What is under the floor
Single-story tract homes from the 1950s through the 1970s were often built on concrete slabs poured directly over the drain lines. In many of them the building drain is cast iron, joining the outside lateral near the foundation. Because the pipe is buried in or under concrete, it is out of sight until something goes wrong.
Cast iron corrodes slowly from the inside. Scale builds up and narrows the pipe, and the bottom of the pipe, where water runs, thins over time. Eventually the bottom can crack lengthwise or wear through, and waste water starts leaving the pipe into the soil under the slab.
Signs that point under the slab rather than out in the yard
- Repeated clogs in a bathroom or kitchen branch, not just the main line
- A sewer smell inside with no obvious source
- Damp or musty flooring, or loose tiles near a bathroom
- A camera showing heavy scale or a rough, uneven pipe bottom before it leaves the house
- Cracks or settling in the floor along a drain path
Common ways to fix cast iron under a slab
| Method | How it works | Trade-offs |
|---|---|---|
| Descaling and lining | Scale is cut away and a liner is cured inside the old pipe | Avoids breaking floors where conditions allow; needs enough pipe left to hold a liner |
| Spot repair through the floor | Concrete is cut at one location and a section replaced | Good for one localized failure; floors must be patched |
| Re-routing | A new drain path is run where it avoids the worst sections | Can reduce floor demolition; depends on layout |
| Full under-slab replacement | Floors are opened along the drain runs and new pipe installed | Most disruptive; replaces everything at once |
How a careful diagnosis proceeds
Camera each branch
Inspect from cleanouts and fixture openings to map which runs under the floor are affected, not only the main line.
Locate the runs
Mark the path and depth of the pipes on the floor so any cutting is limited to the right spots.
Consider a test
Some plumbers can isolate and test drain sections to confirm a leak. Ask whether that applies to your house.
Plan finishes
Before any cutting, agree on who restores concrete and flooring, and what happens to cabinets or tubs over the line.
Ask before any concrete is cut
- Exactly where will you cut, and how large will each opening be?
- Is lining an option for these runs, and which ones?
- Who patches the concrete and who replaces the flooring?
- Does this work need a permit and inspection from the city?
Common questions
Can cast iron under a slab be lined?
Often, if enough of the pipe remains to support the liner and the runs can be accessed. Badly broken sections may still need to be opened.
Is under-slab drain work done by the same plumber as the yard lateral?
It can be. Ask whether the quote covers the building drain inside, the lateral outside, or both.
Will my house have to be vacated?
It depends on the scope. Ask how long each bathroom or the kitchen will be out of service.
General information for La Habra, not a quote or professional advice. In an emergency, call 911.