Under-slab drain problem versus a simple dry trap
The two smell alike, but they behave differently over a couple of weeks.
Suggests a broken drain under the slab
- Smell persists even after every trap is filled and stays full
- Odor is strongest along a crack in the floor or at the base of a wall
- Slow drains or gurgling in the lowest fixtures
- Damp or discolored concrete in a line across the floor
- Smell rises after heavy water use upstairs, like laundry day
Suggests a trap or seal problem
- Smell stops within a day of running water
- Odor is right at one drain opening
- Fixture goes unused for long periods
- No drainage symptoms
- Toilet base or floor drain cover is loose
Why this happens in east Denver's postwar housing
Cast iron was the standard drain material for decades. It is strong and quiet, but the inside of the pipe corrodes over time, especially along the bottom where water flows. Eventually a pipe can develop channel rot, a lengthwise crack or a failed joint. Under a basement slab, nobody sees it happen.
When a drain under the floor breaks, two things can follow. Wastewater leaks into the soil beneath the slab, which can produce a damp, sewage-tinged smell and sometimes settlement. And because a broken drain is open to the sewer system, gas can migrate through gravel and soil and enter the basement through floor cracks, the gap at the base of walls, or openings around plumbing penetrations.
Many Hilltop, Crestmoor and Mayfair homes from the postwar boom still have original under-slab drains, even if the visible plumbing above the floor has been replaced. Houses that were remodeled often had new fixtures connected to old lines below the concrete.
How a plumber usually narrows it down
Rule out the simple causes
Fill all traps, check toilet seals and floor drain covers, and confirm the vent system isn't blocked at the roof.
Camera inspection
A sewer camera is run through a cleanout to look at the line from the inside, recording cracks, offsets, roots, bellies and corrosion. Ask for the video and the footage locations.
Smoke test if needed
Harmless smoke is pushed into the drain system to show where it escapes. Smoke rising through a floor crack or wall is strong evidence of a leak.
Locate and mark
A locator can pinpoint the depth and position of a problem, so any concrete cutting is limited to the right spot.
Discuss repair options
Options can include spot repair through the slab, lining the old pipe, or replacing a section. Each has tradeoffs in disruption and longevity; ask for them in writing.
Before agreeing to any under-slab work
- Can I see the camera footage and the exact location of each defect?
- Is the problem inside the house, or in the lateral out to the street?
- Is lining a reasonable option for this pipe's condition, or is replacement better?
- How much of the finished floor will be disturbed, and who restores it?
- Does this work require a permit from the City and County of Denver?
- Once the line is repaired, should the soil underneath be treated or left to dry?
Common questions
Can odor treatment fix a smell from a broken drain?
No. Until the pipe is repaired, sewer gas and wastewater will keep producing odor. Treatment only makes sense after the plumbing is fixed.
Is the sewer line to the street my responsibility?
In many cities the property owner is responsible for the service line from the house to the main. Ask Denver's wastewater agency, or your city if you live outside Denver, how responsibility is split for your property.
General information for Hilltop, not a quote or professional advice. In an emergency, call 911.