessary.
Avoid these common under-sink repair mistakes
Do not keep tightening a nut after it becomes resistant.
More force is not always better. Overtightening can deform a washer, damage threads or crack a fitting.
Do not wrap every connection in plumber’s tape.
Some plumbing connections seal using washers or compression fittings rather than thread tape.
Do not use glue, caulking or epoxy to patch a failing supply hose.
A damaged pressurized hose should be replaced.
Do not disconnect a supply line while it is pressurized.
Turn off the water first.
And do not assume the leak has stopped simply because the cabinet looks dry several hours later.
Test it again before returning stored items.
Should I repair the faucet or replace it?
A repair usually makes sense when the faucet is in otherwise good condition and the failure is limited to a serviceable component.
That could include a:
- cartridge
- O-ring
- gasket
- washer
- supply hose
- mounting connection
- sprayer hose
Replacement becomes more attractive when the faucet has several problems at once.
For example:
- significant corrosion
- damaged faucet body
- repeated leaks
- failing handles
- unreliable sprayer
- unavailable replacement parts
- loose or deteriorating mounting components
Age alone does not determine whether a faucet should be replaced.
Condition, repair-part availability and the cost of repeated service matter more.
The replacement faucet also has to fit the existing sink configuration, mounting holes, countertop thickness and available cabinet clearance.
What about under-sink leaks in Scarborough condos?
If you live in a condominium, identifying where the leak originates is especially important.
A fixture, faucet or supply hose located entirely inside the unit may be different from a problem involving shared plumbing or building infrastructure.
If water is spreading beyond the cabinet, entering another unit or appears to originate from piping you cannot isolate, notify the appropriate property manager or building representative.
Documenting the leak with photographs before repair can also be useful.
Do not continue running the sink simply to reproduce a leak if water is already travelling into the floor, wall or neighbouring unit.
When is a faucet leaking under the sink an emergency?
An occasional drip and an active pressurized leak are not the same situation.
Seek urgent plumbing assistance when:
- water is spraying from a supply line
- a flexible hose has split
- the shut-off valve will not stop the water
- the cabinet is filling quickly
- water is spreading across the flooring
- water is entering the ceiling below
- the leak is reaching electrical equipment
- water continues flowing even when the faucet is not being used
A slow leak does not usually require the same emergency response, but it should still be repaired.
If the cabinet is already swollen, stained or developing an odour, the leak may have been present much longer than it appears.
Faucet and Sink Repair
When you see water beneath the sink, the puddle only tells you where the water ended up. It does not necessarily reveal where the plumbing failed.
The source could be a supply hose, shut-off valve, faucet body, mounting seal, sprayer connection, drain flange, tailpiece or P-trap.
Start by controlling the water. Dry the area completely. Then test one component at a time.
If the leak is pressurized, difficult to isolate or connected to corroded plumbing, professional diagnosis is usually the safer option.
Saving Plumbing is based in Scarborough and provides plumbing and drain services for homes and businesses throughout the area.
Service is available in communities including Agincourt, Malvern, Birch Cliff, Cliffside and surrounding Scarborough neighbourhoods, with additional coverage throughout Toronto and the GTA.
For faucet and sink problems, you can also visit our faucet and sink repair and installation services in Scarborough
If the water is coming from the drain rather than the faucet, see our drain repair services.
Call (416) 271-6745 or request a free quote.
Frequently Asked Questions
Why is my faucet leaking underneath the sink but not from the spout?
The water may be escaping from a supply hose, shut-off valve, faucet connection, internal hose, mounting gasket or sprayer connection rather than the spout itself.
Dry everything beneath the sink and watch where moisture first returns. If water continues appearing while the faucet is off, inspect the pressurized supply side first.
How do I temporarily stop a faucet leak under the sink?
Place a bucket under the leak and gently close the appropriate hot or cold shut-off valve.
Remove anything stored inside the cabinet and dry the area.
If the shut-off valve is leaking or will not stop the water, shut off the main water supply if you can do so safely and arrange plumbing service.
How can I tell whether the faucet or sink drain is leaking?
Dry the entire area first.
Run the faucet while preventing water from immediately draining away and inspect the faucet and supply connections.
Then release water through the drain.
If the plumbing remains dry until the water begins draining, inspect the drain flange, tailpiece and P-trap.
Why does my kitchen sink only leak underneath when the faucet is running?
A leak that occurs only while the faucet is operating may involve the faucet body, internal hose, cartridge area, pull-down sprayer connection or mounting seal.
Watch the plumbing underneath while someone operates the faucet above.
Why does my sink only leak underneath when water drains?
That pattern usually points toward the drain rather than the faucet.
Possible sources include the sink strainer, drain flange, tailpiece, slip-joint washer or P-trap.
Can I replace a leaking faucet supply line myself?
An accessible supply line can sometimes be replaced after the water is shut off and the correct connection type has been confirmed.
Corroded shut-off valves, seized fittings, awkward cabinet access or leaks that continue after replacement are better handled professionally.
Should I replace an old faucet that keeps leaking?
Repeated leaks can make replacement more practical, especially when the faucet is corroded, repair parts are unavailable, the sprayer or handles are also failing, or previous repairs have not lasted.
A structurally sound faucet with a failed cartridge, seal or supply hose may still be worth repairing.
Who repairs leaking faucets in Scarborough?
Saving Plumbing provides faucet, sink, supply-line and related plumbing repairs in Scarborough and surrounding Toronto and GTA communities. For an active leak, provide the plumber with details about whether the water appears continuously, while using the faucet, while operating the sprayer or while the sink drains.
