Hairline cracks are thin, shallow, and often cosmetic; structural cracks are wider, migrate through layers

https://knoxodpo629.cavandoragh.org/how-to-tell-the-difference-between-hairline-and-structural-cracks

Hairline cracks are thin, shallow, and often cosmetic; structural cracks are wider, migrate through layers, or shift over time. Measure width, monitor growth, and inspect nearby framing to determine repair urgency.