Crack rater: structural, or just ugly?
Four properties decide it, and width is only one of them. Direction matters more, and progression matters most.
Why direction beats width
A vertical crack in a poured wall is usually shrinkage: concrete cures, loses volume, and cracks. Extremely common, structurally boring, and if it leaks, injection fixes it for a few hundred dollars.
A horizontal crack is a different animal. A basement wall is a retaining wall; soil pushes on it laterally. A horizontal crack, particularly in the middle third of a block wall, is that wall failing in bending. Width is almost irrelevant to the diagnosis — a 1 mm horizontal crack in block warrants an engineer, while a 3 mm vertical one in poured concrete often warrants nothing.
Stair-step cracking follows mortar joints because that is the weak path, and it means one part of the foundation has moved relative to another. That is settlement or heave, and on expansive soil it is heave more often than homeowners expect. Why expansive clay does this.
The measurement that actually decides it
Movement over time. A crack that has been the same width since 1998 is a scar. The same crack growing 1 mm a year is an active failure. Nobody can tell which from a single photograph — including the person quoting you. Mark the ends with pencil, write the date on the wall, measure the width at a marked point with a crack gauge or callipers, and log it. The tracker does the arithmetic.
Page data updated