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Growth on My Roof: Damaging or Just Ugly?

William Kibbel III, The Home Inspector

I read about preventing moss and mildew in your Wood roofs article. I have smooth, light green blotches on one side of my roof that isn't dark and fuzzy like moss. My roof is only 11 years old. Will this stuff harm the roof? If so, can it be safely removed without damaging the shingles? They're asphalt shingles, not wood.

From your description, you may have lichen growing on the surface of your roof. It's a combination of algae and fungus and usually has a light, mint-green color. It doesn't trap as much water against the surface of the roof as moss, but it can be acidic and has strands that can penetrate into the shingles. These are probably not good things for your composition shingles.

A more common type of growth on roofs is, of course, moss. Moss is darker green, much thicker, holds more water, and also grips into the surface of the shingles.

The third most common type of growth on a roof is algae. It doesn't have the thick growth like moss and lichen and it looks like stains on a roof, usually in streaks of black or sometimes green. In the past twenty years, the black-streaking algae, Gloeocapsa magma, has spread to many roofs throughout all climates, where in the past it seemed limited to mostly warm, humid areas. From my own observations, this algae doesn't seem to cause any damage to shingles, but it can sure make a quality roof look nasty, dirty and worn out.

Treatment

If the roof is in good condition, the shingles aren't worn, buckling or brittle, I usually recommend that moss and lichen be removed and the roof surface be treated and cleaned.

Here are some steps that can be taken to remove that unwanted roof carpet:

  • Thicker moss and lichen will need to be carefully scraped. A wide plastic scraper, used to remove excess wax from snowboards, works well. A plastic pastry/bowl scraper also works, but don't tell my wife. It's best to round the sharp corners with a file, so as to not gouge your shingles
  • Any remaining or thinner growth can then be treated with an herbicide. There are commercially available products that are specifically for moss, lichen and algae. Most have "zinc" in the ingredients list (zinc sulfate or zinc chloride). If you have copper gutters, valleys or flashings, don't use anything with zinc. Find something with copper sulfate instead. It won't corrode the copper like zinc. Bleach based products can also be used, but need to be kept wet after application to be effective. With any herbicide, bleach or other chemical treatments, be cautious of overspray and run-off that could harm plants and don't let the stuff get in the fish pond
  • After the lichen or moss dies off and loses its grip on your shingles, a low-pressure wash, aimed down the slope of the roof, will likely remove the rest of the growth

Prevention

Periodic reapplication of the chemical treatment can often effectively prevent the return of moss, lichen and algae, but there are other eco-friendly options to consider:

  • Trimming overhanging tree branches can reduce the shade that might be keeping the surface of the roof damp enough to support the growth
  • Installing strips of zinc (or copper) just below each side of the roof’s ridge, then a strip every 10-12 feet parallel to the ridge, can be a perpetual preventative self-treatment
  • If re-roofing is necessary, several composition shingle manufacturers are producing algae resistant shingles that contain copper granules

Cleaning, treatment and maintenance should only be performed on roofs that are not too steep to be safely walked on, unless you're a professional with the right equipment. Take it from me, this stuff can be wet, slimy and very slippery. If there's a risk of harm to a person or the roofing material, leave it alone and let people point and say "Oh look, it's a Chia-house."

About the Author
William Kibbel III is a home inspector and restoration consultant specializing in historic residential and commercial buildings. He is vice president of Tri-County Inspection Company, serving Southeastern Pennsylvania and Central New Jersey.


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