Why does my bathtub leak through the ceiling below?

Write down when the stain grows. Pay attention to three moments: while the tub fills, while someone showers, and when nothing is running at all.

It drips while the tub is filled or full. That points below the waterline: the drain, the overflow, or the pipe connecting them. Oatey describes the parts this way: "A bath waste and overflow system is a critical plumbing component that connects your bathtub to your household drain line." It has a drain at the bottom and an overflow opening near the top, and each one seals with a gasket or a ring of putty or caulk. When one wears out, water slips past it every time the tub holds water, then runs down the pipe and into the ceiling below. If it leaks only when the tub is full or nearly full, the overflow gasket is the prime suspect.

It drips only while someone showers. That points above the waterline: the caulk line where the tub meets the tile, cracked grout, a failing tub spout seal, or a shower door track that lets spray onto the floor. Water gets behind the tub surround, soaks the wall framing and runs down to the ceiling. Nothing leaks when the tub is only filled, because the water never reaches those spots.

It drips even when nothing is running. Then the leak is on a pressurized line: a supply line to the faucet, the shower valve or a fitting behind the wall. This one runs day and night. Read leaking shower valve behind the wall for the signs, and check your meter with all fixtures off. How to do a water meter leak test shows how.

It drips after the tub drains or after someone flushes. A drain or trap joint under the tub, or the toilet next to it, is the suspect. Oatey's wax ring guide says: "If you're replacing the wax ring due to a leak at the toilet base or water stains on the ceiling below, identify the root cause before buying materials." A toilet with a failing wax ring can stain the ceiling below just as a tub can, so check both. See toilet leaking at the base if the floor around the toilet is wet.

Do these one at a time, and give each about 15 minutes. Put a bucket or towels under the drip and keep a light on the ceiling so you can watch.

1. 1The tub-full test. Close the stopper and fill the tub to just under the overflow opening. Wait 15 minutes. If the ceiling below starts to drip, the drain shoe or its gasket is leaking.
2. 2The overflow test. Fill the tub a little further until water begins to run into the overflow opening, then stop the water. Oatey's procedure for checking a bath waste is a draining test: "closing the stopper and filling the tub with cold water until it starts to flow through the overflow plate, and then shut off water and inspect all connection points behind the tub." If a stain grows now and not in step 1, the overflow gasket or the pipe from the overflow is leaking.
3. 3The drain-down. Pull the stopper and let the tub empty while you watch the ceiling. A drip while it drains points to a loose drain joint or trap connection under the tub.
4. 4The shower test. With the tub empty and the stopper open, run the shower for five minutes, aiming at the tub walls only, not the door or curtain. A drip now points to caulk, grout, the tub spout or the shower arm. Then aim the spray at the door and curtain to check the edges.
5. 5The dry test. If nothing above produced a drip, and the ceiling still gets wet when nothing is running, a supply line or valve is leaking. Check the meter.

Each step isolates one part. When a test produces the drip, stop and don't keep running water. Write down which test did it, because that one line saves a plumber a lot of searching. Photos of the stain before and after each test are worth taking too.

Some fixes are yours. Old caulk between the tub and tile can be scraped out and replaced once the area is dry. Cracked grout can be regrouted. A loose tub spout can be tightened or resealed. These are the simplest and most common causes when the leak only shows during showers.

The drain and overflow are a different matter. The gaskets sit under the tub, so reaching them means an access panel behind the tub, which many second-floor bathrooms have on the wall of the adjoining closet or hallway, or opening the ceiling below. A leaking shoe or gasket is a straightforward repair when there is access. If there isn't, the repair may mean cutting an access opening. Don't over-tighten plastic connections, because cracking one turns a slow drip into a steady one.

If the leak is behind the wall, from a supply line, valve or drain pipe you can't reach, stop and have it located before anyone cuts tile. A pro uses moisture meters and thermal imaging so the opening is small. HQ Plumbing & Air offers leak detection for exactly this. We're licensed and insured, we give you a free estimate, and you get the price up front before work starts. If water is flowing and you can't stop it, we're available 24/7.

A stopped leak doesn't finish the job. EPA advises: "It is important to dry water-damaged areas and items within 24-48 hours to prevent mold growth." The stain is only what shows. The insulation and drywall above it can stay wet for days. EPA also warns that "Absorbent or porous materials, such as ceiling tiles and carpet, may have to be thrown away if they become moldy." Soft, sagging or crumbling drywall should be cut out and replaced rather than painted over.

Two safety points. If the water is dripping near a light, fan or smoke detector, treat it as an electrical hazard, and read water dripping from a smoke detector or ceiling light before you touch anything. And if you rent or live in a condo or townhome, the pipe may not be yours to fix; see who fixes a leaking pipe in a condo or townhome.

Here's the short version. Match the timing to the cause, run the tub tests in order, and stop as soon as one produces a drip. Then dry the ceiling, because that is the part that turns into mold.