Callers often arrive here the same way: a hydrostatic pressure test was required before foundation work could be signed off, and it failed. That's a common route into this problem. Foundation repair companies, home inspectors and insurance adjusters often ask for the test because water from leaking drains can undermine a slab.
What a hydrostatic test checks
Drain lines aren't pressurized like water supply lines. To test them, a plumber blocks the line at the point where it leaves the house, then fills the drains under the slab with water up to floor level. If the water level drops over a set period, water is escaping somewhere in the drain system under the floor.
It's a pass-or-fail test. It tells you a leak exists, not where.
Why under-slab drains fail
- Cast iron drains, common in older houses, can corrode and crack over decades
- Joints can separate as the slab or soil moves
- Roots can force their way into joints
- Older plastic drains can crack under settling
Finding the leak
The usual next step is a camera inspection. A waterproof camera on a cable goes through a cleanout and shows the inside of the pipe on a screen. Cracks, separated joints and roots are visible. Some plumbers test sections separately, plugging different points to see which part loses water.
Mention drain concerns when you call so the right equipment comes. Supply-line slab leak detection uses different tools.
Repair options
| Option | What happens | When it fits |
|---|---|---|
| Spot repair | The floor and slab are opened over the damaged section and the pipe is replaced | One bad section in otherwise sound pipe |
| Lining | A liner is cured inside the old pipe, forming a new pipe within it | Cracks and leaky joints where the pipe shape is intact |
| Reroute or replace | Drains are rerun on a new path, sometimes through a tunnel under the slab | Widespread failure or collapsed sections |
Ask how each option affects the test result. If the test was required for foundation work, ask whether a retest is part of the job.
Supply leak or drain leak?
If your water meter moves with every fixture off, you likely also have a supply leak, which is a separate problem. Run the water meter leak test to check. Drains only leak when water goes down them, so the meter won't catch a drain leak.
Related reading
Will a slab leak damage the foundation explains why water under a slab matters to the structure, and the home page covers supply-line slab leaks in Westminster homes.
Two more questions
Is a drain leak the same as a slab leak?
Both are under the slab, but they're different systems. A supply leak loses pressurized water constantly. A drain leak only leaks when water goes down the drain, so the signs are slower and less obvious.
Can a drain line under the slab be fixed without breaking the floor?
Sometimes. Lining the pipe from the inside can avoid digging in suitable cases. Broken or badly offset pipe may still need a section opened.
Keep reading
- Signs of a Slab Leak, Ranked From Strongest Clue to WeakestWhich clues point straight at the slab, which ones only hint at it, and the meter test that settles it.
- Slab Leaks and Homeowners Insurance in California: The Three Parts of a ClaimWhy insurers split a slab leak into access, pipe and damage, and what to document before any work starts.