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Leveling a Concrete Slab

Help for a garage floor that's home to puddles

Ask This Old House Crew
Photo by Matt Kalinowski

I have a 6-year-old garage floor with a sealed surface that slopes toward the garage doors. It looked great at first, but I soon realized that there are some low spots where water tends to puddle. Is there some product I can use to fill these areas in?
— Ed, Schoharie, NY


Tom Silva replies: The product you need is concrete patching compound. It comes in a variety of formulations that can be applied by trowel to fill depressions in concrete. The compound is also good for repairing damaged concrete and other masonry. I usually go with 2-part epoxy formulations because they're extremely durable and have compressive strengths greater than the concrete itself. You have to protect your skin and ventilate the work area, but once cured, the stuff's inert.
The only question I have in your case is whether the sealer you've used will interfere with bonding between the concrete and the patch. After six years, I doubt this is a problem, but you might want to check with the manufacturer just to be sure.
To apply the patch, you need to follow the manufacturer's instructions. Here's what's involved, generally speaking: First, clean the concrete in the repair area with a grease-cutting detergent to remove dirt and loose material. Then scuff up the area with a coarse wire brush, and vacuum. Additional preparation steps depend on the product. For instance, some manufacturers say their compound can be feathered out at the edges, while others recommend chiseling or grinding a small, 1/16-inch-deep shoulder around the boundary of the patch to increase the thickness, and strength, of the compound.
After mixing the compound's resin with the hardener in the exact proportions required, apply it with a hand trowel. For a very smooth finish, wait until the patch starts to stiffen, then trowel it again, dipping the blade in water before every pass. Some compounds can be mixed with fine sand, which improves slip resistance and makes them thick enough to be applied to vertical surfaces.
Let the patched area cure for 8 to 20 hours. Its color probably won't match the surrounding area exactly, but it will be close. And at least you won't get your feet wet every time you get out of your car.


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