What a tub overflow drain does and why it fails

Your bathtub overflow drain is the small opening near the top rim of the tub, usually with a chrome or brass cover plate. When water rises above that opening, it drains into a pipe that runs behind the tub wall and connects to the main drain below. Over time, the rubber gasket inside the overflow assembly cracks, the metal corrodes, or the internal flapper valve stops sealing—and water starts leaking into the wall cavity instead of down the drain.

A failing overflow drain shows itself as water dripping from underneath the tub, soft spots in the wall behind the tub, or mold smell near the overflow opening. Replacing the assembly takes two to four hours and costs between $15 and $50 for the part itself, depending on whether you choose basic brass or a stainless steel model. You will need access to the overflow from behind the tub—either from a basement below or a wall cavity you can open from the back.

Key Takeaways

  • The overflow drain assembly sits behind the tub wall and connects to the main drain below; you must access it from behind, not from inside the tub.
  • Shut off the water supply to the house and open a faucet to release pressure before you start work.
  • Remove the overflow cover plate from inside the tub, unscrew the assembly from behind, and pull it straight out along with the rubber gasket.
  • Install the new assembly with a fresh gasket, hand-tighten the connection to the main drain, and test for leaks before closing the wall.
  • If the overflow pipe is corroded or cracked, you may need to replace the section of pipe running to the main drain as well.

Gather your tools and materials before you start

You will need an adjustable wrench or slip-joint pliers, a screwdriver (usually Phillips head), a flashlight or headlamp, and a bucket to catch water that drains from the old assembly. Have a rag or towel ready to wipe up spills. Buy a replacement overflow drain kit that matches your tub's drain size—most are 1.5 inches in diameter, but measure the main drain opening to be sure. The kit should include the new assembly, a rubber gasket, and mounting hardware.

If you are working from a basement below the tub, you may also need a reciprocating saw or hacksaw to cut into the overflow pipe if it is stuck or corroded. Wear work gloves and safety glasses, especially if you are cutting old metal pipe. Have a bucket underneath the overflow connection point before you loosen anything—old assemblies often hold standing water.

Turn off water and locate the overflow assembly from behind

Shut off the main water supply to the house at the meter or shut-off valve. Open a faucet in the lowest part of the house (usually a basement sink or outdoor spigot) to release pressure in the lines. This prevents water from spraying when you disconnect the overflow pipe.

Go to the space behind the tub—either the basement directly below or the wall cavity accessible from an adjacent room. Shine a flashlight up at the underside of the tub to locate the overflow pipe. It runs horizontally from the overflow opening on the tub wall down to a vertical section that connects to the main drain. You will see a large nut or compression fitting where the overflow assembly threads into the tub. This is what you need to unscrew.

Remove the old overflow cover plate and assembly

From inside the tub, unscrew the chrome or brass cover plate that sits over the overflow opening. It is usually held by one or two small screws. Set the plate aside. You may see a rubber gasket or flange behind it—this often comes out with the cover plate.

Go back behind the tub. Use your adjustable wrench or pliers to grip the large nut at the base of the overflow assembly where it threads through the tub wall. Turn counterclockwise to loosen it. Once it is loose enough to turn by hand, unscrew it the rest of the way. Gently pull the entire overflow assembly straight out toward you. A small amount of water may drip out—catch it in your bucket. Inspect the rubber gasket that was sandwiched between the assembly and the tub wall. If it is cracked, hardened, or flattened, it has failed and must be replaced.

Disconnect the overflow pipe from the main drain

Look at where the overflow pipe connects to the main drain below. This is usually a slip nut or compression fitting. Use your wrench to turn it counterclockwise. If it is very tight or corroded, explore penetrating oil (like WD-40) and wait 10 to 15 minutes before trying again. Once loose, unscrew it by hand and set it aside. The old overflow pipe should now slide out completely.

If the pipe is stuck or corroded solid, you may need to cut it with a reciprocating saw or hacksaw. Cut as close to the main drain connection as possible so you have room to work with the new assembly. Wear safety glasses and work slowly to avoid damaging the main drain fitting itself.

Install the new overflow assembly and gasket

Slide the new overflow pipe up through the opening in the tub wall from behind. Position the rubber gasket so it sits flat against the inside surface of the tub wall, centered on the overflow opening. The gasket should be smooth and free of wrinkles or folds. Screw the large nut onto the assembly threads by hand first, then use your wrench to tighten it firmly—snug but not so hard that you crack the tub. A quarter-turn past hand-tight is usually correct.

From inside the tub, screw the chrome cover plate back on over the overflow opening. Make sure it sits flush against the tub wall with no gaps. If the gasket is visible around the edges, the cover plate is not seated properly—remove it, reposition the gasket, and try again.

Reconnect the overflow pipe to the main drain

Go back behind the tub and connect the new overflow pipe to the main drain using the slip nut or compression fitting. Hand-tighten it first, then use your wrench to snug it down. Do not over-tighten—these fittings are straightforward to crack. The connection should be tight enough that no water leaks when you run water into the tub, but not so tight that you strip the threads.

If the old pipe was corroded and you had to cut it, you may need to install a new section of PVC or metal pipe between the overflow assembly and the main drain. Most overflow kits include a short section of pipe, but if yours does not, measure the distance and buy a replacement section at a plumbing supply store. Dry-fit it first to make sure the connections line up before you tighten them permanently.

Test for leaks and close up the wall

Turn the main water supply back on. Fill the tub slowly and watch the overflow opening as the water level rises. The water should drain smoothly through the overflow without backing up or leaking around the cover plate. Let the tub fill all the way to the overflow and let it drain for at least one minute. Go behind the tub and check for any drips at the connection points. Wipe everything dry with a rag and wait 10 minutes, then check again to make sure no new water has appeared.

If you see water dripping from the connection at the main drain, tighten that fitting another quarter-turn. If water is leaking around the cover plate from inside the tub, the gasket is not seated correctly—drain the tub, remove the cover plate, and reposition the gasket. Once everything is dry and holding, close up the wall cavity or basement access. If you opened a wall, patch and finish it according to your wall type (drywall, plaster, or tile).

Frequently Asked Questions

Can I replace the overflow drain without turning off the main water supply?

No. Turning off the water and opening a low faucet prevents pressure from building up when you disconnect the overflow pipe. Without this step, water will spray from the open connection and make the job much messier and harder to control.

What if the overflow assembly is stuck and won't unscrew?

explore penetrating oil around the nut and let it soak for 15 to 30 minutes. Tap the nut gently with a hammer to help break the corrosion loose, then try again. If it still will not budge, you may need to cut the assembly out with a reciprocating saw and replace the entire section of pipe.

How do I know what size overflow drain to buy?

Most bathtubs use a 1.5-inch overflow drain, but measure the diameter of your main drain opening to be sure. Bring that measurement to the plumbing supply store, or measure the old assembly before you remove it. The replacement kit should match your tub's drain size exactly.

Is it normal for a little water to drip from the overflow when the tub is full?

No. The overflow should drain water silently and completely. If you see dripping or hear gurgling, the gasket is not sealing properly. Remove the cover plate, check that the gasket is flat and centered, and reinstall it. If dripping continues, the gasket may be defective and need replacement.

Can I use plumber's tape or caulk to seal the overflow connection instead of replacing the gasket?

No. The rubber gasket is designed to compress and create a watertight seal under pressure. Tape or caulk will not hold up to the constant water pressure and temperature changes inside the tub. Always use a new gasket when you install a new overflow assembly.