What is the difference between a control joint and an expansion joint?
Sunday, July 25, 2021
A control joint is used to minimize cracking in the surface of a material. One example would be the control joints used for installing stucco over a wood frame wall. Because the stucco expands and contracts at a different rate than the wall under it, control joints create a flexible gap to absorb the movement and avoid surface cracking. Shown below is the bottom of a stucco wall where the control joint was covered over during a repair. The repair was likely needed because the joint was installed improperly originally, and covering it with more stucco simply made the situation worse, with new cracks certain to appear soon. See our article Why is my stucco cracking? to learn more.
Another type of control joint is the groove cut into large concrete slabs, often visible at garage floors. It does not keep the slab from cracking but, because it makes a weakened line in the slab thickness, the groove encourages the inevitable minor cracks to form there–where they are less visible and disturbing to a homeowner. Go to our article Why are there score line grooves in the concrete floor of the garage for more on this.
Unlike a control joint, an expansion joint extends all the way through a structure to create room for both sides of it to move independently. Expansion joints are typically necessary in large buildings and bridges. Although the expansion/contraction rate of a building material may be only a tiny fraction of an inch per foot of length, when that is multiplied 40, 50, or 100 times, the movement becomes a problem needing expansion joints at regualr intervals. The easiest place to observe expansion joints is when crossing a long bridge. They also occur in large buildings, but are often artfully concealed.
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To learn more about exterior walls and structures, see these other blog posts:
How To Look At A House
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