Sewer Line Replacement Under a Slab House: Cost

Why Slab Houses Present a Unique Challenge

In a slab-on-grade home, the sewer line exits the house by passing through or under the concrete slab foundation. In many slab homes, especially those built before the 1980s, several feet to several dozen feet of sewer pipe run beneath the slab before exiting at the foundation edge. When this under-slab section needs replacement, accessing it requires either going through the slab or going under it.

Homes with basements or crawl spaces have exposed or accessible sewer pipes inside the structure. Slab homes do not, making under-slab sewer work one of the most expensive and disruptive plumbing repairs a homeowner can face. The method you choose depends on the length of pipe under the slab, the type of damage, and whether you want to preserve the slab or reroute the line entirely.

Option 1: Tunneling Under the Slab

Tunneling (also called pipe tunneling or horizontal boring) accesses the pipe from outside the foundation by digging a pit at the foundation edge and tunneling horizontally beneath the slab to reach the damaged pipe. The tunnel is typically 3 to 4 feet wide and follows the pipe path.

Tunneling costs $8,000 to $20,000 and is the most expensive option, but it preserves the slab and all interior flooring. No concrete cutting, floor demolition, or floor repair is needed. This makes tunneling the preferred method when the under-slab pipe run is 10 to 30 feet and the home has finished flooring (tile, hardwood, or other surfaces that would be expensive to replace after slab cutting).

The tunnel is backfilled after the pipe is replaced, and the entry pit at the foundation edge is restored. The process is labor-intensive because the tunneling is done by hand or with small pneumatic tools in a confined space beneath the slab.

Option 2: Cutting Through the Slab

Slab cutting involves saw-cutting through the concrete floor inside the house, removing the cut section, excavating to reach the pipe, replacing the pipe, backfilling, and repouring the concrete. The cut area is typically 18 to 24 inches wide, following the pipe path through the house.

Slab cutting costs $5,000 to $15,000 for the plumbing work, plus $1,000 to $5,000 for slab repair and floor restoration. The total depends on how much slab needs to be cut, the type of flooring that must be replaced, and whether structural reinforcement is needed after cutting.

This method is the most direct way to access under-slab pipes and is often the least expensive approach for short runs (under 10 feet). However, it is disruptive to the interior of the home, creates dust and debris, and requires flooring repair that may not match the existing floor perfectly. For homes with concrete floors or carpet that is due for replacement, slab cutting may be the most practical option.

Option 3: Rerouting Around the Foundation

Rerouting abandons the under-slab pipe entirely and installs a new sewer line that exits the house through the foundation wall above the slab or through a new penetration at a different location. The new line runs along the exterior of the house at grade before connecting to the existing sewer lateral in the yard.

Rerouting costs $4,000 to $12,000 and avoids both tunneling and slab cutting. The abandoned under-slab pipe is left in place (usually filled with sand or grout to prevent future collapse). This method works well when the under-slab run is long and the exterior routing is feasible without excessive pipe length or grade problems.

The main limitation of rerouting is that the new above-slab line must maintain proper drainage slope, which depends on the elevation of the interior plumbing connections relative to the exterior grade. In some homes, the interior fixtures are too low relative to the exterior grade to achieve proper slope without a sewage ejector pump ($1,500 to $3,000 additional).

Trenchless Options for Under-Slab Pipes

Trenchless methods can sometimes be used for under-slab pipe sections, avoiding both tunneling and slab cutting. CIPP lining is particularly promising for under-slab work because it can be inserted through an interior cleanout or from outside the foundation, lined through the under-slab section, and cured in place without any excavation beneath the slab.

CIPP lining for under-slab pipes costs $4,000 to $12,000, making it comparable to rerouting and less expensive than tunneling. However, the pipe must be structurally intact enough to serve as a host for the liner. Collapsed or severely deformed pipes under the slab are not candidates for CIPP.

Pipe bursting can also work under a slab in some situations, but access pit locations must be carefully planned because the bursting head creates outward force that could damage the slab if the pipe is too close to the underside of the concrete. A minimum of 12 to 18 inches of soil between the pipe and the slab underside is typically required for safe bursting.

How to Decide Which Method to Use

Start with a camera inspection to determine the extent of damage, the pipe material, and the exact location of the problem under the slab. If the damage is localized to a short section, a targeted slab cut or spot repair may be the most cost-effective approach. If the entire under-slab run needs replacement, compare quotes for tunneling, rerouting, and CIPP lining.

Consider the value of your interior flooring. If the home has expensive tile, hardwood, or other high-value flooring above the pipe path, tunneling or CIPP lining preserves the floor and may be worth the premium over slab cutting. If the home has concrete floors or carpet that needs replacing anyway, slab cutting becomes more practical.