Prevent water pollution when treating a mossy roof

Mossy roof
Large trees that overhang roofs hold moisture and encourage moss and algae growth that damage shingles. (Flickr/ Helen Cook)
Last Updated: 
July 10, 2014

CORVALLIS, Ore. – Many people pay more attention than usual to their roofs in the summer and fall as trees overhang, leaves pile up and fall rains are just around the corner.

If crops of moss or lichen have sprung up, it pays to get rid of them to keep the roof in good shape. The trick is to keep chemicals that kill them from polluting the environment – out of storm drains and groundwater.

Many of the chemicals sold to kill roof moss and lichen contain copper, zinc and iron sulfate that can eventually wash into the environment and are harmful to aquatic life. Oregon State University horticulturist Ross Penhallegon suggests alternatives to help ease the potential of environmental impacts.

Large trees that overhang roofs, leaves and debris hold moisture and encourage moss, fungal and algae growth that damage shingles. Check for debris and leaves on the roof every couple of months and remove them by sweeping, hand removal or with very careful, low-pressure power washing.

If a new roof is needed, non-organic roofing materials such as fiberglass shingles or sheet metal roofing resist moss and algae growth.

To treat a roof, Penhallegon suggests doing the following:

  • Disconnect your downspouts from the gutters when applying liquid treatments. The runoff will filter through the soil, tie up in the organic matter and slowly break down instead of going into street gutters and directly to the nearest surface water. After the roof is treated, wait at least three rainfalls before reconnecting the downspouts.  Check your local ordinances about downspouts.
  • Treat roofs only in dry weather to give the treatment time to soak into the roof rather than wash off in the rain.
  • Remove hanging branches or other plant material that shade the roof. Clearing during spring and fall will significantly reduce the moss that grows there.
  • Use a least-toxic product designed to protect the environment. Read the labels carefully before you buy the product. If you hire others to treat your roof, ask what kind of treatment they use and how they handle runoff.
  • Use the minimum concentration as recommended on the product label.
  • Copper and zinc strips are great methods to control moss and algae. These metal strips allow small amounts of zinc and copper to leach onto the roof, keeping moss from growing.
Author: Daniel Robison