As an interior wall covering, drywall is superior to lath and plaster, which it has largely replaced, for several reasons. Among these are the facts that it is less expensive, easier to install, more fire-resistant and less prone to cracking. When it does crack, it's usually along the joints between sheets, and it can be the result of wall or roof movements, improper installation or even moisture, depending on where the cracks appear.
Minor Seam Cracks
Cracks may appear along drywall seams shortly after installation. If the wall is new, the framing may experience movements of expansion and contraction until it settles into equilibrium. If the seams aren't well taped, these movements can crack the joint compound, which also is subject to a certain amount of shrinking as it dries. The cracking may also be the result of using too few nails or screws to secure the drywall. While not necessarily normal, a certain amount of cracking in a new wall is no cause for alarm, and you can usually repair them without much difficulty.
Wall and Roof Movements
More serious cracks can develop as a result of the movement of the walls or ceiling. Foundation settling is one cause of such movements. It skews the walls, which puts stress on the drywall sheets, and in extreme situations, jagged cracks may appear, usually accompanied by popped nails. If cracks appear in the corners between the walls and the ceiling, it's often the result of stress developing in the roof trusses because of temperature differentials or uneven moisture content between the top and bottom of the roof. If the cracks are severe, an engineer should inspect the trusses.
Wood tends to expand more than drywall in conditions of high humidity, and the Drywall Finishing Council found in a 1997 study that drywall cracks can result from relative humidity changes of more than 50 percent. These cracks often appear where a feature, such as a door, skylight or drop ceiling, protrudes into a large drywalled area. The stress of the movements of the entire house are focused on these transition areas, and the seams frequently crack. It is especially common in high-desert areas where humidity fluctuations are greatest and is very difficult to prevent.
Some drywall cracks may appear as the result of poor installation or taping technique or unfavorable conditions during installation. Seams are vulnerable to cracking if the drywall sheets aren't well secured to the framing, if they are misaligned or if the gaps between them are too large. If you apply joint compound when the temperature is too low, it doesn't adhere well and may lift off. Moreover, if you install and tape drywall under conditions of high humidity, the edges of the drywall can swell, the joint compound may fail to dry or the framing may shrink drastically after installation. All can eventually produce cracks.
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