How to Find and Repair Hidden Plumbing Leaks

Updated: Jan. 12, 2024

Troubleshoot and fix minor drips before they cause major damage.

Next Project

An hour or less




Less than $20


We show simple ways to diagnose and fix common plumbing leaks before they cause rot and other expensive damage. You'll find most of them around bathtubs and showers, drains, sinks and toilets.

Tools Required

  • 4-in-1 screwdriver
  • Adjustable wrench
  • Caulk gun
  • Flashlight
  • Slip joint pliers
  • Utility knife

Materials Required

  • Caulk
  • Drain plug
  • Gaskets and seals
  • Silicone caulk
  • Toilet shims
  • Wax ring

Project step-by-step (8)

Step 1

Spotting “splash leaks”

Splash leaks are simply water escaping past a shower curtain or a shower door. Plumbers tell us it’s the most common type of bathroom leak.

Although it may sound minor, this can cause major damage when water seeps into the subfloor where the flooring meets the tub or shower. Before long, the vinyl flooring or tiles begin to loosen. Even worse, the plywood subfloor delaminates and rots, requiring a huge, expensive tearout and replacement project.

Signs of trouble:

  • Curling vinyl flooring or loose tiles next to the tub.
  • Peeling paint or flaking, chalky-looking wood finish near the shower.
  • Water stains on the ceiling or joists below.
  • Mold spots on the wall or floor near the tub or shower.
  • If you use a curtain, look for standing water on the floor after you shower.

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How to find the source of a leaking bathtub drain:

  • If you have a shower door, splash water all around the door and frame. Leaks around the frame may take five minutes or longer to show up.
  • If the door has rubber gaskets or a rubber door sweep, check them for gaps.
  • Also check for any gaps in the caulk where the shower or tub meets the flooring.

How to fix it:

  • Be sure to overlap sliding doors correctly when you close them. The inner door should be closest to the faucet.
  • If you have a shower curtain rather than a door, make sure you close it completely when you shower, or add a splash guard.
  • Seal a leaking frame by running a small bead of tub and caulk around the inside of it. Force the caulk into any gaps between the frame and the shower surround. Quickly wipe away all the excess caulk. When the caulk dries, test for leaks again.
  • Replace any worn gaskets or door sweeps. Bring the old one to a home center or plumbing supply store and look for a matching replacement. Be sure to get the correct size to fit.
  • If the old caulk along the floor shows gaps, scrape it out and run a new bead.

Homeowner story:
“I noticed the floor tile along the tub was coming loose. I pushed on it and it crunched down into the underlayment, which was totally rotten. We ended up replacing all of the tile and part of the subfloor.”

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Common tub and shower leaks

The most damaging tub and shower leaks occur when water splashes out of the enclosure. Tile and drain leaks can also cause major damage.

Step 2

Tub and shower: Drain leaks

Drain leaks allow water to sneak around the outside of the drain where it’s connected to the tub or shower. This is especially common with plastic or fiberglass tubs and shower pans, since these materials flex slightly when you stand on them, often breaking the seal around the drain.

These leaks can stain or destroy the ceiling below or rot floor joists. In the case of a tub set on a concrete slab, the leak will ruin flooring in the bathroom or adjoining rooms.

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Signs of trouble:

  • Water stains on the ceiling or joists below.
  • Loose flooring near the tub or damp floors in adjoining rooms (if the tub is on a concrete slab).

How to find the source:

  • If you can see the underside of the drain through an access panel or open ceiling, partially fill the tub and then release the water. In a shower, plug the drain with a rag and then release the water. Check the drains and traps for leaks from below through the access panel.
  • If you don’t have access to the underside of the drain, plug the drain and add enough water to form a small puddle around it (photo). Mark the edge of the puddle by setting a bottle of shampoo next to it. Then wait an hour. If the puddle shrinks, the drain is leaking.
  • Don’t rely on your tub stopper for this test; it may leak. Remove the stopper and insert a 1-1/2-in. test plug (find them at home centers). Remove the grate and use a 2-in. plug for a shower.

How to fix it:

  • To repair a tub drain, unscrew the drain flange from above. Then clean the flange and apply silicone caulk. Also remove the rubber gasket under the tub’s drain hole and take it to a home center to find a matching gasket (be sure to get the correct size to fit). Slip the new gasket into place and screw in the drain flange.
  • You might have to access to a shower drain from below. If this is the case, tighten the ring nut that locks the drain to the shower pan. Replace the drain assembly if that doesn’t work. If you don’t have access beneath the drain, cut a hole in the ceiling below or replace the drain assembly with a WingTite drain.

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Close-up of test plug

Measure the drain diameter and buy a test plug to fit.

Step 3

Tub and shower: Tile leaks

Tile leaks occur when water seeps through deteriorating grout or caulk and gets into the wall behind the tile (Figure A). Depending on the materials used to set the tile, this can lead to tile falling off the wall, severe rotting of the wall framing, and damage to the subfloor, joists or ceiling below.

Signs of trouble:

  • Loose tiles.
  • Persistent mold.
  • If the shower is against an exterior wall, you may find an area of peeling paint outside.
  • Stains on the ceiling under the shower.

How to find the source:

  • Examine the grout and caulk joints for gaps. You almost always find mold here.
  • If you have loose tile behind the tub spout or faucet, open the access panel behind the faucet and look for dampness or stains.

How to fix it:

  • Remove the old grout, caulk and loose tiles.
  • If the surface behind the tile is still solid, you can reattach tiles, regrout and recaulk using tub and tile caulk.
  • If more than a few tiles are loose or if the wall is spongy, you’ll have to install new backer board and tile, or a fiberglass surround.
Step 4

Toilet flange leaks

These leaks occur where the toilet meets the waste pipe below. They allow water to seep out at every flush, which will wreck flooring, rot the subfloor and joists and damage the ceiling below.

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Signs of trouble:

  • Water seeping out around the base of the toilet.
  • Loose or damaged flooring.
  • Stains on the ceiling below.
  • A toilet that rocks slightly when you push against it. This movement will eventually break the wax seal between the toilet and the closet flange.

How to find the source:

If you have ceiling stains below the bathroom, measure from stacked walls (Photo 1) before you go through the hassle of removing the toilet. In most cases, the stain occurs close to the source. If the stain is near the toilet, a leaking flange is the most likely source.

Unscrew the toilet bolt nuts and remove the toilet (Photo 2). Look for these leak sources:

  • The flange is level with or below the surrounding floor surface.
  • Cracks in the flange.
  • Bolts or the slots they fit into are broken.
  • The flange is loose, not screwed solidly to the subfloor.
  • Cracks around the toilet’s horn.

How to fix it:

  • If you don’t find any of the problems listed above, reinstall the toilet with a new wax ring.
  • The flange might be too low. Fix it by installing a plastic flange riser over the existing flange.
  • If the flange or bolt slots are broken, install a metal repair flange.
  • If the toilet rocks because the floor is uneven, slip shims under the toilet when you reinstall it.

Homeowner story:
“I had noticed the toilet rocking slightly for a few years, but I kept putting off the repair. Then one day the ceiling under the toilet fell in. It turned out that the wax ring had been leaking for years. The subfloor around the toilet was rotted, and the more it rotted, the more the toilet rocked and the more water leaked out with every flush.”

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Toilet leaks

Toilet leaks can come from the water supply or tank, but the most damaging leaks occur at the flange and wax ring.

Step 5

Sinks: Sink rim leaks

Dribbling water around the sink rim and faucet base with a spongeTMB Studio

Test for rim leaks

Dribble water around the sink rim and faucet base with a sponge. Then look for leaks below using a flashlight.

Sink rim plumbing leaks allow water to seep under the rim or the base of the faucet. They will gradually destroy your cabinets and countertops in kitchens and bathrooms.

Signs of trouble:

  • Puddles, dampness or water stains inside the cabinet.
  • Loose plastic laminate near sink.
  • A loose faucet base.
  • Deteriorating caulk around sink.

How to find the source:

  • If you have a plastic laminate countertop, examine the underside of the countertop using a flashlight. Look for swollen particleboard or other signs of water damage.
  • Dribble water around the sink rim and look for leaks (photo).

How to fix it:

  • Tighten the faucet base by turning the mounting nuts underneath it.
  • If the sink rim is caulked, scrape away the old caulk and recaulk.
  • Tighten the clips under the sink rim that clamp the sink to the countertop.

Homeowner story:
“The caulk around my cast iron sink was in bad shape for years, but I didn’t worry about it until the plastic laminate in front of the sink began to loosen. I stuck my head under the sink and saw that the particleboard under the sink rim was black and swollen.

“When I poked at the rotten particleboard, it crumbled like charred wood. I had to replace all my countertops at a cost of $800. But I’m glad the old countertop gave me a warning sign. With a few more months of rot, that 100-lb. sink could have fallen into the cabinet below.”

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Figure C: Sink leaks

Sink leaks can occur at any plumbing joint. But the most common plumbing leaks happen at the sink rim, shutoff valves, supply line connections and slip joints in the waste lines.

Step 6

Undersink plumbing: Supply leaks

Wiping Water Supply Tap Under The Sink with A Dry TissueTMB Studio

Check for supply leaks

Don’t rely on your sense of touch to find tiny undersink plumbing leaks. Wipe each connection with a dry tissue. Then look for a wet spot on the tissue.

Supply leaks under the kitchen sink or bath vanity can go unnoticed for a long time since they’re usually at the back of the cabinet. Water can run down the pipes into the floor or subfloor, rotting the sink base, the floor and the framing.

Signs of trouble:

  • Puddles, dampness or water stains under sink plumbing and inside the cabinet.
  • Stains on the ceiling below.

How to find the source:

  • Dab shutoffs and connections with a dry tissue or paper towel and look for wet spots (photo).
  • Run the dishwasher and check for plumbing leaks under it.

How to fix it:

  • If the valve stem on a shutoff valve drips, tighten the packing nut. If the leak doesn’t stop, replace the valve.
  • For other leaks at the shutoff valve or at the faucet, try tightening the compression nut first. If that doesn’t stop the leak, disassemble the fitting, coat the ferrule or gasket with Teflon joint compound (available at home centers and hardware stores) and reassemble the connection.
Step 7

Sinks: Drain leaks

Drain leaks in kitchens and bathrooms usually occur at the drain or at the slip joints in the drainpipe. Hidden behind boxes and bottles, these leaks can damage flooring, cabinets and even ceilings below before you notice them.

Signs of trouble:

  • Puddles, water stains or a dank odor in the cabinet.
  • Loose or damaged flooring in front of the sink.

How to find the source:

  • Fill the sink bowls. As they drain, check all joints from the sink to the wall with a dry tissue (photo).
  • Run and drain the dishwasher and check the waste hose connection.

How to fix it:

  • For a slip joint leak, first tighten the slip nut. If that doesn’t work, disassemble the joint, coat the washer with Teflon joint compound and reassemble.
  • For a leak from the basket strainer, tighten the ring nut under the sink. If the leak continues, disconnect and remove the basket strainer. Reassemble it using plumber’s putty as a sealant under the basket’s rim.
Step 8

Two slow plumbing leaks that signal catastrophe

If you notice puddles near your water heater, check the pressure relief valve and the drain valve. If either is dripping, replace the valve. Otherwise, the tank is leaking and you need a new water heater. Don’t delay. Tank leaks often start slow and then suddenly burst days or weeks later, causing a major household flood.

The same goes for washing machine supply hoses. If you notice a tiny leak in the hose itself or at the crimped metal fittings at the ends, replace the hose. Otherwise the hose may eventually burst, releasing a continuous flow of water.

Test for hidden plumbing leaks

The vast majority of leaks occur at or near plumbing fixtures like tubs, sinks and toilets. But if you suspect a leak in the water supply system, there’s a simple way to check it— even if the pipes are hidden inside walls.

First, turn off all your faucets. If you have drippy faucets or a toilet that runs between flushes, close the shutoff valves. Then go to your water meter and check the position of the “1-cu.-ft.” dial. Check the dial again two hours later. If the dial has moved, you have a leak in the water supply.

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