Cracks in the ceiling can seem to appear overnight. In some cases this can be true, depending on the environment in which you live. Heavy moisture, cold weather, hot weather and the ground on which the home is built could all be factors in creating a cracked ceiling. Another factor that can be a cause for cracked ceilings is the load placed on the home. In some cases, heavy loads placed on wooden floors especially second stories of homes can be the main cause of ceiling cracks.
Seams and Joints
Most cracks appear at the seams, joint and corners of the ceilings. Seam can come from a number of reasons. Improper drywall finishing techniques can be a factor. When drywall is finished, there should be a well laid coat of drywall compound below the seam tape so the paper can adhere well to both sides of the joint. If the compound was applied sparingly, even the smallest amount of movement can create a crack line. Cracks that appear in the upper corners, where walls meet the ceiling, could be the result of collected moisture. Moisture can work its way into a corner joint and begin to affect the bond of the drywall compound. Generally, if the corners are cracking from moisture a small area may be turning black from mould growth.
Too Much Weight
Heavy loads placed on floors and ceilings can create movement in the wood framing, causing the drywall to shift. The addition of bookshelves along side a wall can place an extremely heavy load on the floor. If the floor is a wood framed floor, it can create a sag in the framing members if they're not properly supported. This can cause the walls to move ever so slightly and crack joints in the corners and above the floor that has the heavy load placed upon it. A heavy load placed in a storage area or room above the first floor can cause the ceiling joists to bow and sag also, which in turn will shift the framing members to crack drywall joints.
Improperly constructed foundations can also cause ceilings and walls to crack. If the foundation for a home is not properly constructed with correct depth and drainage, the load bearing part of the footer may move. This movement can create permanent damage to the foundation and cause ceilings to buckle and crack. Excessive moisture in the ground can actually cause a foundation to "float" from the water table lifting and moving the foundation. This is generally caused by poor drainage surrounding the foundation of the home. If the home is an area of extreme cold weather, the frost can "heave" a foundation up and down with the seasons. This movement is again from an improper depth of foundation. In areas that are use to dry climates, a heavy rain can force water into the ground and cause it to swell. This swelling can force movement on slabs and foundations, creating the concrete to shift slightly, translating into cracks in the ceiling and surrounding walls.