how to fill a void under a concrete slab (DIY)

This article will guide you on how to fill a void under a concrete slab. Two methods are presented in this article to repair the damaged concrete with the step-by-step instruction on how to do it yourself. Futhemmore, it provides discussion on which method is better to use depending on the area of the void underneath your slab.

how to fill a void under a concrete slab

Void in your concrete can lead to sloping sinkage, and cracking of your concrete. Concrete sinks if the supporting soil is weak or prone to compression. Also, another possible reason is improper drainage underneath your concrete allowing water to be accumulated. The weight of the slab, therefore, for this case cannot be supported by the supporting soil underneath creating sink or crack depending on the pressure exerted by the upper slab.

If you noticed your front porch, sidewalk, garage are sinking, you should immediately repair the damage area. This sinkage is noticeable in concrete steps and stairs. Here are some of the methods you can do to fill the void under your concrete slab:

  • Slab Jacking

Slab jacking involves drilling holes in your concrete slab then filling those holes with special mixture using a portable pump and flexible hoses. 

Advantage of Slab Jacking

  • Compared to changing your concrete slab, it involves less convenience and little mess when doing the project.
  • It maintains the integrity of your slab sometimes further stregthening the slab. It provides a stable base. Furthermore, it maintaing the color of your concrete.
  • There is no noise and waste generated as compared to slab replacement. Noise from breaking the concrete is associated with dust and debris generation. The slab that will be replaced will require to be disposed of in a landfill.
  •  Area surrounding the project site will not be greatly affected, thus no demolition and re-landscaping are required.
  • The repair can be done on any weather condition.

Equipment or tools you need:

  • Drill equipped with masonry bit

Common drill used is a gas-powered rotarty percussion drill with 1 to 2-inch diameter. It should have four points and can drill holes that are not “out-of-round”.

Should have at least 7-cubic foot capacity with 10 horsepower engines. It should be able also to withstand heavy use.

  • Grout pump

You can use either positive displacement hydraulic piston pumps or hydraulically operated progressive cavity pumps. The pump to be used should be able to deliver pressure from 1 to 100psi and able to fill the void with grout in a uniform and consistent manner.

Step-by-step instruction

Step 1. Drill holes

Using a drill equipped with a masonry bit to drill at least four holes through the slab. The number of holes may vary depending on the size of the slab. Smaller slab requires at least one or two holes in the middle of the slab. Larger slabs, on the other hand, may require three holes.

Also, the thickness of the slab affects the distance of your holes. Commonly, the holes are drilled three to eight apart, but no closer than 1 foot from the edge and as much as possible, should be drilled approximately equal from each other. The usual range of the hole size is 1 to 2 inches in diameter and placed in a manner that the holes will be strategically 


Step 2. Prepare the fill material.

There are various fill materials you can utilize. The most common fill materials are listed below:

  • Sand-Cement Grout

Sand-cement grout is a mixture of Portland cement, sand, and water. Sand is the best option instead of pea gravel because it can be easily compacted. Also, this is the most

cost-effective fill material, however, disadvantage of using this material includes inextensive void penetration and will require a significant amount of time for the filled concrete to be cured (usually it will take two to three weeks depending on the air-flow.

Using a spade, mix the fill material in a wheelbarrow. You can add an expander to prevent the shrinkage of the material during the curing phase.

  • Expanding Polymer Foam

Utilizes a polyurethane, a high-density material to be injected through the holse. The fill material will undergo chemical reactions that will initiate the expansion of the foam sealing the void defects. Polyurethane foam achieves slab stability and non-erosive. It does not require curing, thus, the replaired slabs are usable after application. It also penetrates hard to reach even the deepest and smallest sinking gaps.

Step 3. Connect a hose to slabjacking pump.

Slabjacking pumps will be used to fill your slabs beneath. Run the hose connected to the slabjacking pump to the holes leading to the void.

Step 4. Pump the fill material to fill the void.

Pump in the fill material and set your pump to a 10 psi pressure. Fill in the void under your slab by pumping the material through the hose.

Step 5. Switch to other holes.

Fill the void evenly by switching the hose to other holes. This is to keep the fill material level underneath your slab to be as even as possible. Continue switching from one hole to another until the void is completely filled.

Step 6. Fill in the holes.

Fill in the holes with a mixture of concrete mortar. Make sure to level the mortar to the slab using a trowel.  Then allow the concrete to cure for two to three weeks if you used sand-cement grout. If you use expanding polymer foam, allow tthe foam to sit first for a few minutes for the chemical reaction to occur.

  • Sectional Slab Repair

Alternative method for filling the void under your concrete slab is to repair discrete sections of the damaged concrete slab. It is advisable to use this method if there is a large area of void underneath your slab.

Step-by-step instruction:

Step 1.Identify first the area where there is a void underneath.

The area of your slab with void can be determined commonly with a sound. If the area of your void is large it can be easily determined by tapping your concrete and if it produces a hollow sound.

Step 2. Break up the concrete then fill it with sand.

Raising the slab is the first option to create access to the void. However, for this case when the void is large already, the concrete slab will be prone to breakage, thus, it is better to just break the defective area of your concrete slab using jackhammers. Then fill it with sand-cement grout, you don’t have to pump it in since there is already a relatively enough area to pour the grout.

Step 3. Ensure even filling of fill material.

Ensure that the filling is properly compacted because if not it will just allow for void to reform again. You can use a trowel to evenly apply the grout on the void.


Void underneath your concrete when not repaired immediately will lead to extensive damage to the slab that slab jacking nor sectional slab repair can repair the concrete. This may require the slab to be entirely replaced requiring major construction work. To prevent this from happening, it is important to install a proper drainage system under your concrete and ensure that the supporting soil underneath is properly compacted. This will prevent absorption of water inside the concrete preventing void formation.

FAQs: DIY Fill Void under Concrete Slab?

Can you use spray foam under concrete slab?

Yes, you can usre spray foam under a concrete slab. Spray polyurethane foam (SPF) offers advantages over installation of rigid foam under your foundation and basement slabs. However, onle closec-cell SPF can be used in subslab insulation using open-cell foam does not provide adequate compressive strength.

What is the best fill under concrete?

Granular material or removed earth,open-graded materials such as pea gravel of uniform size should be backfilled beneath concrete flat-work. These materials are self-compactive and still require the top 6 to 12 inches to be compacted with well-graded gravel or crushed material to reduce settling over time. In addition this kind of material allows water accumulated in your foundation to be drained into a sump that will allow it to be pumped to a storm sewer.

Can I use sand under concrete?

Yes, you can pour sand under concrete. Concrete slabs thens to float on the soil especially to uneven surface, Thus, soft grounds or voids underneath may create crack when subject to heavy weight like vehicles. Sand or gravel over clay of about 4 inch thick or other poorly draining soils can be placed under the slab to provide even support.

Should you put plastic under concrete?

6-mil Visqueen (polyethylene plastic) had ever since been used as vapor retarders to barred vapor under concrete slab. However, recent studies suggest that it is seldom effective because plastic allows lots of water vapor to pass through and it can get damaged during installation of concrete reinforcement. This damage will create holes under your concrete slab allowing accumulation of water vapor.

Is clay a good base for concrete?

When your soil is clay-heavy one, pouring concrete can be difficult for clay tends to compress when subject to weight causing it to sink or shift over time. It can also absorb and leach moisture cfro the concrete itself that will result in a brittle base and uneven curing. It is still possible to pour your concrete above clay soils without encountering problems once there is proper preparation.


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