DEAR TIM: What appears to be a roof leak has developed in the ceiling of my 5 x 7 foot bathroom. A professional inspection of the roof revealed no problems. I looked into the attic and noticed that the bath ventilation fan pipe simply discharges air near a roof vent. The noisy fan doesn't seem to move that much air. Could this be a problem? How can it be corrected? F.E.

DEAR F. E.: Your bathroom fan is very likely the cause of your wet ceiling. My guess is that this problem became noticeable when the outdoor and attic temperature began to drop. Moist, humid air from bath activities was exhausted into your attic. This water vapor then condensed on the cold surfaces (rafters, roof sheathing, exposed nails) in the attic space. This condensed water dripped onto your ceiling and produced the phantom leak.

Bathroom ventilation is extremely important. Many people think that you need to simply exhaust the steam from hot showers or baths and unpleasant odors. Residual surface moisture on towels, carpeting, wash rags, and shower walls can significantly raise the humidity level in a bathroom after you have dressed and left the house. This moisture can be removed by controlling the bathroom fan with a humidistat rather than a standard on and off switch. The humidistat will automatically turn the fan off when the humidity has dropped to a satisfactory level.

The moist, humid air from bathrooms and kitchens must be ducted to the exterior of a house. This ducting can be achieved through an exterior wall, a soffit or eave, or the roof. Many fan manufacturers make special fittings that allow you to extend ductwork to the exterior of your house.

Your existing fan may not be moving enough air for several reasons: it's not sized properly, the exhaust pipe is too long, sized improperly, or is obstructed, or the fan is simply worn out. Many of these things you can check.

Fans are sized depending upon how many cubic feet of air they can move each minute at a given pressure (static pressure). This quantity of air is often expressed as cubic feet per minute (CFM). The Home Ventilating Institute recommends that bathroom air be totally replaced a minimum of 8 times per hour. Kitchens require a minimum of 15 air replacements per hour.

Your bathroom contains 280 cubic feet of air (5x7x8). Using the above recommendation, you need to exhaust 2,240 CF in an hour (280x8). Purchasing a fan with a capacity of 37 CFM will meet this minimum standard (2,240 / 60).

It is vitally important to follow manufacturer's instructions regarding exhaust ducting. If you use the proper size duct pipe, minimize bends, tape all joints, and exhaust your fan to the exterior, you should have a dry ceiling in the future.