Homes built on a concrete slab in the 1950s through the 1970s usually have cast-iron drain lines set in the soil before the slab was poured. Those lines collect the kitchen, bathrooms and laundry and leave the house as the building drain, which then becomes the lateral to the street.
Cast iron corrodes from the inside. Over decades the bottom of the pipe, where wastewater runs, wears thin and rough, and the top can crack where gases and moisture collect. Scale narrows the opening and catches paper, so slow drains often show up long before an actual break.
Signs the trouble is under the slab
- The camera finds heavy scale, a split bottom or a break before it reaches the cleanout outside
- Backups start at fixtures in the middle of the house rather than near the exterior wall
- A persistent sewer smell inside with no dry trap or vent problem found
- Damp or lifting flooring in one area with no water supply leak found
- Repeated clogs that clear but return quickly in the same branch
Two broad approaches
Repair in place
- Lining from inside the existing pipe through a cleanout or access opening
- Little or no floor removal when the pipe still holds its shape
- Depends on the pipe's condition and the number of branch connections
- Cannot correct a sag or a collapsed section
Replace or reroute
- Cut the slab and trench to the bad section, or tunnel under the foundation from outside
- Or abandon the old line and run new drain pipe through walls, attic or around the house
- Solves sags and collapses with new pipe at proper slope
- More disruption to floors, cabinets or landscaping depending on the route
How the main methods trade off
| Method | Where the disruption is | Main limitation |
|---|---|---|
| Slab cut and trench | Inside the house along the pipe route | Flooring and sometimes cabinets must come out |
| Tunneling | A pit outside, digging under the foundation | Access space and soil conditions matter |
| Lining | Usually at cleanouts or a small opening | Pipe must be round enough to hold a liner |
| Rerouting | Walls, attic or exterior route | Requires a workable path for new drain lines |
Questions for the plumber
- Exactly where under the floor is the damage, and how deep?
- Is the rest of the cast iron in the house in similar condition?
- Which fixtures connect to the damaged section?
- What will be removed and restored, and who handles the flooring?
- What permits are needed for this work? Check with the city if unsure.
Common questions
Is under-slab sewer damage the same as a slab leak?
Not quite. A slab leak usually means a pressurized water line leaking under the floor. Drain damage under a slab is a gravity sewer problem, found with a sewer camera rather than pressure testing alone.
Do I have to replace all the cast iron at once?
Not always. If damage is limited to one run, that section can sometimes be repaired alone. The camera footage and the age of the rest of the system guide that decision.
Will lining shrink my drain?
A liner adds a thin wall inside the pipe, so the opening is slightly smaller, but the smooth surface usually flows well. Ask the plumber how the liner thickness compares with your pipe size.
General information for Placentia, not a quote or professional advice. In an emergency, call 911.