A rainy day should be a time to curl up with a book and some tea -- not curl up in a corner watching a waterfall come down into your home. Water leaks coming from the ceiling can be tough to figure out because the original source of water could be coming from so many places. Take care of ceiling leaks as soon as possible as they can rapidly grow worse, leading to structural damage.
The basic cause behind a ceiling leak is water either dripping through an existing crack or water pooling somewhere and creating a crack by wearing through the material it's on. Depending on the strength of the material, the water could take some time to wear through. Water can also travel along a surface inside the attic or crawlspace to leak through into a room that's nowhere near the original outside leak.
Rain and Wind
Too much rain on the roof is an obvious cause of ceiling leaks. The rain pelts and wears away at the roof, eventually dripping into any attic or crawlspace, where the process repeats itself on the layer that acts as your ceiling. Tim Carter of Ask The Builder notes at least 10 places on a roof where water can break through, including valleys between sections of the roof, cracks in the "roof field" of shingles and flashings. A strange but very real possibility is that wind has blown water into areas, such as under eaves or in the connection between floors, pushing water into any cracks.
Leaky Pipes and Overflow
Ceiling leaks can occur on any floor, not just the top one. A major culprit in these cases is the pipe network for the building's water system. If a pipe develops a crack or breaks, the water dripping into the wall can pool along the ceiling's upper surface and wear through the ceiling materials. Overflow from a tub, sink or toilet, or even excessive water dripping from clothes you've handwashed and left to dry can create a ceiling leak for the story downstairs if enough water pools or finds its way to any cracks in the floor. One other possibility is condensation -- look around pipes, vents, skylights and anything that could possibly form a layer of condensation. eLocal Plumbers notes that even condensation on a toilet can lead to a leak.
Always check a structure for mould if you've discovered a ceiling leak. While mould can be cleaned up, some types require professional intervention -- it isn't always typical bathroom-type mildew. Drenched building materials can also become weaker, sometimes to the point of collapse. This can happen regardless of the type of building; for example, KGET in Bakersfield, California reported in December 2010 that a leaky roof caused a bedroom ceiling to collapse on a toddler; the "Hartford Courant" in Connecticut reported in January 2011 that a high school classroom collapsed due to a leak while "The State News" in East Lansing, Michigan reported tiles in a cafeteria ceiling fell on students after the ceiling began to leak in 2009.