Both bathrooms sit above your garage.
At random times, you find water on the garage floor.
You can see the plumbing in the garage ceiling.
But you cannot spot the leak source.
Here is the key clue.
It leaks during showers, not baths.
It also stops when nobody stands in the tub.
Quick Q&A
Q: Why does it leak only when someone stands in the tub?
A: The tub or shower base can flex under weight and open a tiny gap at the drain, overflow, or seam.
Top causes that match your symptoms
- Tub flex at the drain shoe. Weight shifts the drain seal.
- Overflow gasket seep. Splash hits the overflow and finds the gap.
- Door or curtain leak. A person changes the spray pattern.
- Grout or caulk the crack. Flex opens a hairline seam.
- Tub spout diverter drip-back. Water runs behind the wall.
Find it fast with these simple tests
Do these tests one at a time.
Keep each test under ten minutes.
Test 1: Rule out splash and door leaks
Run the shower with a person inside.
Tape a plastic sheet to the tub rim.
Let it drape inside the tub.
This forces all water into the drain.
If the garage stays dry, it is splash or door-related.
Check the door sweep and side seals.
Also check the curtain gap near the valve wall.
See guidance on shower door sweeps here:
Family Handyman: common shower leak fixes
Test 2: Check the overflow gasket
A standing person makes a bigger splash.
That splash can hit the overflow plate.
Dry the overflow area first.
Then aim the shower at the overflow for two minutes.
Watch below for drips.
Overflow gaskets are cheap and common.
Test 3: Catch a drain shoe leak under load
Put dry paper towels under the drain trap.
Stand in the tub and shift your weight.
Do not run water yet.
Now run the shower for two minutes.
Check towels for the first wet spot.
That first wet spot is your best clue.
Test 4: Check the tub spout and shower arm
Remove the spout and inspect the pipe joint.
Also, check the shower arm at the wall.
Water can follow threads and drip inside walls.
This Old House shows common leak paths here:
This Old House: how to fix a leaky shower
What usually fixes this
- Replace the drain shoe gasket and reset the drain.
- Replace the overflow gasket and plate seal.
- Re-caulk the tub-to-wall joint with 100% silicone.
- Add support under the tub if it flexes.
If the tub moves, the seal will keep failing.
Support may mean mortar, foam, or blocking.
Follow the tub maker’s install notes if possible.
I’m Chris Mayer, writing for Plumbing 101.