Q. In recent years, moss and lichen growth on asphalt roof shingles seems to have become an increasing problem in our area. What’s the best way to prevent recurring organic growth on roofs and, when preventative measures fail, what’s the best way to remove the growth?
A. Mike Guertin, a builder and remodeler in East Greenwich, R.I., and frequent presenter at JLC Live, responds: I have spent a fair amount of time dealing with moss, lichens, and algae on roofs. It’s a problem here in Rhode Island, especially at my own house, and it doesn’t seem to be limited to a particular type of asphalt shingle. Laminated shingles may have more nooks and crannies where debris can settle and provide food for moss, and the edges may make it easier for spores to hang tight until they begin growing, but I don’t know of any studies on whether three-tab or no-cutout single-tab shingles are less prone to organic growth than architectural shingles.
Prevention. As far as I know, there aren’t any “moss-resistant” shingles, though there are algae-resistant ones made with granules that contain copper, such as Owens Corning’s StreakGuard shingles. When it rains, copper ions wash out of the granules to inhibit algae growth. To help resist moss growth on a new roof, you can install copper or zinc strips according to the manufacturer’s instructions. Depending on the local climate, sun exposure (north-facing roofs are more prone to organic growth than south-facing roofs), and tree cover, you may need only one strip of metal near the ridge to inhibit moss growth, or you may need to install strips along shingle courses every 3 to 4 feet up the roof slope. Again, ions from the copper or zinc strips wash down the roof each time it rains, and it’s those ions that inhibit moss growth.
Removal. Once moss has gained a foothold on a roof, there’s no fast and easy way to get rid of it. Manually removing it—with a stiff-bristled broom or even a hard-edged tool—can cause more damage than the moss does. I’ve found that an effective solution is a product called Spray & Forget Roof Cleaner (sprayandforget.com), a liquid concentrate that comes in a container that can be attached to a garden hose. It’s not a quick fix; when I used it the first time, it took about three or four months before the moss on my roof turned brown and started falling off on its own. Now I spray the roof with the solution every three years, and it keeps the lichens, moss, and algae at bay.
There are other, similar products that probably will work as well. What they all have in common is that they’re not as aggressive as the bleach/detergent/water mixes you’ll find recipes for online. Those bleach mixes will kill the moss in a few days, but what washes off onto plants below can kill more than you intended. Spray & Forget is bleach-free and, since it’s applied in a mist, not much reaches the ground, and it doesn’t seem to have much of an impact on plantings below.
I have noticed more moss, lichens, and algae in my area than I used to, but whether that is the result of a warming climate is hard to say. It could be that most asphalt shingles last longer than they did in the 1970s and ’80s. Back then, we replaced roofs every 15 to 20 years; now, it seems we replace them every 30 to 35 years or longer. My roof was moss-free for 15 years, then it started growing. To eliminate the problem altogether, my next roof will be metal.
