How to Level a Garage Floor

Garage Interior with Cracked Slab

Juan Silva / Getty Images

You may first notice that your garage floor is out of level when something begins to roll—a tennis ball, a baseball bat, a child's wheeled toy. Or you'll see spilled water that doesn't pool up but instead develops long streaks as it makes its way downhill. But those are annoyances, more than anything.

An out-of-level garage floor may require leveling when any or all of three critical scenarios happen:

  • Gapped Garage Door: The garage floor may be so tilted that the garage floor does not meet the bottom edge of the door. When you're inside the garage, especially when the garage is dark and it's light outside, you'll see that telltale crack underneath the door.
  • Garage Floor Tilts to the House: If the garage floor has begun to tilt toward the house, water from the driveway, often boosted by overflowing gutters and downspouts, may flow toward the house itself.
  • Critical Services Are Affected: Garages often contain three appliances that absolutely need to be level: the washer, dryer, and water heater. Washers and dryers may stop working when they are out of level. Water heaters may tip over, especially in the event of an earthquake.

In most cases, leveling a garage floor will require the services of a professional. We take a broad overview of these services here to help you make an informed decision on which route to take.

Slab Jacking With Slurry

Slab jacking is one method that can correct the tilt of an entire concrete slab floor. You cannot do this yourself and it is expensive. But it is still cheaper than demolishing and re-building your garage. It utilizes your existing concrete slab, with only minor touch-ups required if the slab jacking efforts succeed.

Also called mudjacking or concrete raising, a matrix of holes, each about 1-2 inches in diameter, is drilled into the concrete. Then a slurry of sand, gravel, ash, and water is ​injected into the holes. 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 (however, this can be regulated by the technicians by adding different proportions of lime or sand). Slab 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. The slurry injections require larger injection holes and more of them, plus more product needs to be injected under the slab.

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 3-4 pounds of polyurethane ​is equal to 100-150 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 will cost about four times more than slurry-based jacking, mainly due to the cost of the source product.

Using Leveling Compound for Smaller Issues

Leveling compound or mortar can fix small areas of an uneven floor. In fact, this is regularly done for interior floors before the floor covering is installed.

But pouring out leveling compound or troweling down mortar across an entire garage floor isn't advisable. Masonry patch products tend to crack when applied thinly. Instead, apply leveling compound only as a spot fix if you might have a section that has cracked off and is now sloping down. Dips and sags are also great candidates for concrete leveling compound, as they are fairly localized.

All areas that will receive the leveling compound must be thoroughly clean for this cement product to adhere properly. Since motor oil can be difficult and even impossible to clean from concrete, abrading the concrete may be necessary.

A similar option is to build platforms from plastic or wood sleepers underneath 3/4-inch plywood in select areas that need to be level.

Article Sources
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  1. Water Heater Strapping Requirements. County of San Diego, Planning & Development Services.