The causes for drywall cracks will vary considerably depending on the circumstances. The age of a home, materials used and quality of workmanship are all factors.
Cracks In New Homes
New homes will get cracks during the first year or two after constructions. One reason for this is settling of the material on the foundation. When the structure settles unevenly, it will create stress points in the drywall.
The drywall will crack at these stress points. Hairline cracks are nothing to be concerned about. Cracks that cause the drywall to separate may mean a more serious issue.
Drywall is installed in flat panels that are typically from eight to twelve feet long. Between the panels there are joints. Tape and drywall compound is used to finish the joints.
Joints should not be place at the sides of doors or other stress points. Tape and mud by themselves are not strong enough to keep from cracking. A heavy door that is slammed can crack drywall if there is a joint in the wrong spot.
Many older homes have plaster walls instead of drywall. Plaster can be more brittle than drywall making it prone to cracking.
Plaster will sometimes develop a multitude of spider cracks. The same stress point issues that apply to drywall also apply to plaster. Cracks in plaster walls can be fixed the same way that you do on drywall.