(Photo: Hugh Cairns)

Moss thrives in dark damp conditions so if your roof is sheltered by overhanging branches and foliage then it can be prudent to trim back surrounding trees so that more natural light falls on the roof during the daytime. Your gutters will be happier too.

Roofs are generally big surfaces. Most people just live under them, walk around them and rarely look up at them. One of the main purposes of our roofs is to get the precipitation that lands on them drained away as fast as possible. Good roof slope is extremely important to do that. Taking steps to remove debris and surrounding vegetation helps prevent moss growth.

Moss likes to grow on cool damp roofs. When the roof dries out the moss goes dormant and it will survive until the moisture returns. Roofs that are shaded by large trees or larger structures often provide longer periods of moisture, and that encourages moss build up. Moss build up impedes the flow of precipitation off the roof. Moss can damage roofs by attacking the shingles, forcing them apart and allowing leaks to occur.

Removing moss from roofs mechanically rarely eliminates the problem. Moss is pretty hardy stuff. Removing moss from roofs often involves scrapping or brushing, but it has to be done gently in order not to damage the roof covering. The granules on asphalt shingles protect them from the sun. Removing them will shorten the service life of the roof. Don’t power wash asphalt shingles, the high pressure is sure to harm the covering.

Since it’s hard to physically remove all the moss roots and small pieces from a roof surface, the moss and algae will re-grow from whatever is left. A number of products can be used to kill the remaining plant material or discourage new growth. They are widely available in gardening and home improvement stores and are usually applied as a powder but can be dissolved in water and sprayed with a basic garden sprayer.

You may also consider zinc or copper strips and flashing. Metal strips are placed in strips just below the ridge cap or installed as a ridge cap. When precipitation contacts the strips, the result is the production of a mild chemical that will reduce moss. Many asphalt shingle manufacturers now offer products with the granules having content of these metals.