In this guide
- Typical Cost to Fill a Void Under a Concrete Slab
- What Determines the Price?
- Void size and depth
- Whether the slab needs to be lifted
- Slab thickness, weight, and condition
- Access and injection-hole locations
- Cause of the void
- Common Methods and Their Costs
- Polyurethane foam injection
- Mudjacking or cement-grout slabjacking
- Concrete replacement
- Cost by Slab Location
- Other Costs That May Be Added
- How to Tell Whether a Void May Be Present
- How the Repair Process Works
- How to Compare Contractor Quotes
- Ways to Manage the Cost
- Frequently Asked Questions
- Is filling a void cheaper than replacing concrete?
- Does void filling always lift the slab?
- How long does slabjacking take?
- Will the repair last?
The cost to fill a void under a concrete slab often ranges from a few hundred dollars for a small repair to several thousand dollars for a large or difficult job. Price depends on the repair method, the size and accessibility of the void, whether the slab must be lifted, and what caused the soil to settle. Understanding those variables helps homeowners compare quotes and avoid paying to fill a space that is likely to form again.
Typical Cost to Fill a Void Under a Concrete Slab
For many U.S. residential projects, contractors charge roughly $3 to $25 per square foot for slabjacking or void filling. Smaller jobs may fall below or above that range because companies often have a minimum service charge. A modest repair may cost about $500 to $1,500, while extensive work beneath a driveway, garage floor, patio, or foundation slab can reach several thousand dollars.
These figures are planning estimates, not guaranteed prices. A quote may be calculated by square footage, the number of injection holes, material volume, or a minimum project fee. The total also depends on whether the contractor is simply filling an empty space or raising settled concrete back toward its original elevation. Those are related but distinct services.
| Repair scope | Common planning range | What the estimate may include |
|---|---|---|
| Small, accessible void | $500–$1,500 | Inspection, a few injection points, and limited grout or foam |
| Moderate slab repair | $1,000–$3,500 | Multiple injection points and partial lifting or stabilization |
| Large or complex area | $3,000–$10,000 or more | Extensive material placement, difficult access, or related repairs |
| Per-square-foot estimate | About $3–$25 | Varies substantially by method, location, and project minimum |
Ranges overlap because a small but hard-to-reach void can cost more per square foot than a larger, straightforward repair. Regional labor rates and contractor availability also affect estimates. Homeowners can use cost guides from Angi as a starting point, but a site-specific inspection is necessary for a reliable bid.
What Determines the Price?
Void size and depth
The larger the gap and the more material needed to fill it, the higher the likely cost. A shallow void beneath one corner of a patio is different from a long, deep cavity under a garage floor. Contractors may use probing, visual inspection, elevation measurements, or other diagnostic methods to estimate the void’s extent. Because the space is hidden, the final amount of grout or foam may not be known exactly before injection begins.
Whether the slab needs to be lifted
Filling a void can stabilize a slab without correcting its height. If concrete has settled, the contractor may need to inject material in stages to lift it as well. Lifting requires careful monitoring because excessive pressure can crack the slab, raise it unevenly, or affect nearby concrete. A quote should state whether it covers void filling only, slab lifting, or both.
Slab thickness, weight, and condition
Thicker, heavier slabs may require more material or more injection points. A cracked, deteriorated, or poorly supported slab may not be a good candidate for lifting. If the concrete is badly damaged, replacement or partial replacement may be more appropriate than injecting beneath it.
Access and injection-hole locations
Most repair methods require small holes drilled through the slab. Limited access, obstructions, finished flooring, low clearance, or nearby utilities can make the work slower and more expensive. Contractors should identify where holes will be drilled and explain how they will patch them afterward.
Cause of the void
Common causes include soil erosion from leaking plumbing or poor drainage, poorly compacted fill, soil shrinkage, and changes in groundwater. If the cause continues, a newly filled void may reappear. Repairing a leaking pipe, redirecting runoff, or correcting a drainage problem can add to the project cost but may protect the slab repair.
Common Methods and Their Costs
Polyurethane foam injection
Polyurethane foam is injected through small holes and expands beneath the slab. Contractors use it to fill spaces and, where appropriate, lift concrete in a controlled manner. It is lightweight, cures relatively quickly, and typically requires smaller holes than traditional mudjacking. Its cost is often toward the higher end of slabjacking estimates, with many projects planned around $5 to $25 per square foot or a contractor’s minimum charge.
Foam is not automatically the best choice for every void. The appropriate product, injection pressure, and placement depend on the slab and soil conditions. Homeowners should ask what type of foam is being used, how the contractor determines that the space is filled, and whether the material is intended to lift or only support the slab.
Mudjacking or cement-grout slabjacking
Mudjacking pumps a cement-based grout or similar slurry through larger holes beneath the concrete. The material fills the void and can raise a settled slab. It may cost less than polyurethane in some markets, with broad planning estimates often around $3 to $8 per square foot. The actual price can vary with material volume, access, and the contractor’s minimum fee.
Grout is heavier than foam and may require larger drill holes. It can be a practical option for some exterior slabs, but the added weight and the condition of the supporting soil matter. Neither method should be selected on price alone; the contractor should explain why the proposed material suits the site.
Concrete replacement
If the slab is broken into pieces, severely cracked, too thin, or unsupported over a large area, removing and replacing it may be the more durable solution. Replacement costs depend on demolition, disposal, thickness, reinforcement, access, and finish. It can cost substantially more than filling a small void, but comparing replacement with repeated repairs is useful when the slab’s condition is poor.
Cost by Slab Location
The location affects access, surface finish, and consequences of settlement. These examples are broad estimates for planning, not fixed prices.
| Slab location | Common considerations | Possible cost pattern |
|---|---|---|
| Sidewalk or walkway | Trip hazards, narrow access, and local rules | Small repairs may meet a contractor minimum; multiple panels increase cost |
| Patio | Drainage slope, landscaping, and decorative finish | Often moderate; finish damage may require additional work |
| Driveway | Vehicle loads, long sections, and water runoff | Can rise with area, access limits, and the need to correct drainage |
| Garage floor | Interior access, stored items, and floor coatings | May cost more if contents or finished surfaces must be moved or protected |
| Foundation slab | Structural movement, plumbing, and soil conditions | Requires careful diagnosis; engineering or plumbing work may be additional |
Foundation-related settlement deserves particular attention. A localized void under a nonstructural patio is not the same as movement affecting a home’s foundation. If doors or windows stick, walls show new cracks, floors slope, or movement continues, the homeowner should seek an evaluation from a qualified foundation professional or structural engineer rather than assuming that simple injection will solve the problem.
Other Costs That May Be Added
- Inspection or evaluation: Some contractors offer free estimates; others charge for diagnostic work or engineering review.
- Leak detection and plumbing repair: A leaking water or sewer line may need repair before the void is filled.
- Drainage improvements: Downspout extensions, grading, or drain work can help limit recurring erosion.
- Concrete patching or replacement: Cracks, broken edges, surface damage, or damaged finishes may need separate attention.
- Site access and preparation: Moving stored items, removing obstacles, or working around landscaping can increase labor.
- Permits or engineering: Requirements vary by location and project. Confirm with the contractor and local building department when structural work is involved.
How to Tell Whether a Void May Be Present
A hollow sound when tapping concrete can indicate a gap, but it does not prove that a void exists or show its size. Other clues include a slab that rocks underfoot, a visible gap at an edge, settlement near a downspout, or concrete that has dropped relative to an adjoining surface. Cracks can have many causes, so they should be assessed in context.
Water is a frequent contributor to soil loss. Check whether downspouts discharge beside the slab, whether the ground slopes toward the house, and whether irrigation or plumbing may be leaking. Homeowners should avoid drilling or injecting materials themselves without confirming utility locations and understanding the soil and slab conditions.
How the Repair Process Works
- Assessment: The contractor examines the slab, settlement, cracks, drainage, and access. The contractor should ask about when the problem appeared and whether it is changing.
- Repair plan: The proposal identifies the method, injection locations, intended result, and any work needed to address the cause.
- Drilling and injection: Small holes are drilled, and grout or foam is pumped beneath the slab. For lifting work, elevation is checked as material is placed.
- Hole patching and cleanup: Injection holes are filled, and the work area is cleaned. The appearance of patches may differ from the original concrete.
- Follow-up: The contractor explains curing or access restrictions and what to monitor for signs of renewed settlement.
Published technical resources from the American Concrete Institute and the Concrete Network describe concrete repair and slabjacking practices. A contractor’s proposed method should be appropriate to the specific slab; general cost guides cannot determine whether a particular repair will work.
How to Compare Contractor Quotes
Obtain at least two or three written estimates when practical. Ask each contractor to inspect the same area and explain whether the price is based on square footage, material use, or a minimum charge. A low bid may exclude lifting, drainage corrections, patching, or follow-up, so the total scope matters more than the headline price.
- Does the quote cover void filling, lifting, or both?
- What material and injection method will be used, and why?
- How will the contractor locate the void and monitor slab elevation?
- Is there a maximum price or a process for approving additional material?
- Are hole patching, cleanup, and related repairs included?
- What warranty applies, and what conditions or exclusions does it contain?
- Is the contractor licensed and insured where required, and can the company provide local references?
Be cautious of promises that a slab will be returned to an exact original elevation or that settlement can never recur. Soil conditions and water movement can change, and old concrete may not tolerate lifting. A sound estimate describes the expected outcome and limitations clearly.
Ways to Manage the Cost
Addressing a small settlement problem early may prevent a trip hazard or further damage, but a contractor should first confirm that the slab is suitable for injection. Fixing the source of erosion before or alongside the repair can reduce the risk of paying for the same work again. Keeping the area accessible and removing stored items before the crew arrives may also reduce labor time.
Homeowners should compare repair with replacement when the slab has extensive cracking or deterioration. They should also review warranties carefully: coverage may apply to workmanship or material, while excluding new settlement caused by drainage changes, leaks, or soil movement. The best value is not necessarily the lowest initial quote; it is a repair plan that matches the cause, slab condition, and intended use.
Frequently Asked Questions
Is filling a void cheaper than replacing concrete?
For a sound slab with a localized void, injection is often less disruptive and less expensive than demolition and replacement. If the concrete is badly damaged or the soil problem is extensive, replacement may be more practical. A site inspection is needed to compare the options.
Does void filling always lift the slab?
No. Some projects are designed to support the slab without changing its elevation. The quote should specify whether lifting is included and what result the contractor expects.
How long does slabjacking take?
Many residential injection jobs can be completed in a few hours, although preparation, access, slab size, and repairs to the underlying cause can extend the schedule. Ask the contractor when the surface can be used again.
Will the repair last?
Durability depends on the material, installation, slab condition, soil, and whether the original cause is corrected. Filling a void without controlling ongoing water erosion or settlement may not prevent future movement.