A crack in brickwork is a record of something that moved. The useful question is never how long it is, which is what everyone measures first. It is which direction it runs, whether it is still opening, and what it is running through. Two cracks of identical length can mean a normal seasonal cycle in one wall and a failing footing in another.
Read the direction before the width
Masonry is strong in compression and weak in tension, so it splits along whatever line the movement puts in tension. A wall that has settled at one end cracks diagonally toward the low corner. A wall that has expanded along its length cracks vertically near the ends, often at a corner or an opening. A wall being pushed inward, by soil against a foundation, cracks horizontally. Direction narrows the cause faster than any other single observation, and it costs nothing to look.
The cracks that come with the material
Some cracking is the wall behaving normally. Clay brick grows slowly and permanently after it leaves the kiln, and it also expands and contracts with temperature, so long runs of brickwork without an expansion joint will eventually relieve themselves somewhere. Fine cracks confined to mortar joints, hairlines above a window or door where the lintel deflects slightly, and shrinkage cracking in a concrete block wall from curing are all common and usually stable. The tell is that they do not grow and they do not go through units.
Stair-step cracks and what is underneath
A crack that steps diagonally through the mortar joints is the classic settlement signature: the joints are the weak path, so the crack follows them. A narrow, old, painted-over stair-step crack that has not moved in years is usually a settlement that finished long ago. The ones worth acting on are the ones with clean fresh edges, width that increases toward one end, or a jog where one side of the crack has moved out of plane with the other. That last one, where you can feel a step with your fingertip across the crack, means the sections are no longer acting as one wall.
Vertical cracks through the units are a different story
When a crack runs through brick faces rather than around them, the force was strong enough to break the strongest part of the assembly. That is worth taking seriously wherever it shows up. Horizontal cracks are the other category that should not be watched casually, especially in a basement or foundation wall, because they generally mean lateral pressure from soil or water rather than settlement, and the wall can bow before it does anything dramatic.
Marking a crack so it tells you something later
The cheapest diagnostic in this trade is a pencil and a date. Draw a line across the crack at its widest point, mark the ends, write the date on the wall, and photograph it with something for scale. Look again after a full seasonal cycle. If nothing has changed in twelve months, the movement is historic and the repair is cosmetic. If it has widened or extended, you now have evidence of an active problem and a rate, which is exactly what an engineer needs and what turns a vague quote into a scoped one. Glass tell-tales and plastic crack monitors do the same job with more precision if the crack is one you are genuinely worried about.
Frequently asked questions
How wide does a crack have to be before it matters? Width alone is a poor test. A hairline horizontal crack in a foundation wall is more serious than a wider vertical crack in a garden wall. What matters is direction, location, whether it passes through units, and whether it is still moving.
Should a crack be filled while it is being monitored? No. Filling it removes the evidence. Monitor first, establish whether it is active, deal with the cause if there is one, and repair the masonry last, otherwise the repair cracks again and nothing has been learned.
Do cracks always mean foundation problems? No. Thermal movement, normal clay brick expansion, minor lintel deflection, and shrinkage in block all produce cracks in walls with entirely sound footings. Foundation movement is one cause among several, and it usually announces itself with a diagonal pattern plus doors and windows that have started to bind.