Stucco is a mixture of aggregate (sand) and Portland cement. It is a brittle material and it has very little ability to flex. Occasional hairline cracks are fairly common, however, more extensive and larger cracks can become a problem. Two common problems are:
Internal stress: as Portland cement hardens or cures, it shrinks. This may result in cracking at the point where it is least resistant or at the center of large panels that have no relief.
External stress: Forces such as ground swelling, settlement, windloads, or thermal expansion and contraction can act on the cured stucco resulting in cracking. Moisture may enter stucco wall systems through cracks and where it abuts other materials such as window frames and doors.