The situation
Illustrative example. Not a real job or customer.
An owner in a 1960s slab home reads about slab leaks after a high water bill. Their meter indicator moves slowly with everything off, and they are ready to book leak location.
- Before calling, they follow a rule-out list and close the irrigation valve. The meter still moves.
- They add food coloring to each toilet tank and wait without flushing.
- In the back bathroom, color appears in the bowl within minutes.
- They close that toilet's angle stop and recheck the meter. The indicator stops.
A worn flapper in a rarely used toilet was letting water run continuously. Replacing the flapper ended the leak.
The food coloring test
- Lift the tank lid.Set it somewhere safe; tank lids are heavy and break easily.
- Add color.Add several drops of food coloring to the tank water.
- Wait.Do not flush for fifteen to thirty minutes.
- Look in the bowl.Any color in the bowl means the flapper is leaking.
- Flush.Flush afterward to clear the color.
Why this was easy to mistake
| Sign | Slab leak | Running toilet |
|---|---|---|
| High bill | Yes | Yes |
| Meter moves with everything off | Yes | Yes |
| Meter stops with house shutoff closed | Yes | Yes |
| Meter stops when that toilet's stop is closed | No | Yes |
| Warm floor | Possible on the hot side | No |
What the owner did afterward
After replacing the flapper, the owner repeated the meter test with every fixture off and saw no movement. They added the toilet check to their quarterly routine and compared the next bill with the same period the year before to confirm it was back to normal.
Rule-outs that come before slab leak location
- Every toilet passes the food coloring test.
- Irrigation is off at its supply valve.
- The water softener is bypassed or idle.
- The water heater relief valve discharge pipe is dry.
- Outdoor hose bibs are not dripping.
Common questions
Is this a real story?
No. It is invented to illustrate a common false alarm.
Can a rarely used toilet still run?
Yes. Flappers can wear or warp whether or not a toilet is used often, and a quiet trickle is easy to miss.
Should I still get a plumber after fixing the toilet?
If the meter is still with everything off, the leak is likely solved. If it still moves, continue with the meter and shutoff tests.
General information for Fullerton, not a quote or professional advice. In an emergency, call 911.