How can I determine if cracks in the garage floor are a structural problem?
Tuesday, September 18, 2018
Most garage floors have cracks in them. It’s something we see every day and they are rarely a structural defect like in the photo above. Two factors contribute to cracks opening up in a garage floor slab: shrinkage of the concrete as it cures and settlement of the ground under the slab. All slabs naturally contract as part of the hardening process over the first couple of months after being placed, and many contractors cut score lines in the surface of the slab to encourage the minor cracks resulting from shrinkage to happen in the lines—where they are less obvious. See our blog post Why are there score line grooves in the concrete floor of the garage? for more on this.
We do not define a crack as “structural” unless it has one or both of these two characteristics:
- An opening of 1/8-inch or more across (wide enough to stick two side-by-side quarters into it), or
- Has “differential,” which is where one side of the crack is raised above the other.
Some cracks will have both traits, but even meeting our definition of a structural crack does not necessarily mean it is a serious problem. The important factor is whether a crack continues to widen over time, because there is typically a little settlement of the ground in the first two years after construction as it reacts to the new loads piled onto it. After the initial settlement, the site stabilizes and not much further happens.
Two-car garages also have a concentrated load at the corners of the garage door due to the transfer of a large area of roof weight to the sides of the door opening, which also often occurs near two outside corners of the house. This tends to create more settlement at those corners, with a resulting crack that runs diagonally between a side wall and the garage door, creating a triangle shape with a point at the outside corners with the extra weight on them. The photo below shows this typical crack pattern, with red lines added next to the cracks to highlight them.
Unless the cracks are unusually large, like in the photo below, we just recommend keeping an eye on them. You can also measure the width occasionally if they appear to be growing.
Because of consumer worries about concrete floor slab cracks—not just in garages—some builders in our area are now using a post-tension system to strengthen a slab’s resistance to cracking. Rods are set in the formwork before the concrete is placed, and stretched later to create a vise-like grip on the concrete at the ends and prevent cracks from forming. There is also a notice that is required to be posted at a home with a post-tension slab warning to not tamper with the slab. Also, see our blog posts Why is a garage floor sloped? and How do I recognize serious structural problems in a house?
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To learn more about exterior walls and structures, see these other blog posts:
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