Those long cracks in your ceiling can be an eyesore, but they may be alerting you to a more serious problem. Dripping water may be causing the cracks. If they are in a downstairs room, the problem may be leaking pipes and if upstairs, the cracks may signify a leak in the roof. Neither prospect is a happy one. On the other hand, the cracks may simply be the result of vibrations from a ceiling fan.
Why Drywall Cracks
Drywall is most susceptible to cracking along the seams, which are the joints between adjacent sheets that have been covered with tape and joint compound, or mud. Mud doesn't have much structural stability and cracks easily when it is subjected to stress. It is also water-soluble and becomes weaker when it gets wet. Drywall sheets are similarly vulnerable to moisture, especially in a ceiling, where dripping water can form pools and stand long enough to permeate the drywall. Cracks from moisture are distinguishable from stress cracks because drywall also deforms when it gets wet.
If you have a leak, close inspection of the cracks should reveal the presence of moisture. Suspect a broken water or waste pipe if you are on a lower floor, especially if the leak is under a bathroom. If you're on the top floor, check in the attic for signs of a leak in the roof. The source of the leak may not be obvious. Water sometimes travels along the roof under the shingles until it finds an opening, and then it can follow another path for some distance before it begins dripping.
Vibrations can cause dry cracks in drywall, and they usually appear along the seams. The source may be a ceiling fan, exhaust fan, air conditioner or other appliance. It usually isn't difficult to identify, but it can be mysterious if it is an appliance such as an old refrigerator on the floor above the one where the cracks are occurring, and the floors constitute separate living spaces, as in an apartment building or condominium. Impacts to the floor above a ceiling can also cause cracks to the drywall as well as consternation for those on the floor below.
Even though foundations are designed for stability, it isn't unusual for a certain amount of settling to occur as a house ages. Settling is hardly ever symmetrical, and it distorts the framing, which in turn puts stress on the drywall. The stress tends to separate the sheets and pop the nails, and cracks may form on the seams. They are hardly ever confined to the ceiling, developing on the walls as well. You can sometimes retard the development of such cracks by taping the seams with fibreglass mesh tape and hot mud, which a setting-type joint compound that resists cracking.