Most Likely Causes (Ranked)
The tub drain connects through a shoe fitting underneath the tub. Plumber's putty seals the drain flange to the tub surface, and a gasket seals the shoe to the drainpipe. Both fail over time. The putty dries and cracks, and the rubber gasket hardens. Every time you fill the tub, water seeps past the failed seal. This is the #1 bathtub leak cause.
The overflow opening (behind the trip lever or pop-up plate) has a rubber gasket sealing it to the tub body and a pipe connecting it to the drain assembly. This gasket compresses from the clamping bolts and dries out from infrequent contact with water. When you fill the tub near the overflow level, water leaks behind the tub wall.
The tub faucet valves inside the wall develop the same issues as any faucet — worn cartridges, degraded seals, corroded seats. The difference is that tub valves are inside the wall, so a leak may run down inside the wall cavity before appearing. Hot-side failures are more common due to heat.
Fiberglass and acrylic tubs can crack from impacts, heavy loads, or insufficient support underneath. Stress cracks typically develop at the drain area or along the floor where flex is greatest. Cast iron and steel tubs don't crack but can develop rust-through in 30-50+ years if the enamel is chipped.
The joint between the tub and the tile surround (or fiberglass surround) is sealed with caulk. When caulk fails, shower water runs behind the surround into the wall cavity. Grout on tile surrounds also fails, allowing water penetration. This is often misdiagnosed as a plumbing leak because the water shows up on the ceiling below.
How to Fix It
Reseal the Drain Assembly
$5-15 DIY / $150-250 Pro- 1Remove the drain flange (use a drain removal tool or insert needle-nose pliers into the crosshairs and turn counterclockwise).
- 2Clean old putty from both the flange and the tub surface.
- 3Apply a fresh rope of plumber's putty around the flange. Press into place and screw back in.
- 4From below (access panel or ceiling access): check the shoe gasket. Replace if hardened or cracked.
- 5Fill the tub and check for leaks from below.
Replace Overflow Gasket
$5-10 DIY / $100-200 Pro- 1Remove the overflow plate (1-2 screws). Pull out the trip lever mechanism if applicable.
- 2Behind the plate, you'll see the gasket between the overflow pipe and the tub body.
- 3Replace the rubber gasket (take the old one to the store to match). Apply a thin layer of silicone to the tub side.
- 4Reinstall the plate and tighten screws evenly — snug, not gorilla-tight (overtightening cracks the tub).
- 5Fill the tub to the overflow level and check for leaks.
Recaulk Tub-Surround Joint
$5-10 DIY- 1Remove all old caulk with a utility knife and caulk removal tool. Clean with rubbing alcohol.
- 2Fill the tub with water (this weights the tub down so the caulk won't crack when used).
- 3Apply 100% silicone caulk (not latex) in a smooth, continuous bead. Smooth with a wet finger.
- 4Let cure 24 hours before using the shower. Keep the tub filled during curing.
When to Call a Professional
Water stains on the ceiling below a bathtub require opening the ceiling for inspection — the drywall is likely compromised.
Soft or spongy flooring around the tub means the subfloor is rotting — repair is $500-2,000+.
Never use latex caulk in wet areas — it molds quickly. Use 100% silicone.
If the tub floor flexes when you stand in it, the subfloor underneath is damaged or the tub needs better support.
How to Prevent It
Recaulk the tub-surround joint every 2-3 years.
Regrout tile surrounds every 5-7 years or when grout cracks.
Don't stand on the edge of a fiberglass tub — it flexes and cracks.
Check below the tub annually through an access panel (if available) or check the ceiling below for stains.
Frequently Asked Questions
Test each separately. Fill the tub without using the shower → drain or overflow. Use the shower without filling the tub → surround seal. Fill to the overflow level → overflow gasket.
There should be — look in the adjacent room or closet on the wall behind the faucet. If there isn't one, cut a 12x12 inch access hole in the adjacent drywall. It's much cheaper than tearing out the tub surround.
Drain or overflow reseal: $150-250 pro. Recaulk: $5-10 DIY. Faucet valve: $150-350 pro. Cracked tub: $300-500 refinish, $1,500-5,000 replacement.
If the stain is dry and not growing, the leak may be intermittent. If it's wet or growing, the leak is active. Either way, address it within a week to prevent mold and structural damage.