Cracks in Your Garage Floor? Here’s What To Do


The first time I saw a crack in my concrete garage floor, I didn’t know what to think. Was this just an unsightly gap? Or was my foundation about to crumble and my whole house collapse? If you have one or more of these cracks at your place, let’s figure out what causes them and whether you should worry. I’ll then tell you what I found out about how to fix them up.

Should You Worry?

Garage floor cracks may look like your house is going to be a special effect in an earthquake movie. But even if they’re a bit unsettling, they’re very common. You do not have to worry about most of them.

It May Not Be a Crack

Let’s get one thing out of the way first. Is it really a crack that you’re looking at? It could be a joint. Unlike unpredictable cracks, joints are intentionally put in by your home builder. I have joints in my garage and there are two main kinds:

  • A cold joint forms when a worker pours a section of concrete next to another section that has already hardened.
  • Because contractors realize that concrete is going to crack, they want to control where the crack will happen. They do this by putting control or contraction joints in the floor about 10 to 20 feet apart in both directions. A typical two-car garage can have joints separating the floor into four separate slabs. These joints weaken the concrete at specific points, so any cracks can happen at the joint in a straight line, instead of wildly across the entire floor.

What to Look At

A tiny crack that is 1/8-inch wide or less isn’t a concern unless you think it might fill up with dirt, oil, and automotive fluids. You also don’t have to worry about surface gouges or a shallow pit, which can happen if you accidentally drop that heavy power tool you borrowed from your neighbor.

You do have to check things out more if

  • The crack is wider than 1/8-inch,
  • The floor has many small cracks across it,
  • Or one side of the crack is lower or higher than the other side.

What Causes Cracks in Your Garage Floor?

Cracks appear on your garage floor for many reasons.

  • Shrinking. When concrete hardens, it shrinks, which makes cracks appear. These are no problem, so you can move on.
  • Bad installation. The garage worker screwed up by using low-quality materials or by not reinforcing the concrete with enough steel bars or steel mesh. You may need to do something about these cracks soon.
  • Poor drainage. If the soil under your garage has heavy clay or is denser than normal, it would retain moisture that can freeze during winter. That frozen soil then expands, spreading the concrete apart from itself and making it crack. You want to fix this before cold weather strikes.

Settling a Problem

I’m breaking this out into a separate section because it’s a biggie.

Over many years, the ground under your foundation can move or settle. This movement produces small cracks on a concrete slab, usually within the first years after a house is built. These blemishes are normally nothing to fuss about.

Major shifts in the soil can produce major cracks in your garage floor. These cracks can accumulate water and get bigger, with different sections of the floor sinking unevenly.

This type of major settling doesn’t just happen under the garage. You can find out if it’s going on in other parts of your home by laying a marble or small ball on a floor without carpet, such as in the kitchen or bathroom. If the ball moves on its own, the area it rolls to is lower than the area where it started. You’ve potentially got a major issue that can get worse over time.

If you suspect your foundation is becoming uneven, don’t try out to figure it out on your own. Call for a pro to check this out right away or your house may still show up in that earthquake movie.

How to Fix Cracks in Your Garage Floor

For most garage floor cracks, the good news is that you don’t need a pro that can cost plenty. You can do the job yourself if you follow these simple steps.

1. Examine the Crack

Use a level to make sure that both sides of the crack are even. If they are, you can repair it on your own. Now check the grade and drainage around your house. If the slope is not obvious, turn on a garden hose away from the foundation. You want the water to be flowing away from the foundation. If it flows toward your home, it may be getting under the garage and making the crack worse. You want to address this issue before you repair the floor because it will mess up your work.

Measure the size of the crack, which can determine the product and method for filling it.

  • Small cracks that are half-an-inch or smaller can be filled with an epoxy concrete crack filler.
  • Large cracks over a half-inch may require a mortar mix.

2. Pick the right materials.

Avoid using the water-based or latex-based repair products that you typically find in home improvement stores. While they are inexpensive and easy to use, they shrink over time and pull away from the concrete. They also do not accept sanding, paint, or epoxy very well.

Also avoid self-leveling sealants. They’re used by a builder for control or contraction joints that I’ve already talked about. They won’t work for repairs.

Rely on compounds that are based on epoxy, polyurethane, or poly-urea. They’re formulated to harden stronger than concrete and prevent water from leaking through. You can either sand or grind them so that they are flush with the floor, and cover them with paint, a clear coat, or epoxy.

2. Prep the Area

You want to clean the area in and around the crack so that the repair material bonds solidly to the concrete. That material won’t do that if the crack is full of dirt, loose concrete, and auto fluids. Also, dirt and dust can mix in with the filler, which will weaken it.

Start by parking your car outside the garage. You’ll need to keep the repaired area free of any weight for a few days, at least.

Clean the entire garage of any dirt and debris. Don’t forget the shelves and the top of any cabinets. You don’t want any dust from a cabinet floating into the concrete repair material and weakening it.

Keep the garage door closed! I made the mistake once of cleaning my garage with the door open to let in light and air on a hot day. A sudden gust of wind blew in leaves and dirt, undoing all my hard work.

A small crack may only need cleaning and vacuuming before it is prepped. A larger one may need you to prep the filler area by breaking off loose pieces. Always wear eye protection before doing this because concrete chips may fly into your eye.

  • You can break away any chips using a cold chisel, which is made for hard materials like concrete. They call it that because blacksmiths used it to cut cold metal instead of other tools for cutting hot metal.Put the blade of the chisel at an angle that is in line with the crack. Then with a hammer, hit the top of the chisel to break away the loose material. Do this on both sides of the crack until you’ve prepped its entire length. Scrub the crack with a stiff wire brush and sweep out the debris. Then vacuum what’s left.
  • You can also use a circular saw or diamond blade to cut into the sides of larger cracks. Aim for a groove that is at least half an inch wide.

If there’s any oil or other fluid in the crack, you need to clean that off. If the oil is wet, use kitty litter, sand, or baking soda to absorb any liquid. After the oil has dried, vacuum or sweep up the absorbent material. Scrub the area with water and a paste of baking soda. Wash the area and let it air dry. If needed, vacuum any remaining particles.

The crack is now ready to accept the filler.

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3. Fill It In

Nobody likes to read instructions, even me. But take time to read what’s printed on your filler material. Each one mixes and applies differently. Some may even require that you add in substances like sand. You want to get it right the first time and make the repair permanent, especially since your car will be rolling over your repair every day.

  • With a small crack, pit, or surface gouge, all you may need to do is apply a bead of filler in the area that is lower than the floor.
  • With a large crack, use a small trowel to apply the filler. Depending on the material you’re using, you can apply more material right away if it’s looking a bit thin, or may need to let it dry before adding another layer.

Don’t worry if the filler that you apply spills out over and around the crack. You’ll take care of that in the next stop.

4. Smooth It Out

With the trowel, smooth out the top of the filler material until it is level with the floor. Allow the filler to dry thoroughly and cure completely, which may take a few days if you have low temperatures and high humidity.

Sand the top of the filler until it is as smooth as the rest of the floor. Sweep and vacuum any loose material. You can now finish the repair with paint, epoxy, or a clear coat sealer. Allow that to dry as well.

If you did it right, your repair will blend in with the rest of the concrete floor and be hard to spot. You’ll be able to drive your vehicle right on top of it with no further problems.

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