Ice damming can also cause water to enter the house if you don't have an adequate shingle underlayment or if the flashing (the aluminum or steel used over roof joints, like chimneys and pipes) has deteriorated.
The air in the attic needs to flow freely from the soffit to the ridge to reduce humidity.
Make sure there is a vent in areas such as the kitchen or bathroom.
Dryers should vent outside.
Inadequate ventilation could also be the culprit, which causes excessive moisture conditions in which mold and mildew thrive.
One way to combat the problem is to put a vapor barrier between the insulation and the inside of the house, which can reduce the amount of moisture that goes into a wall and help the moisture escape from the wall.
This doesn't mean you need to replace your roof, but these stains can reduce curb appeal.
Cleaning the shingles is an option, but that could shorten their life. Fortunately, StainGuard protection is available on many GAF Shingles, including Timberline Cool Series Shingles, Timberline Natural Shadow Shingles, and Timberline ArmorShield II Shingles.
(StainGuard protection is available only on shingles with StainGuard-labeled packaging. See GAF Shingle & Accessory Ltd. Warranty for complete coverage and restrictions.)
In both hot and cold weather, moisture in the attic can become trapped and condense on the rafters if it's not insulated properly. This moisture can drip down onto the insulation and reduce its effectiveness, potentially increasing energy costs.