With the thaw we experienced recently, many home owners noticed water leaking from the ceiling on the upper floor of their home. This is caused from frost forming in your attic, not your roof leaking, at least in the winter months. The reason for the frost is that warm humid air (relatively speaking) is escaping into your attic and freezing there, through attic bypasses. These can be the attic access panel (obvious), or through plumbing vents in the walls (less obvious). The good news is there are a couple of easy ways to fix it.
#1 If you have a bathroom fan, run it at least an hour after your shower. This will allow the humidity levels to even out with the rest of the house. If you don’t have one, in the spring get one. Building code doesn’t require it if you have a window in the bathroom, but are you really opening that window when it’s -5 F outside? Make sure it is vented properly through the roof, not just into the attic. If you have a vent fan, and it isn’t vented through the roof, you will really have attic frost problems. Your next question about how to turn it off because you aren’t home for an hour after your shower. Get a licensed electrician to wire in a timer switch.
#2 Run the exhaust fan over your stove when you are cooking. If your hood only recirculates this won’t help, but you probably don’t need to have a whole house humidifier either.
#3 turn the whole house humidifier off. This is only for those of you that have a forced air furnace, and have a humidifier installed. The colder it is outside the lower the recommended setting is by the time we are below 0, it should be at its lowest setting anyway. Just turn it off and if as noted above, your stove exhaust hood recirculates, you probably don’t need one.
#4 Make sure the supply and return ducts in your house are balanced. This is probably the hardest to do/verify as a home owner, and will probably require a professional HVAC contractor to fully verify. That is why I put it last. Honestly though if you can verify the following you are probably OK. Is your combustion/make-up air, directly plugged into a return duct, it shouldn’t be. This was done to heat the air when it is cold outside, and to help keep the basement/utility room warmer. I believe it was an attempt to increase energy efficiency, that I don’t believe works. By doing this though you can over pressure your upper floors compared to the basement and force more air in to the attic. On a side note over pressurizing your upper floors and creating lower pressure in the basement, is a great way to increase the Radon level in your home. Don’t do that.
This is not an exhaustive list, but is a great place to start. In the Twin Cities, I would be more than happy to come out and see if you have any other causes/fixes for your attic frost. Once spring time rolls around, I would recommend checking that your roof ventilation is adequate so that the roof structure dries out, other wise the structure and decking can begin to decay, and there is the potential for mold growth.