There are plenty of innocent reasons for cracks to form on ceilings, and a few serious reasons as well. Cleaning cracks out, reattaching loose plaster and patching a ceiling is often all that's required to restore a ceiling's smooth surface.
How Ceilings Hang
Ceilings attach to the frame of the house on the bottom members of trusses or joists between floors. Plaster coats cover wallboard, blue board--composition board formulated to bond with plaster--or lathe--2-inch strips of raw lumber nailed across truss rails to form a grid for plaster. Cracks in the plaster that covers the ceiling often result from the movement or improper treatment of one or more of these elements. Most ceiling cracks involve only finish coats of plaster. Ceiling cracks caught early can often be easily repaired.
All houses respond to their environments. As lumber cures and during cold winters, it contracts, but ice dams cause expansion of wallboard and truss boards. It expands during hot, humid summers. Rough, unfinished lath expands during summers and contracts during winter for its entire lifetime. Soil that's heavy with clay expands when wet and contracts during dry spells. As these small expansions and contractions take place over the years, thin or uneven places in plaster come under more stress and eventually give way. Movement cracks may begin near the centre of trusses or ceiling joists due to the effects of humidity and wander along the joist. Other cracks form as soil pushes on floor joists at ground level.
Older homes spring little cracks along the edges of wallboard, or as the keys--bits of plaster shoved between lathe to hold the ceiling close to the lathe framework--lose their grip. As plaster or joint compound dries, cover coats may crack along the sides and around the corners underlying materials. Key failure causes irregular cracks, due to the pull of gravity on unsupported, brittle plaster. In addition to movement due to weather and settling, joists and lathe move as the load they carry shifts--for example, when a large family moves into a house previously inhabited by a couple. As weight increases and shifts, the joists bend and shift to accommodate it, perhaps beginning small stress cracks in the ceiling below.
A few builders still work without specifications from an architect, but the practice was commonplace a century ago. Sub-standard materials, inexperienced craftsmen or poorly engineered structures can result in ceiling cracks that reappear, even after repairs. Cracks also appear when previous repairers failed to clean out, or key, cracks, giving new material a wide-topped trapezoidal opening to grab hold of in the plaster. Plasterers who fail to match repair plaster, perhaps patching lime plaster with gypsum plaster, risk delamination that will signal its presence with cracks. Cracks caused by materials problems are common in older homes, and cracks that reoccur should be investigated for underlying problems.