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:

• Is the stucco on a wood frame house allowed to extend down into the ground? 

• Does stucco need expansion joints?

• What are common problems with stucco? 

• Why is stucco that goes into the ground a problem at a wood frame house?

• What are the signs of stucco wall leaks? 

• What causes raised white lines of residue on a block wall that are crusty and crumbling? 

• What is the difference between soil subsidence, heave, creep, and settlement? 

• How much ventilation is required for the under-floor crawl space of a home? 

• What causes stair-step cracks in a block or brick wall?

• What causes a horizontal crack in a block or brick wall? 

• How can I tell if a diagonal crack in drywall at the corner of a window or door indicates a structural problem?

• What causes the surface of old bricks to erode away into sandy powder? 

• What are the pros and cons of concrete block versus wood frame construction?

• There's cracks running along the home's concrete tie beam. What's wrong? 

• What would cause long horizontal lines of brick mortar to fall out?

• How do I recognize serious structural problems in a house?

• What is engineered wood siding?

• Should I buy a house that has had foundation repair? 

• How can I tell if cracks in the garage floor are a problem or not? 

• What do you look for when inspecting vinyl siding?

• Why is housewrap installed on exterior walls under the siding? 

• Why did so many concrete block homes collapse in Mexico Beach during Hurricane Michael? 

• How can I tell if the concrete block walls of my house have vertical steel and concrete reinforcement?

• Should I buy a house with structural problems? 

• How can I tell if the exterior walls of a house are concrete block (CBS) or wood or brick?

• What are the common problems of different types of house foundations? 

• What are the warning signs of a dangerous deck?

• How can I tell whether my house foundation problems are caused by a sinkhole or expansive clay soil?

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