Large basement concrete slabs naturally expand and contract. This type of movement will inevitably cause cracked basement floors. If it was built and poured properly, the builder should have used control joints within the slab to help actively determine where and which way these cracks are going to occur. If it wasn’t, then there’s no telling where or even how they may spread.

Actively planning for cracks on a basement floor requires the use of control joints. Unfortunately if your slab is already poured and hard this is going to be pretty difficult to do if not impossible. But if it’s part of a new construction, having control joints in your basement slab will allow for cracks to form uniformly and expectedly due to movements caused in the drying and shrinking process as well as for contraction and expansion over time and temperature changes. These control joints are hidden and have to be made before the concrete cures. What happens is that the crew finishing your slab will actually slice through the entire thickness of the slab in a grid-like fashion. After it’s all sliced, they’ll re-trowel it to make it all smooth again giving the appearance of a completely solid piece. When it all cures and the slab eventually moves it will crack along those grid lines. In other words, when the concrete does crack as we all know it eventually will, you’ll know where it will crack and that it will also crack in a straight line instead of randomly all over the slab.

How big does the crack have to be to worry about?
Again, there’s some general guidelines amongst basement contractors that they tend to follow. If you have a crack on your basement floor that either separates or gives a vertical displacement of more than 3/16ths of an inch, then you’re likely to have a problem that needs some attention. You better look at correcting this situation before you start looking at any basement flooring ideas.

What do you do about cracked basement floors?
There are literally a number of things that you can do to remedy the cracks in concrete floors, but they are really dependent on what type of crack and what kind of flooring application you plan on installing over it. In all actuality though, probably the most prolific one is to do nothing at all. Take for example that you are just going to be laying a simple pad and carpet over the floor. The separations will be completely unnoticed and the differences will simply just disappear. But if you are going to install a much thinner type floor covering, something like vinyl tiles or linoleum, those same cracks and lifts in the floor could could cause some unwanted dips and spikes in the flooring itself. If you want to fix it you’re going to need to go rent a grinder from a local tool depot, as I’m sure you probably don’t just have one laying around and besides, it’s not like you need to buy one as you’re not going to need it again after this job, and just grind down all those irregularities that are shooting out of your concrete slab. You can then take a floor leveling compound and work the floor back into a level state of being.

Now if your cracks are a bit more serious or you’ve got a fairly large spot that sticks out quite a bit higher than the rest of the floor, these will entail quite a bit more work and it’s often quite hard to pinpoint it down to a certain cause. The good thing about this is, you’re not likely to have this type of situation as they are not that common. If you do happen to live in an area that has a fairly large substrate of clay underneath, like the Midwest, you may be a little more proned to some problems with your floor. The clay alone can play a major role in breaking up a concrete floor quite easily as it sucks up all that moisture and expands. To fix something like this can be difficult. There’s really only one or two things you can do. Get rid of the clay, or, you need to find a way to divert that water so it doesn’t get sucked up through the clay around your home. Either one can be a very painstaking task and you’d want to make sure that that is what’s causing the problem so I’d recommend checking with a professional not only to see that this is the cause, but what it would cost to get it corrected. Might turn out to be the best money you’d ever spent on your basement before you ever did anything in your basement.

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