A crack is more serious when it is between five and 15 millimeters wide (0.5 to 1.5 centimeters or even half an inch), since the cause could be more serious than simply dry plaster or a house that is settling. Before you panic, most cracks are harmless and occur in almost every home, even in new construction. A fine crack is simply a sign that the house is settling, which is purely cosmetic and can be easily fixed with putty and paint. To determine the severity of a wall crack, it is best to examine the shape of the crack and the direction in which it runs down the wall.
If the crack is vertical and starts near the apex where the wall and roof meet, it could be a sign that it was created when the foundation was laid after construction. Vertical cracks run in the same direction as drywall, which generally makes them less severe. If the crack is horizontal or extends at an irregular 45-degree angle, it could mean that there is a more serious problem, such as a serious change in the foundation or water damage. Surface imperfection cracks can often be repaired with drywall putty, sanding tools, and a fresh coat of paint.
Most serious cracks usually require professional help to determine the exact cause and may include some reconstruction to prevent further damage.