What Is a Slab Foundation?
A slab foundation is a single continuous concrete pour - typically 4-6 inches thick - that sits directly on the prepared subgrade. Most homes built in the South, Southwest, and parts of the Midwest use slab construction. Post-tension slabs (which have steel cables embedded under tension) are common in Texas and California.
Slabs are generally durable but are directly exposed to ground movement. Any significant change in the moisture content of the soil beneath - from drought, flood, plumbing leaks, or vegetation - can cause the slab to shift, crack, or heave.
Why Slab Foundations Fail
Understanding the cause of failure is essential because different causes require different repair approaches.
Soil Settlement
When the soil beneath a slab loses moisture (drought, root uptake, poor drainage), it shrinks and compresses. The slab loses support and can crack and sink in affected areas. This is the most common cause of slab foundation problems in Texas, Oklahoma, and the Southwest.
Expansive Soil Heaving
Expansive clays - particularly Blackland Prairie clay in Texas and Bentonite clay in Colorado - absorb water and swell. If moisture reaches the soil beneath the slab unevenly (from a plumbing leak, inadequate drainage, or irrigation), the slab can heave upward in isolated areas. Heaving and settling require different repair approaches.
Plumbing Leaks Under the Slab
Post-tension slab homes have plumbing embedded in or under the concrete. A slow slab plumbing leak can saturate and erode the underlying soil, creating voids beneath the slab that eventually cause it to crack and drop. Plumbing leaks must be repaired before foundation repair can be effective.
Poor Initial Compaction
Slabs built on inadequately compacted fill can settle as the fill material compresses under load over time. This is common in newer developments where cut-and-fill grading was done without adequate compaction testing.
Drainage Problems
Water pooling against the foundation or inadequate positive grade (slope away from the foundation) allows sustained soil saturation, which accelerates both expansion and erosion at the slab perimeter.
Slab Foundation Repair Methods
Drilled Pier Underpinning (Most Common)
Drilled piers - either helical piers or push/resistance piers - are installed through the slab edge or through holes cut in the slab. The piers are driven or screwed down to stable bearing capacity, then a bracket is used to lift and stabilize the slab.
Helical piers are screwed into the ground and can be installed in tight spaces. They’re suitable for lighter loads and are often used for interior settling.
Push piers are driven hydraulically into the ground using the weight of the structure. They’re typically used on the perimeter and require a minimum load to drive properly.
Cost: $1,000-$3,500 per pier installed. Most residential projects require 6-12 piers for a full repair; $6,000-$25,000 is a typical range for complete slab stabilization.
Mudjacking (Concrete Leveling)
Mudjacking injects a cement slurry or polyurethane foam beneath the slab to fill voids and lift sunken sections. It’s less expensive than pier installation and suitable for slabs that have minor settling but haven’t lost bearing capacity.
Limitations: Mudjacking does not address the underlying soil instability. On expansive clay soils, results may be temporary. It works better in stable-soil areas where the slab has sunk due to void formation rather than ongoing soil movement.
Cost: $500-$3,500 depending on area and number of injection points.
Polyurethane Foam Injection
A modern alternative to mudjacking, foam injection fills voids more precisely, weighs less (reducing further settling risk), and cures faster. Used for smaller slab areas and interior floors.
Cost: $500-$2,500 depending on extent.
Drainage Correction
For heaving slabs caused by uneven soil moisture, drainage improvement is the primary treatment. This includes regrading, French drain installation, gutter extension, and irrigation management. Structural repair is ineffective if the moisture problem isn’t resolved first.
What to Expect During Slab Repair
- Assessment: Contractor assesses the slab with a laser level to map differential movement. They may check for plumbing leaks with a pressure test before structural work.
- Access: For perimeter pier installation, small access holes are dug at the slab edge. Interior piers require 12-18 inch core holes drilled through the slab.
- Pier installation: Piers are driven or screwed to refusal depth. A bracket is attached to the footing.
- Lifting: Hydraulic jacks lift the structure to target grade. Not all slabs can be lifted to original grade - aggressive lifting can crack plumbing connections.
- Grouting and patching: Core holes are filled with concrete. Access pits are backfilled and compacted.
- Cleanup: Most contractors leave the work area broom-clean. Landscaping disturbed during access may need restoration.
Slab Foundation Repair Cost Factors
| Factor | Effect on Cost |
|---|---|
| Number of piers needed | Primary cost driver - more piers = higher cost |
| Pier type (helical vs. push) | Helical piers typically cost 10-20% more |
| Interior vs. perimeter piers | Interior piers require slab coring - adds cost |
| Post-tension slab | Cutting cables requires engineering coordination - adds cost |
| Access difficulty | Tight clearance, landscaping, fencing increases labor |
| Local labor market | Texas/Oklahoma typically lower; California/Northeast higher |
When to Get an Inspection First
A foundation inspection ($200-$750) is recommended before signing any repair contract. An inspector can:
- Confirm the movement is in the foundation (vs. framing, soil shrinkage at finish materials)
- Map the extent of settlement with a laser level
- Identify plumbing leaks that must be addressed first
- Recommend a repair approach without being the one selling the repair
Inspectors who don’t do repair work provide the most unbiased assessment.