Information on Roof Moss
In moist climates or even in warm climates with north-facing roof slopes unwanted moss and other plant life can grow on roofs. Moss and related growth can damage many roof types especially composite shingles and wood shakes. Homeowners can prevent moss and other growth in several ways.
First, trim overhanging tree limbs that prevent sunlight and wind from drying the roof surface. Shade-providing trees benefit and harm roofs. The shade helps keep roofs cool and the buildings they cover cool. However, moss, mildew, algae and fungus will grow on roof areas that are frequently moist and shady. It isn't necessary to remove trees or tall shrubs that might be shading a roof, but it is necessary to trim and thin trees that are providing shade over large sections of the roof. Remove just enough foliage to allow some light and adequate airflow on the roof's surface.
Second, regularly remove excess debris (leaves, needles, twigs, etc.) from the roof. Any material that holds moisture will provide an opportunity for moss and plant growth. The valleys of roofs are especially prone to moss growth because they are likely to collect leaves and pine needles throughout the year.
Third, place a zinc or copper strip under the edge of the ridge cap. The zinc or copper releases small amounts of zinc sulfate and coppersulfate down the roof when it rains so that moss growth is reduced and/or stopped. Some composition materials come with zinc granules already embedded in the shingles. The zinc and copper strips are made of metals that are naturally found in nature, but in very limited quantities due to their potential toxicity to other plant and animal life. Therefore, the use of these metal strips should be limited to moss and algae-prone areas of the roof. Copper tends to be more expensive, but it does last longer than the zinc strips.
Fourth, improve ventilation on the inside of the roof, to minimize moisture build up in the rafters and sheathing that supports the roof materials. With increased airflow, shingles and shakes are more likely to dry out and less likely to become a moss garden.
If moss is already growing on the roof, cleaning agents can stop the existing moss growth. This, however, is not a quick fix, and may require repeated applications to control the problem. These products can be very toxic, and should be used only according to the manufacturer's instructions. If you don't want to use a toxic cleaner (usually a zinc sulfate solution), you can wash the roof with water and a light brush to remove the moss growth. Whichever method you choose to kill/remove moss, work gently from the top-down to minimize the stress to shingles.
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