A crack is movement made visible
Something moved. Framing shrank as it dried, the structure settled, a temperature or humidity swing expanded and contracted the assembly, or a joint was never properly reinforced to begin with.
Filling that crack puts a rigid material across a joint that is going to move again. It will crack again, usually within a season, along exactly the same line.
Why tape is the whole answer
Tape carries tension across the joint. That is its entire function, and it is why a properly taped seam holds for decades while an untaped one fails quickly.
The durable repair is to cut the crack out along its length, bed tape across it in wet compound, then rebuild the surface in thin coats feathered wide.
When a control joint is the right answer instead
Where movement is significant and ongoing, particularly on long wall runs, large ceilings, and hillside properties, repeatedly repairing the crack is treating a symptom.
A control joint is a deliberate break in the surface that lets the assembly move without cracking the finish. It prevents the crack rather than repairing it on a cycle.
When to worry
Most cracks are cosmetic. Worth a structural engineer opinion are cracks wider than about a quarter inch, cracks that keep widening, diagonal cracks running across multiple surfaces, and cracks accompanied by doors that no longer close properly.
