Water stains on the ceiling around your bath fan may indicate a leak coming from the vent cap on your roof, but condensation is the more likely culprit. If bath fan ducting isn’t properly insulated, the moist air from your house will condense inside the duct.
The first step is to head to the attic. You may find that the insulation simply needs to be refastened. If you see that your duct isn’t insulated at all, pick up duct insulation at the home center. Use zip ties or aluminum tape to fasten the insulation.
If your ducts are properly insulated, another potential cause of condensation is lack of use. Bath fans have a damper designed to keep the outside air from entering in through the fan, but that valve doesn’t stop warm air from escaping. Whether you use your bath fan or not, some warm air will still escape into the ducting. On very cold days, that warm air is likely to condense inside the ducting, especially if the fan is never run to dry it out. Guest baths are particularly prone to this problem.
The bottom line: Before you climb on the roof to look for leaks, make sure your bath fan duct is insulated, and run the fan more often and for longer periods. Switches with built-in timers are available.