Cracks in plaster ceilings occur primarily due to aging, structural issues, poor workmanship or improper curing. Each of these issues has a specific look to it, allowing the owners of the structure the opportunity to identify what may be causing the cracks.
Aging causes cracks to appear in certain formations, depending on the age of the ceiling. At an age of about 30 to 40 years, cracks tend to develop running the longer length of the ceiling. At the 40-to-60-year mark, further cracks appear running perpendicular to the first cracks. Between 70 and 90 years of age, the ceiling develops even more cracks and begins to loosen. At this point the plaster needs to be repaired or replaced to prevent it from falling down in pieces.
Structural damage in plaster ceilings is due in large part to a lack of support between the ceiling and the floor above. As the floor above a plaster ceiling flexes because of weight or movement, the flexing transfers to the plaster covering, causing it to develop cracks. Structural damage also occurs as a house settles and moves in place or because of the expansion and contraction of the wooden lath holding the ceiling in place, due to moisture in the air.
The final major causes of cracks in plaster ceilings are poor workmanship and improper curing of the plaster. If the plaster is mixed incorrectly, or if there is a conflict in the base and finish materials, then the plaster can become brittle, causing it to fail early and develop a series of cracks throughout the ceiling.