Step 2: Read the direction
- Vertical, thin, in poured concrete: the most common crack there is. Usually the concrete shrinking as it cured.
- Diagonal from a window or wall corner: usually settling or stress concentrating at the corner.
- Stair-step through block mortar joints: part of the foundation is moving. Often settlement at a corner, or pressure on the wall.
- Horizontal, along the wall: the one to take seriously. Soil pressure is usually pushing the wall in.
Step 3: Check for the warning signs around it
A crack with any of these next to it deserves an inspection, whatever its width:
- One side of the crack sits higher or further out than the other
- The wall looks curved or leans in when you sight along it
- Water or damp soil comes through the crack
- Doors upstairs started sticking, or floors started sloping
- Cracks in drywall above the same spot
Step 4: Track it through a winter
In Minnesota, foundations move most during freeze and thaw, so one measurement tells you less than the same measurement taken again in spring.
- Draw a pencil line across each end of the crack and write the date.
- Photograph it with a ruler or coin in the shot, from the same spot each time.
- Re-check every month or two, and right after the spring thaw.
- Want precision? A plastic crack monitor (two overlapping plates with crosshairs) shows movement down to a millimeter.
A crack that grows past its pencil marks or gets wider between checks is an active crack. Get it looked at before the next winter.
When to call
Call for an inspection if: the crack is horizontal, it's 1/4 inch or wider, it's getting bigger, one side has shifted, or the wall is bowing. For everything else, measure, mark, and watch.
Next steps: how cracks are repaired, bowing walls, settlement, and what repairs cost.
Sources
- This Old House, “What to know about foundation wall cracks” (structural engineer Jim Graham on crack width and direction).
- InterNACHI, “Inspecting Foundation Walls and Piers.”
- U.S. Mint coin specifications (quarter thickness 1.75 mm); ISO/IEC 7810 ID-1 card thickness (0.76 mm).
- Avongard crack monitor documentation.