What does a horizontal crack at the brick ledge or slab edge mean?

Reviewed by Preston, Oklahoma foundation repair consultant · Updated 2026-10-14

In short: A horizontal crack along the base of brick veneer or the outside edge of a slab can come from the brick and concrete moving against each other, from the slab edge lifting or dropping with the soil, or from frost or soil pressure on shallow walls. The crack alone does not tell you which; its width, any offset, and an elevation survey of the floor do.

What does this crack look like?

The brick ledge is the narrow shelf at the outside edge of a slab or grade beam (a thickened concrete beam around the perimeter) where the first row of brick sits. This sign is a crack that runs roughly level along that line, either through the top of the concrete edge or in the mortar joint just above it. It is common on slab-on-grade and post-tensioned houses and also appears on stem walls and basement walls.

Why does it happen?

Brick and concrete move in opposite directions

Fired clay brick takes on moisture and expands permanently, with most of that growth in the first weeks after firing and a slower rate for years ¹. Concrete does the opposite and shrinks as it cures ¹. Where brick bears directly on concrete, that mismatch creates shear stress that can crack the concrete or pull the brick away at a corner unless a bond break or flashing separates them ¹. Brick industry guidance calls for a bond break, such as flashing, building paper or polyethylene sheeting, between foundations and brick walls for this reason ². This is a materials issue, not soil movement; see material shrinkage and thermal cracking.

The slab edge lifts or drops with the soil

Oklahoma sits in a region where expansive clay is widespread ³. Expansive clay swells when it gets wetter and shrinks when it dries. Under a slab, the perimeter can rise as the soil swells and fall as it shrinks, so the edge may move up or down through the seasons ³. Because the corners of a uniformly loaded slab press on the soil with about one-fourth the pressure of the center, they tend to rise relative to the middle when clay swells ⁴. A slab edge that rotates or rises this way can open a horizontal crack at the ledge. This pattern is described under perimeter wetting heave (edge lift).

Frost and soil pressure on shallow walls

Swelling or frost heaving of saturated soil can bend shallow masonry or block walls and produce horizontal cracks ⁵. Inspection guidance suggests suspecting frost heave when damage sits above the local frost depth or appeared after a cold snap, and checking for broken downspouts or poor drainage when expansive soil is suspected ⁵. On basement or stem walls, the same horizontal crack can come from soil pushing inward; see lateral earth and hydrostatic pressure.

How are the causes told apart?

How is it documented?

• Width and offset. Record the crack width and whether one side sits higher or farther out than the other. A clean crack edge suggests recent movement, while a dirty or previously filled crack may be inactive ⁵.
• Change over time. A crack monitor can record movement; cyclical movement may take six months or more to measure ⁵.
• Floor elevations. Engineering guidance treats a relative floor elevation survey as the step that goes beyond a visual first impression ⁶. A plotted survey shows whether the edge is high or low compared with the interior. See elevation surveys.

Related

• Perimeter Wetting Heave (Edge Lift)
• Frost Heave
• Freeze-Thaw and Frost Depth
• Distress Pattern Library
• Symptom Matrix