Water stains are not uncommon in homes. Spotting these early and acting quickly is the best way to avoid expensive repair costs.

Roof

A wet spot in the ceiling or walls may not always be a problem with the plumbing system. If the spot has been growing after heavy rain, there may be issues in the roof, such as loose or missing tiling, or a leaking vent. For example, in time of extreme weather such as ice or snow build-up, areas of the roof can freeze, expand, or crack. Melted ice can subsequently leak into the ceiling.

Ridge vent

The ridge vent is another potential area for leaks. In times of extreme weather, such as a big storm or snow storm, ice or water can blow into the ceiling through the ridge vent, causing a leak in the ceiling.

Shower and bathroom leaks

If the ceiling water stain is expanding during certain times of the day, the problem may reside within the home rather than the roof. For example, a ceiling stain that expands in the morning or evening may be attributable to the morning or evening shower or bath time. The bath and shower should be checked for leaks in this case.

Source:http://plumbinghelptoday.com/denver-plumbing-repair-blog/2013/07/why-is-there-a-water-stain-on-my-ceiling/