Hollow Concrete Floor

Why Does Your Concrete Floor Sound Hollow?

In Home Maintenance by Giovanni ValleLeave a Comment

Have you ever heard a hollow thumping sound when walking on your concrete floor? This phenomenon is more common than you might think, and many factors can cause it.

A void in the underlying concrete causes a hollow sound in the concrete floor. This can result from poor installation and maintenance, age-related issues, erosion from excess water, shrinkage from drought, improper soil settlement, temperature variations, and even burrowing animals.

Understanding why your concrete floor sounds hollow is the first step towards addressing the issue and keeping your home safe. This article will discuss the most common causes and how to determine the cause of the hollow sound in your concrete floor.

Causes of Hollow Sounds in Concrete Flooring 

If you hear a hollow sound on a concrete floor, it could indicate an underlying issue with the concrete, such as an air pocket or a void in the subfloor. 

Over time, if the subfloor is not supported correctly, concrete can become weak and brittle, causing the top surface of the floor to separate from the underlying concrete and causing a hollow sound when walking on it. This can signify a void, leading to cracking and other structural issues. 

Many things can cause voids in a concrete floor: 

  • Improper placement of reinforcements. If the rebar, mesh, or other reinforcement material is not placed evenly throughout the concrete slab, air pockets can form, leaving voids in the finished product.
  • Improper mixing of the concrete. If the mix is too wet or dry, it can lead to a weak concrete slab with voids.
  • Improper curing. If wet concrete is not allowed to dry slowly, the surface can dry prematurely, leaving pockets of air.

Other reasons include seasonal changes, flooding, and drought. Improper installation and maintenance can also lead to concrete voids.

Voids in a concrete floor can be a significant source of structural weakness and are not always easy to identify. It is essential to take preventive measures and use proper techniques to avoid voids in the first place.

In some cases, smaller voids can be repaired with resurfacing, while in other cases, the entire concrete layer may need to be replaced. Either way, it’s important to have it looked at as soon as possible to avoid further damage.

How To Fix Your Hollow Concrete Floor

If you have a concrete void that needs to be filled, your best option is to hire a professional. While filling a concrete void yourself is possible, it can be complicated and time-consuming. You don’t want to try it without the proper knowledge and experience. 

You’ll need the right tools and materials for the job, and it’s important to ensure you follow all safety protocols.

Several issues can cause concrete voids, so it’s essential to identify the underlying problem before attempting to fill it.

Once you’ve figured that out, you can choose one of the following options for filling the concrete void.

Sectional Slab Repair

Sectional slab repair is typically used to repair concrete slabs damaged due to shifting soils or poor construction methods. The hollow sound is repaired by breaking up and removing the slabs above it, then filling the underside with a concrete mixture.

This can be done in sections, which is where the name comes from and is best for small voids under concrete slab flooring. 

The new concrete must be adequately compacted and leveled to create a firm, level surface that will last for decades. The new concrete is then sealed with a waterproofing sealant to protect it from water, moisture, and other elements. 

The entire repair process can be done quickly and is often a cost-effective alternative to replacing the whole floor.

Sand-Cement Grout

Another option for filling voids under a concrete slab is to use grout. This involves injecting a high-strength grout mixture beneath the slab and into voids. The grout will expand, filling the voids and stabilizing the slab.

A sand-cement grout is a special mortar used to fill a gap in pre-existing concrete structures. It is made from a mixture of sand and cement and fills cracks, cavities, and other openings. 

Sand-cement grout is a cost-effective material and can be used for both interior and exterior applications. 

It is often used to:  

  • Stabilize footing
  • Patch and repair cracks
  • Fill expansion joints
  • Repair small sections of concrete

Sand-cement grout is ideal for filling voids in hollow concrete blocks and masonry walls. When properly mixed, it can provide a strong, durable material.

One drawback to this technique is that it can take up to three weeks to cure fully. It’s probably not the best option for homes that are lived in. 

Foam Injection

This method involves injecting expanding polyurethane foam into the empty space. The foam is injected through small holes in the concrete and is designed to lower soil pressures, reduce the chance of future movement, and restore the structural integrity of the slab. 

Besides being unaffected by water and saturated soil, foam injection allows the floor to be used immediately after the repair. It can be used in various residential, commercial, and industrial applications.


Mudjacking is a process that involves drilling a few holes in the concrete and pumping a slurry mixture of sand, cement, and water into them to fill the voids beneath the floor. 

It’s an economical and efficient way to lift concrete slabs that are uneven or have sunk due to erosion, loose soil, or water damage. 

Once the slurry is pumped into the holes, it starts to harden, raising the concrete to its original position.

Mudjacking can be used on driveways, patios, sidewalks, and other concrete surfaces. It’s a great alternative to replacing the entire concrete slab, but it can be time-consuming and the mixture can take longer to dry than other methods.


The hollow sound on a concrete floor is a problem many homeowners face. It can happen for several reasons, from improper installation to water damage, and it can be a sign of a problem with the subfloor or foundation.

If you notice this issue and suspect it’s caused by an air pocket or a void in the floor, it’s essential to have a professional inspect your foundation and determine its integrity.

Cracks in your foundation or any other signs of weakness should be repaired immediately.


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