Garage Floor Sounds Hollow – Why And What To Do?

Have you ever observed your garage floor producing a hollow sound and wondered what caused this and what to do about it? So, we researched why this happens and how to deal with it, and here is what we discovered.

When you hear hollow sounds from your garage floor, it can mean that you have a delaminated concrete floor. This suggests that the top surface of the garage floor has split from the underlying concrete.

You can fill the void left by the hollow concrete floor by slabjacking. But it could be a difficult undertaking for the typical homeowner, depending on how big the space beneath your floor is.

Continue reading as we explain more about floor delamination, why it happens, and what other signs your floor is experiencing it. We'll also cover how to perform slabjacking, how long it takes to perform, and how much it costs. Additionally, we'll discuss the dos and don'ts you need to keep in mind to avoid the risk of having a delaminated concrete floor.

Large garage in whie and blue tones, Garage Floor Sounds Hollow - Why And What To Do?

What Is Concrete Floor Delamination?

Floor delamination is the horizontal separation between a floor slab's top surface and the concrete beneath. Depending on the origin of the delamination, delaminated thicknesses can be smaller or thicker than the typical range of 1/8 to 3/8 inches.

Surface delaminations are unfortunately difficult to spot during finishing procedures but become obvious later after the concrete has dried or soon after the floor has been put into use.

What Is Concrete Floor Delamination - Gray old grungy textured plaster wall with black spots.

How Does Concrete Floor Delamination Develop?

In this section, we'll show you how floor delamination develops.

Here is how it's formed:

  • The cement and aggregate settle when you pour and compact new concrete. The lighter components move toward the surface as a result of this natural settlement, which results in bleeding i.e., the displacement of surplus mixed water and trapped air.
  • When finishing activities begin too soon and the surface is sealed off before bleeding is finished, air and/or water might become trapped beneath the mortar of the densified surface.
  • As it hardens, concrete develops subsurface gaps where the trapped water or air is. These gaps weaken the areas directly beneath the surface, which could eventually separate with slab use.

As you try to determine the size of the delaminated region, very thin mortar layers above the delamination may even separate when hit with a hammer.

How Does Concrete Floor Delamination Develop - The texture of natural stone with traces of erosion and delamination on a gray surface

When Should You Worry About Your Concrete Floor Delamination?

Delamination may not impact a concrete slab's performance if it just affects one area. However, if it is prevalent, you have a more significant issue that you need to handle. Delaminations can be mild or severe and take many different forms.

When somewhat sticky mixes with larger percentages of particles are put together, blisters frequently occur. Blisters are tiny, localized delaminations that range in size from one to three inches. They arise in places that are finished too fast and have moderate to high evaporation rates.

A delamination problem can be fairly pervasive and affect bigger zones of a slab surface when finishing procedures are carried out too early and uniformly over the slab.

What Is Concrete Slabjacking?

What Is Concrete Slabjacking - Filled in mud jacking drilled holes in concrete pavement

Slabjacking is used to avoid floor shifting, settling, and unevenness caused by developed hollows on everything from patios to basement floors, porch floors, and garages.

As the name implies, slabjacking involves raising all or a portion of a concrete slab. Instead of using a mechanical jack, in this method, you'll use soil and other materials. In the region where a void is thought to exist, holes are made in the concrete slab, and fill is pumped in. This "jacks up" the slab and fills the gap.

You might be able to perform slab-jacking on your own. Even when you still need help from contractors, slabjacking is frequently much less expensive than tearing out and replacing the slab.

How To Perform Slabjacking To Your Concrete Floor?

The method of slabjacking itself is amazing. Listed below are the steps to completing this task:

  1. Drill many holes in the slab to begin the slabjacking procedure. To successfully raise back the slab into an even position, you need to deliberately design the holes. Space the slabjacking holes two to four feet apart with a diameter of two inches for each hole. Depending on the severity of the concrete damage, a slabjacking operation may require a different number of holes.
  2. Mix cement, water, and additional components like sand, dirt, or limestone aggregate. Then, through the drilled holes, hydraulically inject this mud-like substance beneath the concrete until the pressure of the cement mixture created when the spaces are filled elevates the slab. Monitor the volume of slurry injected into each hole to achieve an even surface.
  3. Pour fresh concrete into the holes after removing the slurry mixture from them.
  4. Seal any potential cracks. You still need to complete this task after the slabjacking procedure. Water can seep under the slab if you don't fill these cracks. This can cause further soil erosion, which might cause more harm to the concrete. You may perform the caulking yourself if you have the necessary supplies and equipment.

How Long Does It Take To Repair A Delaminated Concrete Floor?

The majority of simple slabjacking tasks only take one or two hours to complete. Larger works take longer but often take one day to finish. The slab is usable once the task is done, but you should hold off on applying any considerable weight to the concrete for at least a few hours.

How Much Does It Cost To Slabjack A Delaminated Concrete Floor?

How Much Does It Cost To Slabjack A Delaminated Concrete Floor - Interior of a clean garage in a house

On average, homeowners spend between $500 and $1,200 for concrete slabjacking. Larger jobs and difficult-to-access areas could result in greater fees, which would raise the overall cost.

How To Avoid The Risk Of Concrete Floor Delamination?

How To Avoid The Risk Of Concrete Floor Delamination - Interior of the empty garage in the residential house.

Starting the final finishing of the slab after the bleeding process is done is the simplest technique to avoid delamination. However, this might not be as simple as it seems.

To help you prevent delaminations, you should abide by the following dos and don'ts:


  • If the concrete is air-entrained, finish with care. An extremely dense, hard-troweled surface is not required for air levels of more than 3%. If you offer the surface in outdoor spaces where slip resistance is an issue, with a light steel-troweled finish, or a broom finish, there is less chance of delamination.
  • To encourage a more equal setting of the mix, preheat the concrete or use small amounts of the set accelerator. This will make it simpler to estimate when the project will be finished.
  • When atmospheric evaporative conditions surpass concrete bleeding rates and are high, take extra measures. Bleeding is still happening even though it can't be seen clearly across the entire slab, so place the domed garbage can lid on the surface and wait a few minutes before removing it. After lifting the lid, if you observe bleeding water, there is still bleeding that's occurring.
  • Take action to prevent quick evaporation circumstances, such as wind, low humidity, and high temperatures, from occurring on the project.


  • Don't prematurely seal or close the slab surface. Before beginning finishing operations that seal the surface, let the bleeding finish. Be cautious that sticky mixes containing more cementitious or sand particles will bleed quite slowly.
  • Don't complete slabs that are installed on impermeable surfaces too soon. Bleed water must rise to the top when pouring concrete over an impermeable subbase because it cannot escape out of the slab's bottom. Especially in cold weather, the amount of bleed water will be higher, and the bleed period may be longer compared to slabs cast on a porous sub-base.
  • When the ground is colder than 40°F, avoid pouring concrete on it. The lower portion of the slab takes longer to set because of the chilly soil, which makes predicting when it will finish much harder to predict. Cover the subgrade and keep it until you finish placing the concrete to lessen this differential set retardation.

Final Words

A delaminated floor may cause the hollow sound you hear from your garage floor. Having concrete delamination in small areas of your garage floor is okay. However, you need to fix this problem when it affects bigger areas of your garage.

Additionally, although you can do slabjacking yourself, you need to employ experienced contractors to do the job perfectly and avoid any additional damage.

Check out our other articles here:

Does All Laminate Flooring Sound Hollow?

Why Is My Bathroom Floor Sinking?