Download PDF version (209.7k) Log In or Register to view the full article as a PDF document.
Q.How can I kill moss growing on cedar shake roofs? And after I've killed the moss, what can I put on the roof to keep it from growing again?

A.Paul Fisette responds: Scrub the surface with a mixture of 1/3 cup household detergent, 1 to 2 quarts of household bleach, and 2 to 3 quarts of warm water. That should kill the moss. The detergent helps wet the surface, and the bleach kills the growth.

To prevent the moss from returning, you have to create conditions that won't support its growth. It's not possible to keep all dampness off the roof, but you can enable it to dry out faster by cutting back overhanging trees. You can also poison the food source of the moss by applying finishes that contain zinc, or a solution of copper naphthenate with 3% to 4% metal content. Unfortunately, those finishes don't last more than two years, so they require maintenance.

Another method is to install a ridge cap made of zinc or copper. "Bleed" water running down the roof surface as a result of the normal corrosion from those metals will help reduce moss growth. Tucking strips of copper under the butt end of the shingles every 4 to 6 feet down the roof, leaving 1 inch exposed, may also help.

Paul Fisette is director of Building Materials and Wood Technology at the University of Massachusetts at Amherst and a contributing editor to the Journal of Light Construction