What causes stair-step cracks in a block or brick wall?
Tuesday, June 26, 2018
A stair-step crack is a diagonal crack but, since mortar is usually not as strong as concrete block or brick, the crack migrates to the mortar joints as it zigzags along a path of least resistance. Sometimes a stair-step crack will mostly follow the mortar joints, then take a short-cut through a defective block or brick, before returning to the stair-step pattern.
If you draw a line through the center of a diagonal stair-step crack, then draw a perpendicular arrow to it, you will have the approximate direction of movement—either up or down—of the crack. Many cracks have a “hinge point” at one end and loosely pivot around that point, with the opposite end being wider. Others move more uniformly.
This crack pattern usually indicates settlement of the corner of a structure (blue arrows). The corner of the house is “laying down,” like the severe example shown below. But it can also indicate heaving of the area to the left of the crack (green arrows), and sometimes requires close examination to determine which side is moving.
A pyramid shaped crack of joined diagonals, as in the diagram below, is usually indicative of subsidence (dropping) of the area under the pyramid. Because window openings are weak spots when a wall is having settlement or heaving problems, diagonal cracks emanating from the corner of a window are common.
When the surface of one side of a stair-step crack is forward of the other side, which is what we call “differential,” it indicates a more significant structural problem. One side is rotating, like in the photo below.
Although crack patterns can often lead you directly to the underlying problem, sometimes a more in-depth analysis is necessary. See our blog post How do you analyze a crack from cause to effect? for more on this.
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To learn more about exterior walls and structures, see these other blog posts:
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