Moss and algae will grow wherever it can take hold on a cool damp surface. When the surface dries out these organisms become dormant and survive until the moisture returns. Roof tops in the temperate rainforest areas of British Columbia provide such conditions. Roof areas that are shaded by overhanging trees or other buildings provide longer periods of cool moisture that extends growth time and encourages a more rapid build up. Aside from aesthetics a build-up of moss can impede the flow of water off the roof and, depending on the slope, eventually build up enough water to divert it under water shedding roof systems such as asphalt shingles, cedar, or concrete tiles etc. and into the building. On flat roofs the resulting ponded water can build up to a point where it will cause a leak or lead to structural deterioration and damage.

How to remove moss and algae has been answered in a variety of methods involving physical removal, chemicals or a combination of the two.

The first consideration to reduce moss and algae growth is to reduce the time of available moisture, when moss and algae can grow. These plants become dormant when dry but will restart growth when the conditions are favorable. This usually involves cutting back overhanging tree branches and removing excess debris such as leaves and twigs to facilitate drying. Removal of existing moss can also be done at this time with a broom and/or plastic scraper. Care must be taken to avoid damaging the surface of the roofing materials when doing this. Granules on asphalt shingles protect them from the sun and all materials can be broken if care is not taken when walking on, sweeping, or scraping. Damage to the materials or their surfacing will shorten the service life of the roof. Roofing product manufacturers and associations recommend not using power washing devices. The force of the water can remove the surface or break away pieces of material causing immediate leaks or shorten the service life of the roof system.

It is nearly impossible to physically remove all the moss roots and small pieces from a roof surface, and being quite hardy, 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.

These are widely available in gardening and home improvement stores. They are usually applied as a powder but can be dissolved in water and sprayed with a basic garden sprayer. If applied as a powder the roof surface should be damp but not raining at that time. Application instructions and cautions from the manufacturer should be followed. These types of products release slowly and can be effective on their own for a year or more. Overspray onto adjacent plants and drainage into ponds etc. should be avoided.

Combinations of Trisodium Phosphate and chlorine bleach are widely used in varying concentrations. These kill the moss and algae quickly when applied from a garden sprayer. Concentrations used vary from 5 gallons of water with 1 cup of TSP to 2½ gallons of water with 1 cup of TSP and 2½ gallons garden areas etc. The roof surface will become more slippery as it is wetted so caution to start at the far end from the roof access is advised, avoiding walking on the wet surface.

Organic Fatty acids are a natural soap product that can be used to kill algae and moss. When used as directed it is safe around plants and pets etc. The product comes in a liquid concentrated form that can be diluted into a garden sprayer tank or used with a proportioning hose end sprayer. Most garden supply and home improvement stores carry these products.

These metals when placed in strips just below the ridge cap or installed as a ridge cap flashing will react with the water running over the surface to produce a mild chemical solution that will reduce moss and algae growth for many years. Many asphalt shingle manufacturers now offer products with the granules having content of these metals. The effectiveness of strips placed near the ridge will vary with the surface area of the strip and the amount of water running over it. The affect of the strip on the moss and algae is usually good for at least 10 to 15 feet. Packaged zinc strips with instructions are available in most home improvement stores. When installing zinc strips fasteners through roofing materials should not be left exposed to avoid possible roof leaks.