Foundation cracks: cosmetic vs. structural

Nearly every concrete foundation cracks. Concrete shrinks as it cures, and shrinkage cracks appear in the first year or two of a building's life and then stop. The question is never whether there is a crack; it is whether the crack is still moving.

Usually cosmetic

• ·Hairline cracks, roughly the width of a business card edge or less
• ·Vertical or near-vertical cracks in a foundation wall, evenly spaced
• ·Fine map-pattern crazing on a slab surface
• ·Cracks that have been there unchanged for years and are clean and dry

Worth an engineer's look

• ·Cracks wider than about 1/8 inch, or ones you can slide a coin into
• ·Diagonal cracks running from the corners of windows, doors, or openings
• ·Horizontal cracks in a foundation wall, especially with inward bowing
• ·Stair-step cracking through masonry or brick mortar joints
• ·One side of the crack sitting higher or further out than the other
• ·Cracks that leak water, or show fresh light-colored concrete edges
• ·Any crack accompanied by sloping floors, sticking doors, or new drywall cracks upstairs

Horizontal cracking with bowing is the one to treat as urgent. It suggests lateral soil pressure on a wall that was not designed for it.

Direction tells you more than width

A vertical crack is usually concrete doing what concrete does. A diagonal crack is a hint that one part of the foundation has moved relative to another — differential settlement, expansive soil heave, or a footing bearing on fill. A horizontal crack points at pressure from the side rather than movement from below.

Bay Area specifics

Much of the Bay Area sits on expansive clay that swells in winter and shrinks through the dry season. That annual cycle opens and closes cracks, which is why a crack that looks different in March than it did in September is telling you something real. Hillside lots add downslope creep, and bay-margin fill adds settlement and liquefaction risk.

Monitor before you panic

1. 01Photograph the crack next to a ruler or a coin, and date the photo
2. 02Mark the crack ends with a pencil line and the date
3. 03Repeat every few months, across at least one wet season and one dry one
4. 04A crack that has not moved in a year is behaving; one that widens is not

If the crack is growing, or it falls into the second list above, get it looked at before it becomes a repair. Foundation work is far cheaper designed than discovered.