How to Flatten an Uneven Garage Floor

By
Lee Wallender
Lee Wallender
Lee Wallender

Lee has over two decades of hands-on experience remodeling, fixing, and improving homes, and has been providing home improvement advice for over 13 years.

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Updated on 07/04/22
Reviewed by
Deane Biermeier
Deane Biermeier
Reviewed by Deane Biermeier

Deane Biermeier is an expert contractor with nearly 30 years of experience in all types of home repair, maintenance, and remodeling. He is a certified lead carpenter and also holds a certification from the EPA. Deane is a member of The Spruce's Home Improvement Review Board.

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Fact checked by
Sarah Scott
SS
Fact checked by Sarah Scott
Sarah Scott is a fact-checker and researcher who has worked in the custom home building industry in sales, marketing, and design.
Learn more about The Spruce's Editorial Process
Garage Interior with Cracked Slab

Juan Silva / Getty Images

Project Overview
  • Working Time: 3 - 5 hrs
  • Total Time: 2 days
  • Skill Level: Intermediate
  • Estimated Cost: $50 to $100

Structural problems with a concrete garage floor can be relatively simple, such as a slab that has cracked over time and settled in spots to create an uneven surface. Or, they can be much more major, such as when the entire slab settles unevenly and causes a pronounced tilt in an undesired direction. This can become a problem if the tilting is so major that the garage door no longer seals against the slab when closed, or if the slab begins to tilt toward the house, inviting rain water to possibly infiltrate the foundation.

There are three possible options for correcting these issues.

Before You Begin

Unfortunately, DIYers are equipped to handle only relatively minor slab problems, such as when cracking causes small sections of the floor to settle, creating an uneven surface; or when the slab was not poured and finished carefully, resulting in shallow dips where water tends to collect. This kind of unevenness is relatively easy for almost anyone to fix, using leveling cement and simple tools. Leveling cement is similar to ordinary mortar, but it has a finer texture that makes it more pourable, and it usually includes a latex or epoxy additive to help it bond to the cement slab.

Tip

The number of cementitious patching, leveling, and surfacing products available in stores can be somewhat bewildering. For repairing an uneven concrete garage floor, the best product will be a dry powder leveling cement mix designed for indoor and outdoor applications. It is the same product often used to prepare indoor floors for ceramic tile. Be aware that some leveling cement products recommend a latex or epoxy additive, either mixed when you add water to the dry powder or brushed onto the repair surface before applying the leveling cement. Avoid using products advertised as combination patch/levelers, as these are thicker that can be hard to smooth out. Instead, fill deep areas with patching cement, then use standard leveling cement to create the finished surface.


More major repairs to a slab that has tilted out of level over its entire expanse will call for a professional service to provide a mud-jacking or foam injection repair. These processes use specialized equipment and techniques that are far outside the skill level of most homeowners.

Avoid the temptation to pour mortar or floor leveling cement over the entires slab and spread it out to create a new smooth surface. It is very difficult to successfully apply a uniform layer over the entire surface, and such thin veneers rarely form a secure bond. Within months, your slab may look worse than ever as sections of the veneer flake away.

What You'll Need

Equipment / Tools

  • Broom
  • Shop vacuum
  • Scrub brush
  • 8-ft. 2x4
  • Angle grinder with coarse grinding wheel
  • Pointed trowel
  • Caulk gun (if needed)
  • Plastic bucket
  • Paint brush (if needed)
  • Paint roller (if needed)

Materials

  • Concrete oil stain remover or dishwashing soap
  • Concrete patch (if needed)
  • Leveling cement
  • Bonding additive (if needed)
  • Concrete sealer or paint (optional)

Instructions

How to Flatten an Uneven Garage Floor Using Leveling Cement

  1. Clean the Slab

    Begin by thoroughly sweeping and vacuuming the entire slab. Clean away any oil stains with a commercial concrete oil-stain remover, or with ordinary dish soap and a scrub brush. If stains cannot be removed entirely, abrading the area with a grinder will help the leveling cement adhere.

  2. Mark the Low Spots

    Use a long straightedge, such as long 2x4, to move over the floor, looking for low spots beneath the straightedge. Use light pencil marks to circle low areas you want to fill.

    Tip

    Few concrete slabs are completely flat. Dips and depressions less than 1/4-inch deep are usually not troublesome enough to warrant repair.

  3. Grind Down High Edges

    Use an angle grinder equipped with a coarse grinding wheel to forcefully abrade down any high edges, such as where the slab has cracked and sagged unevenly. Also, grind down any areas with stubborn stains.

    Use the shop vac to remove all traces of grinding dust.

  4. Repair Deep Cracks and Depressions

    If the vacuum has sucked out deep cracks that leave sizable voids, use non-shrinking concrete patch to fill these areas. Concrete patch is available both in premixed tubs and in caulk-gun tubes for application. Use a pointed trowel to force the patch material down into the crack to completely fill gaps.

    Allow the patching cement to fully dry and cure before proceeding. For most products, this requires about 24 hours.

    Tip

    Don't expect the leveling cement alone to fill deep depressions in the slab. It is not designed for that purpose. Instead, always fill major voids with patching cement and allow it to fully dry before applying leveling cement to fill the remaining low spot. Leveling cement alone typically will fill depressions up to about 1/2 inch deep. Anything deeper calls for a patching cement repair before applying leveling cement.

  5. Prime the Area (Optional)

    If the leveling cement directions call for it, brush on a bonding liquid over the area to be filled with leveling cement. This should be done only where the leveling cement label directs you to do so.

  6. Mix the Leveling Cement

    In a plastic bucket, mix the leveling cement according to manufacturer's directions. The mixture should be a thick but pourable slurry. If so directed, add a latex or epoxy bonding liquid as you mix the leveling cement.

  7. Apply the Leveling Cement

    Slowly pour the leveling cement over the depression, and use the 2 x 4 or another long straightedge to screed over the area, ensuring a uniform, flat surface. There is usually no need to trowel the surface, as leveling cement is fluid enough to self-level into a smooth, uniform surface.

    If desired, you can lightly brush over the surface of the leveling cement before it is fully hardened, which will give it a texture to patch the surrounding area.

  8. Allow to Dry

    Allow at least four hours for the leveling cement to dry enough to walk on. Allow at least 24 hours if you plant to seal or paint the slab.

  9. Apply Sealer or Paint (Optional)

    Applying a fresh coat clear sealer to the entire garage floor is a good idea after the patch dries. Or, if the floor was painted, you can now paint the entire floor to blend in the repaired areas with the rest of the surface.

When to Call a Professional

If the issue with your garage slab goes beyond small dips and depressions and instead involves major tilting of the slab, then solving the problem will probably require hiring a professional mud-jacking or foam-jacking service.

Mud Jacking With Slurry

Mud jacking is one method that can correct the tilt of an entire concrete slab. The process begins by the technicians drilling a matrix of holes, each about 1 to 2 inches in diameter, into the concrete slab. Then. a slurry of sand, gravel, ash, and water is ​injected into the holes. This is a tricky process that requires much skill and experience. Technicians do not randomly inject slurry. By test and intuition, they have to make sure that the slab is supported in all areas. If one area gets too much push, it can crack the slab by forcing it upward too much. If one area gets too little slurry, it is left unsupported and can eventually crack and break.​

It takes about one day for the slurry to cure to its eventual strength of about 2,400 psi. Mud jacking with slurry will be far less expensive than polyurethane-based jacking, due to the lower cost of the source product and because it is a well-established method with many companies offering the service.

Slab Jacking With Polyurethane Foam

Foam injections are similar to slurry injections but are considered more effective because technicians can better predict the expansion of the polyurethane foam. Foam slab jacking requires holes that are slightly over 1/2 inch in diameter. After the hose is inserted into the hole, the foam is injected and a calculated amount of time passes while the foam expands to its maximum size. After the injections are made, the holes are covered over with patching cement.

The foam product is far lighter than mud, too. About 3 to 4 pounds of polyurethane ​is equal to 100 to 150 pounds of slurry. One reason for having a lighter base product would be to avoid overburdening the soil with yet more weight. Because of this, it is often used in weight-sensitive areas, such as near bridges.

Poly-based slab jacking costs about four times more than slurry-based jacking, mainly due to the cost of the source product.

Article Sources
The Spruce uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.

  1. How to Fix Low Spots on a Garage Floor. AAA Concrete Raising Company.