Best Ways To Fill A Void Under A Concrete Slab

Voids under concrete slabs are more common than most homeowners realize, and they can quickly lead to sinking, cracking, and uneven surfaces if left untreated. Whether the issue is under a driveway, patio, sidewalk, or interior floor, the right repair method can restore stability and prevent costly structural damage.

Below is a clear, modern guide to the best ways to fill a void under a concrete slab, when each method makes sense, and how to choose the safest, longest‑lasting solution.

What Causes Concrete Voids?

Voids typically form when the soil beneath a slab erodes, settles, or was never properly compacted. Water is the most common culprit – it washes away fine particles in the sub‑base, leaving empty pockets that can no longer support the concrete. Over time, this leads to sinking, cracking, and uneven stress on the slab.

Other contributing factors include:

• Poor drainage around the home
• Expansive or unstable soils
• Tree root decay
• Improper construction or compaction
• Heavy loads on thin or aging slabs

Why You Should Never Ignore a Void

A void beneath a slab may seem harmless at first, but it can quickly escalate into:

• Trip hazards
• Water pooling and drainage issues
• Worsening cracks
• Structural stress on adjacent slabs
• Higher repair costs later

Experts recommend addressing voids as soon as they’re discovered to prevent further damage.

Best Ways to Fill a Void Under a Concrete Slab

Below are the most effective, widely used repair methods ranging from professional solutions to limited DIY options.

1. Polyurethane Foam Injection (PolyLevel®)

Best for: Driveways, sidewalks, patios, pool decks, garage floors, interior slabs.

Longevity: Long‑term | Cure time: Minutes

Polyurethane foam injection is considered the most advanced and durable method for filling voids and lifting settled concrete. A technician drills small holes in the slab and injects a lightweight, expanding foam that fills the void, compresses weak soil, and lifts the slab back into place.

Why homeowners prefer polyurethane foam:

• Expands to fill every gap
• Lightweight, so it won’t add stress to weak soils
• Fast curing (you can use the slab the same day)
• Long‑lasting and water‑resistant
• Minimal drilling and disruption

This method is widely recommended by modern foundation and concrete repair professionals.

2. Mudjacking (Slabjacking)

Pros:

• Lower upfront cost
• Works for large voids

Cons:

• Heavy material can cause further settling
• Larger drill holes
• Longer cure time
• Not ideal for weak or eroded soils

While mudjacking remains a viable option, it is less durable than polyurethane.

3. Foam Concrete / Cellular Concrete

Best for: Large voids, utility trenches, industrial applications

Longevity: Long‑term

Foam concrete is a lightweight, flowable fill material that can be pumped into large voids. It’s more common in commercial or municipal settings, but can be used for residential slabs when voids are extensive.

Pros:

• Fills large or irregular voids
• Lightweight
• Good for deep stabilization

Cons:

• Not typically used for precision slab lifting
• Requires specialized equipment

4. Sand or Gravel Filling (Limited DIY Option)

Best for: Very small, shallow voids

Longevity: Short‑term

For tiny voids near slab edges, homeowners sometimes use sand or pea gravel to fill gaps. However, this is not a structural repair and won’t stop ongoing soil erosion.

Pros:

• Inexpensive
• Simple for small gaps

Cons:

• Doesn’t stabilize soil
• Doesn’t lift the slab
• Not suitable for interior or load‑bearing slabs

5. Full Slab Replacement

Best for: Severely cracked, broken, or structurally compromised slabs

Longevity: Long‑term | Cost: Highest

If a slab is too damaged to lift or stabilize, replacement may be the only option. This involves demolishing the existing slab, re‑prepping the base, and pouring new concrete.

Pros:

• Fresh, clean surface
• Can correct major structural issues

Cons:

• Most expensive option
• Long cure time
• Doesn’t address underlying soil issues unless corrected

How to Choose the Right Method

Here’s a quick comparison to help homeowners decide:

When to Call a Professional

You should contact a concrete repair specialist if:

• The slab is sinking or uneven
• Cracks are widening
• Water is pooling near the slab
• You suspect soil erosion
• The void is deeper than a few inches

Professionals can diagnose the cause, measure the void, and recommend the safest, most cost‑effective repair method.

How Much Does it Cost to Fill a Void Under a Slab?

The cost to fill a void under a slab depends on how big the void is, what caused it, and whether the concrete has sunk along with it. Because every void is different, the best way to determine the cost is to have one of our specialists evaluate it during a free inspection.

During that visit, a System Design Specialist checks how large and deep the void is and what’s behind it, whether that’s shrinking clay soil or poor drainage. They’ll also see if the slab needs to be lifted back to level and how easy the area is to reach. Those answers tell us how much PolyLevel® foam the job will take and how long it will run, and that’s what sets the price.

If a drainage problem caused the void, we’ll make sure that gets addressed first so the repair holds up. In most cases, filling a void costs far less than tearing out and replacing the concrete. Take a look at our PolyLevel vs. mudjacking comparison to see how the options stack up, then schedule your free inspection to find out what your repair will cost. Financing options are available.

How Do You Fill Large Gaps in Cement?

To fill a large gap in concrete, the first step is figuring out where the gap is and what caused it. A gap between two sections of concrete, such as an expansion joint, can usually be cleaned out and sealed. But if the gap is underneath the concrete because soil has washed away or settled, it needs to be filled from below.

When there’s a void under the concrete, we inject PolyLevel® foam through small, penny-sized holes. The foam expands to fill the empty space, supports the slab, and can lift it back to level. Once the slab is stable, we seal the cracks and joints with NexusPro®, a flexible silicone sealant that moves with the concrete and keeps water out.

Simply filling a large surface gap with mortar or caulk may not solve the problem if the concrete underneath is unstable. A free inspection can determine whether the gap needs to be sealed, the area underneath needs to be filled, or both.

Key Takeaways

Filling a void under a concrete slab isn’t just about pumping material into a gap. It’s about restoring stability, preventing future damage, and protecting your home’s value. Modern solutions like polyurethane foam offer fast, long‑lasting results that outperform older methods like mudjacking.