Cracks in a garage floor get more attention than other areas of a home for the simple reason that they are exposed to view in a big, open area, with no floor covering over them. There are two main reasons for garage floor cracks and a cluster of other less likely possibilities:

1) Concrete shrinks as it cures - A concrete slab hardens by a process called hydration, in which the water in the mix reacts with the other components to form a crystalline structure. The whole mass shinks slightly as it cures and the process is rapid on the first few days, then slows down to a crawl over the next year to 18 months that it takes to fully cure. So a few hairline cracks, like in the top of the photo below, are common and not a structural problem.

   Larger cracks of 1/8” thickness or more, where you can stick two quarters held side-by-side into the crack, indicate a potential problem. Also, if the crack has “differential,” meaning that one side of the crack is lower than the other, it adds to the significance of the crack.

    Because these small hairline cracks are inevitable, many builders cut deep score lines across the slab, which encourage any shrinkage cracks to happen in bottom of the score slot, making it less of a concern visually. 

2) Structural settlement - While the shrinkage of the slab is complete within a couple of years after a home is built, the settlement of the soil under the house comes in two phases. There is the initial settlement that happens in the first couple of years as the soil adjusts to the new loads placed upon it, and then there is the later settlement over the ensuing years. This later settlment can be due to an inadequate foundation, tree roots under the home, or any geologic movement. See our blog posts What is the difference between soil subsidence, heave, creep, and settlement? and What should I do about a tree with roots running under my house? to learn more.

    Also, sinkhole activity under or near the home is always a possibility in Florida. Go to our article What are the warning signs of a sinkhole? for details.

    One of the ways that builders have attempted to keep concrete slab cracks away is post-tensioning, a technique that builds extra lateral compression into the slab. Read more about it at Why is there a WARNING! POST-TENSION SLAB sticker in my house?3) Other possibilities - If excessive water is added to the concrete mix to make it more workable or it is troweled improperly, a maze of tiny hairline cracks can form on the surface of the slab. It’s called crazing and, while not a structral problem, it is unsightly and bits of the surface may pop off over time. Also, if loads are put on the slab too soon after placement, or slab reinforcement bars or mesh are missing, not sufficient, or placed too close to the surface, cracking can result. 

    The concentrated dead loads on the slab at either side of a double-car garage door are yet another possible cause of cracking of a garage floor slab. We suggest also reading our post How can I tell if cracks in the garage floor are a problem or not? for more on this. 

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Here’s links to a collection of our other blog posts about STRUCTURE AND ROOMS:

• What are the building code requirements for notching and boring holes in a wall stud? 

• What causes dark or light "ghost" lines on ceilings and walls?

• Can you access or exit a bedroom through another bedroom?

• What is the difference between a carport and a garage? 

• What are simple ways to find the cause of a ceiling stain?

• What is the minimum size of habitable rooms in a house according to the building code? 

• Why is my garage ceiling sagging? 

• How can I identify what kind of wood flooring I am looking at?

• Why does my concrete floor slab sweat and get slippery?

• What is the minimum ceiling height for rooms in a house? 

• How much can I cut out of a floor joist? 

• How can I tell if my floors are sloping?

• Why do the floors slope in this old house? 

• What are the common problems when a homeowner converts a garage to conditioned living space, such as a family room?

• How can I tell if a wall is load-bearing? Which walls can I take out? 

• Can I remove the interior walls under a roof truss?

   Visit our STRUCTURE AND ROOMS and GARAGES AND CARPORTS pages for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.