The short answer

Measure the pattern before buying a filler. A narrow crack through a garage slab is not interpreted the same way as a widening crack that aligns with a foundation wall, carries water, or accompanies sticking doors and uneven floors. A photo and log can be more useful than a one-time visual label.

Build a crack record

Detail How to record it Why it matters
Location Room, wall distance, and nearby joints Patterns can reveal movement or construction joints.
Width Same scale and lighting each time A ruler alone can exaggerate a narrow gap.
Vertical offset Whether one side is higher Displacement changes the concern.
Moisture Dry, damp, staining, or active water Water can be the main repair problem.
Related symptoms Doors, walls, drainage, or seasonal change Context is more useful than width alone.

Do not widen or seal a crack before documenting it if you are trying to understand movement. Keep water away from the area, check gutters and downspouts, and note whether the crack changes after heavy rain or freezing weather. Those observations do not prove a cause, but they improve the service conversation.

Make the next measurement comparable

Choose identifiable points along the crack and label them on a sketch. At each point, photograph straight down with a millimeter scale in the same plane as the surface. Record the instrument used and avoid claiming more precision than it permits. A crack-width comparator can be easier to read than an ordinary ruler on a fine crack. Keep the original photographs; a crop without scale can make a stable crack look larger.

Record the two edges’ relative height separately from gap width. Note whether the observation was after rain, a freeze or a dry spell, and agree a review interval with the assessor if monitoring has been recommended. Do not wait for the next scheduled reading if a new offset, spreading damage or active water entry develops.

NRMCA’s concrete-cracking guidance describes multiple mechanisms, including shrinkage, restraint, settlement and freeze-thaw deterioration. This is why this guide provides no single width that certifies safety. A ground-supported garage floor, a suspended slab and a foundation element have different jobs. If you do not know what the slab supports, establish that before choosing a cosmetic filler.

Choosing a repair scope

Cosmetic filling, flexible joint treatment, drainage work, slab repair, and structural underpinning are different scopes. Ask the contractor to identify the suspected cause, what observation supports it, and what would make the recommendation change. Avoid an estimate that promises structural certainty from a photograph alone.

Limits

This guide cannot classify a crack as safe or unsafe. Rapid movement, significant displacement, water damage, or multiple related symptoms deserve a qualified structural or foundation assessment.