Your roof is designed to repel moisture and move the rain, sleet and snow away from your home. Unfortunately if you’re looking up at your roof and noticing that it’s started to foster new life in the form of moss, algae or mildew it’s a sign that your roof has been retaining water. Moss, algae and mildew often require damp conditions to grow, so if you are suddenly seeing patches appearing you should have your roof inspected to rule out water damage to the sub layers.