Homes, particularly new builds, have to settle into their foundations. Even after this occurs, natural earth movements and erosion underneath the foundation can cause additional settling. Homes, on average, settle less than 1/4-inch per year. While this amount may not seem like much, to the building materials inside your home it is significant.
Plaster, while it creates a very hard surface, is not a flexible material. Once it has set, it isn't going anywhere. When the house shifts naturally, the plaster stays in one spot. Since it cannot move, the plaster cracks as the house shifts. Homeowners with plaster walls typically just keep plaster patching materials around because this phenomenon is so common. It is definitely a downside to this material, but for many, this is offset by its appearance.
Drywall is more flexible than plaster, but even it will crack, especially as the material ages or the house shifts more dramatically. In addition, drywall that is taped with mesh tape or is installed improperly will be more apt to crack. The settling cracks in drywall are typically tiny and more like hairlines than what appears in plaster. While they can be patched, it is likely the cracks will appear again.
Vertical and diagonal cracks above doors and windows are normal. This is the pattern that shows up with normal settling of the house. The depth of the crack will be slight, no more than a 1/4-inch, and the crack will look spidery and uneven in appearance. The exception to this rule is if a drywall seam was not taped and only spackled. In this case, you will see a crack through the entire section of drywall.
When to Worry
Horizontal cracks are definitely cause for concern. In this instance, a foundation specialist should be called in to inspect the home. Overly large cracks more than a foot in length are also cause for concern, unless the drywall was not taped. If the cracks are accompanied by large sections of debris, the drywall or plaster will need to be patched and fixed. Excessive and/or sudden cracking, as defined by three or more cracks, or large cracks that suddenly appear, are worrisome, and a professional should be called in to investigate the issue.