A crack in a brick wall is a record of something that happened. Read properly it tells you what moved, in which direction, and whether it is still moving. Most of the panic I get called into on Westville houses is about cracks that stopped moving before the current owner was born.
The four questions
1. What shape is it? A crack that steps diagonally through the mortar joints is following the weakest path in a wall being pulled apart by differential settlement. A vertical crack straight through units usually means tension across a longer span — thermal movement, an expansion problem, or a missing movement joint. A horizontal crack in a bed joint, particularly in a basement wall, means lateral pressure, and that one is different in kind from the others.
2. Where is it? Cracks concentrate at corners, at openings, at changes in foundation depth, and where an addition meets original construction. A crack running from the corner of a window is almost always about the lintel or the opening; a crack at a re-entrant corner is about the footing.
3. How wide, and does the width change along its length? A crack that is wide at the top and closes toward the bottom indicates rotation — one part of the wall tipping. Uniform width along the run points to straight translation. Width alone is a weak signal; taper tells you the mechanism.
4. Is it moving? This is the only question that determines urgency, and it takes time to answer.
Monitoring, cheaply
A plastic crack gauge costs about $25, screws across the crack, and reads movement to a quarter millimeter. Read it monthly for a year and write the date on each reading. A cheaper version: a pencil line across the crack with the date, plus a photo with a ruler in frame.
What the year tells you:
- Seasonal oscillation that returns to zero — thermal or moisture movement in the wall or in clay soil. Annoying, not structural. Repair cosmetically and expect it to recur.
- Steady progression in one direction — active settlement or an active load problem. This is the one that needs an engineer.
- No measurable change — historic movement that has stopped. Repoint it and forget it.
Twelve months feels like a long wait when a wall looks alarming. It is far shorter than the time it takes to undo an unnecessary underpinning job, and underpinning a house that was not moving is a genuinely common outcome of acting on one dramatic photograph.
What the repairs look like
Static cracks: cut out and repoint in a matched mix, $12 to $30 per linear foot. If the crack passes through units, individual bricks can be cut out and replaced to break up the line.
Cracks that need tying: helical bars bedded in the bed joints across the crack at 18 to 24 inch vertical spacing, $60 to $140 per linear foot. This restores tensile continuity without a visible repair.
Active movement: the crack is a symptom and repairing it is pointless until the cause is addressed — drainage, a tree, a failed lintel, a footing. Distinguishing a step crack from a vertical one matters here, because the two point at different causes; the differences are set out in more detail in the comparison of step and vertical cracking. The Brick Industry Association technical notes at gobrick.com cover movement joint spacing and crack control in brickwork if you want the underlying numbers.
Frequently asked questions
How wide is too wide? As a rough field guide, hairline to 1/16 inch is cosmetic, 1/16 to 1/4 inch warrants monitoring, and above 1/4 inch or any crack with visible offset between the two faces deserves professional assessment. Rate of change matters more than absolute width.
Should I fill a crack while I monitor it? No. Filling it destroys the evidence. Mark and measure it, and leave it open for the monitoring period unless water ingress is causing damage, in which case use a flexible sealant that will not mask movement.
Does a crack mean the foundation is failing? Usually not. Most cracks in older masonry are from historic settlement, thermal movement, or a local problem such as a rusting lintel. Foundation failure is one possible cause among several, and it is the least common in houses that have already stood for a century.