They are called “control joints” or “contraction joints.” Concrete shrinks a little as it cures and hairline shrinkage cracks are unavoidable, especially in large slabs. While the cracks are unsightly, they do not affect the structural integrity of the slab.

   But it can look scary to a homeowner, so contractors cut a score line across large concrete slabs, like in a two-car garage or large driveway apron, running in both directions. It’s usually done with a concrete saw shortly after the concrete is placed and finished, and the typical depth is three-quarters to one inch—just enough to weaken the slab slightly at the score so that any cracks happen at the bottom of the score line, where they are not noticeable.  

   The photo below was taken at the outside wall of a garage, just below the siding. You can see the bottom of the scored notch at the side of the slab and where the hairline shrinkage crack opened below it. Cutting contraction joints in concrete does not guarantee that a meandering crack will not appear, but it reduces the likelihood of it.

Also see our blog post What is the difference between a carport and a garage?

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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 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? 

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