5 min read · Updated August 12, 2026
A slab leak is a pressurised water line failing underneath a concrete floor. What makes it expensive is not the pipe — it is the concrete over it, and the time it usually takes anyone to notice.
The signs
- A warm or hot spot on the floor, usually on a hot-water line
- A water bill that has climbed with no change in how the property is used
- Water pressure that has dropped gradually across the whole building
- The sound of running water when every fixture is closed
- Damp or discoloured flooring, or baseboards that have started to lift
- A persistent musty smell in one area
- Cracking in slab flooring, or a foundation area that stays damp
The test you can run yourself
This takes five minutes and it is worth doing before you call anybody.
- Turn off every fixture and water-using appliance in the building. Ice makers and irrigation included.
- Go to the water meter and look at the leak indicator — usually a small triangular or gear-shaped dial that spins with any flow.
- Watch it for a couple of minutes.
If it is moving, water is leaving the system somewhere. That does not tell you where, but it definitively establishes that there is a leak, which is the question worth answering first.
If you have a shutoff valve at the building, you can narrow it further: close it and re-check the meter. Still moving means the loss is between the meter and the building. Stopped means it is inside.
What happens next
Locating comes before opening anything. The point of a leak-location visit is to make sure that whatever gets opened is opened in the right place, and to give you the option of not opening it at all.
Your realistic options are usually:
Direct repair. Open the slab at the leak, repair the pipe, close it. Right when the system is otherwise healthy and the location is accessible.
Reroute. Abandon the failed section and run a new line above the slab or through a different path. Often the better answer when the leak is under finished flooring, or when it is not the first one.
Repipe. When it is the third pinhole in the same system, repairing the fourth is not a strategy.
The number that decides it
One leak in an otherwise healthy system is a repair. A pattern of leaks is a system problem, and continuing to repair a system problem one hole at a time is the most expensive path available.
Ask for both numbers and the reasoning behind them.
This article is general information about how plumbing systems behave. It is not a diagnosis of your building, and it is not instructions for performing regulated work. Gas, electrical, pressurised systems, sewage contamination and code-regulated work belong with the appropriate licensed trade.