Properly installed and used a bathroom fan should not leak in the summer or in the winter. If the vent does not leak in the summer, especially during the heavy rain that often accompanies thunderstorms, it indicates that the shingles and flashings were installed correctly and shedding water off the roof as intended. Water that drips from the fan during cold winter weather is usually an indication of a problem with the venting system not the roof itself.
During cold weather some moisture will always condense on the inside of the vent pipe, where it passes through the cold attic.
To operate properly a bathroom fan venting system needs to move sufficient air up an insulated pipe and outside of the house to remove excessive moisture from the home. This requires a fan that is rated to move enough cubic feet of air for the size of the room being vented, a vent pipe large enough that this air flow is not restrict and a vent in the roof to allow air out of the home and keep water from leaking in.
Bathroom fans are rated in two ways; by the amount of air they move and by the amount of noise they make. Generally speaking the less expensive the fan the less air it will move and it will make more noise while doing it. Fans are rated from 50 cfm ( cubic feet per minute ) up to 180 cfm or more. The noise that they make is measured in Sones and usually range from .03 up to 4 for less expensive models . A sone is a measurement of sound in terms of the comfortable hearing level for the average listener. One Sone is the equivalent to the sound of being in a quiet kitchen listening to a quiet refrigerator running.
The Vent Pipe
Venting the fans has been done in a variety of ways in the past; metal pipe, plastic pipe, insulated and uninsulated often using the shortest route possible. For many years we dreaded the Chinook winds and the quick rise in temperatures that it brought, we would be flooded with calls from new home owners that had dripping fans. What we eventually found worked was using the proper size insulated plastic pipe for the fan flange and run the pipe horizontally on the ceiling wall board under the insulation for about two feet. Working with the HVAC companies and getting them to run the vent pipe across the ceiling was all we needed to do to eliminate most of these calls. The pipe can then run up to the roof vent with as few sharp corners as possible. This allows small amounts of moisture that condenses on the inside of the pipe to collect on the horizontal run without dripping into the bathroom. When the fan runs this moisture is drawn out with the air without causing any problems.
The Roof Vent
The vents that are used on the roof to vent fans are known as goose necks. They rise up from the roof deck about six or seven inches curving down toward the roof to vent the air down toward the roof deck, looking somewhat like a gooses head. They are larger than the pipe that they vent so that the air flow is not restricted. The opening is covered by a screen or metal damper to keep critters out and allow maximum air flow.
Operation of the fan
The fan should be run for fifteen or twenty minutes after the shower or bath is used. During very cold weather it is a good idea to run all of the fans in the home, even if the bathroom is rarely used, every couple of days for same length of time to remove the moisture that will naturally condense on the inside of the vent pipe from the slow trickle of air up the pipe. Running the fans will prevent excessive frost build up and the drips that can result.
Properly installed and used bathroom vents should give many years of problem free use and make the bathroom more comfortable.