Can a cracked drain line under the slab cause sewer smell?

### Drain pipes run partly full

Sewer pipes are not full of water. Water flows along the bottom, and gas sits in the space above it. A crack near the top or side of the pipe lets that gas out while the water keeps flowing past. That is how a building can have a steady smell with no wet floor and no backup. Oatey lists cracked pipes as one of the main causes of sewer odor, noting that cracks allow gas to escape.

### Where the gas goes

Gas follows the easiest path up. In a slab-on-grade building that means cracks in the concrete, the openings where pipes pass through the slab, expansion joints and gaps at wall bases. Wikipedia's sewer gas summary says the mixture can include hydrogen sulfide, ammonia and methane, so a leak under the floor is a real air-quality issue, not just a nuisance.

### What damages a line under a slab

Old pipe, shifting soil, roots reaching in at joints, an earlier repair that was done badly, and heavy loads over the line can all break a drain. The EPA lists blockages, line breaks and sewer defects among the causes of sewer system failures, which is why a cracked line that only smells today can become a backup later.

### The pattern that points to the line

• The smell is constant or nearly constant, not just near one drain
• The smell is worst at floor level, near slab cracks, or in a room over a known drain run
• Refilling every trap does not change it (see dry floor drain traps)
• Vent checks find nothing wrong (see blocked roof vents)
• Slow drains or repeat clogs on the same line
• Damp soil, settling, or an unexplained wet spot outside near the line
• A pest problem that seems to come up through the floor

### Rule out the free causes first

One sign alone is thin. Several together make a line problem likely. Ruling out traps, vents and the toilet seal first saves you from booking a camera inspection when a free fix was available. Check toilet wax ring smells and the traps before you assume the line.

### What the camera shows

A drain camera inspection feeds a flexible cable with a camera through a cleanout or an open drain, and sends live video to a monitor. It can show visible deposits, roots, offsets, cracks or collapsed sections and help locate them. Deposits, standing water, restricted access and camera orientation can hide defects; a video does not always establish the precise escape path for an odor. That is why it is the usual next step when everything simpler has been ruled out. A commercial building with a recurring mystery odor is the textbook case.

### What to ask for

Ask for the video, the footage of the defect, and the location marked on the floor or measured from the cleanout. That record supports a repair decision, a lease discussion or an insurance claim. It also lets you get a second opinion without paying for a second inspection.

### After the camera: repair options

Where a line runs, how deep it is and what the pipe is made of decide the repair. Options range from a small spot repair to lining or replacing a section. See trenchless sewer repair versus digging for the tradeoffs, and signs of tree roots in a sewer line if roots are involved.

### Cracks, offset joints and low spots are different problems

The camera does not just find a crack. A crack, a joint that has pulled apart or slipped out of line, a section that has collapsed, and a low spot where water sits are different defects with different smells and different repairs. A low spot holds waste that gives off odor even with no leak. A joint that has opened leaks gas at that one point. A collapsed section blocks flow, so it produces backups as well as odor. Knowing which one you have is the difference between a clean-out and a dig.

### What to expect on a commercial job

In a commercial building the line under the slab often serves restrooms, kitchens and floor drains on several branches, and the plumber may need to camera each branch separately, starting from cleanouts or removed fixtures. Plan for some access work, like pulling a toilet or opening a cleanout in a mechanical room. Tell tenants beforehand, since the work can interrupt restrooms for part of a day. If the building has a grease interceptor or a lift station, tell the plumber, because those change where the camera can go and where gas can leak.

### Private line versus city main

Where the line sits matters. On its water services page, Phoenix says the city repairs only broken service lines within the public right of way, and that property owners are responsible for cleaning and maintaining the sewer line from the house to the connection with the city main in the street or alley. A crack inside your slab or on your side of the connection is the owner's to fix. See who is responsible for the sewer line in Phoenix. Other cities set their own rules, so check yours.

### In a lease

In a rented building, review the lease, applicable law and the diagnosed cause to decide who arranges and pays for work. A dated report supports that discussion but does not decide legal responsibility on its own.

### When you also have slow drains or wet floors

A smell together with slow drains, a wet slab, or a bare patch in the soil outside is stronger evidence that the line is failing. Do not wait for a backup. An early inspection can help plan repairs before a backup, and a dated inspection report makes the case to an owner, a landlord or an insurer.

### Why waiting makes it worse

A crack in a drain is a place where soil, roots and water can enter the pipe, and where waste can leave it. Over time the surrounding soil can wash out and leave a void under the slab, and the pipe can sag or separate further. That is how a smell becomes a backup, then a floor repair. Early investigation can reduce avoidable damage, but the needed repair depends on the actual defect and site conditions.

1. 1Rule out the free causes first: refill traps, check the toilet seal, check vents.
2. 2If the smell survives, book a camera inspection of the drain line. Ask for the video and a marked location.
3. 3Keep the report. It matters for a repair plan, a lease dispute or an insurance claim.
4. 4Ventilate the space while you wait, and get out if anyone feels sick. See the first signs of sewer gas poisoning.

We are licensed and insured, and you get a free estimate and an upfront price before any work starts. For the full tracing order, see how to find the source of a sewer smell, step by step.