Boston professionals give answers for keeping roofs clean

Boston professionals give answers for keeping roofs clean

My roof has significant areas of black discoloration that I suspect may be mildew. Is having my roof cleaned necessary? Is it safe for the roof?
— Angie's List member Dora Garrett.

The discoloration is caused by algae, says Steven Hamer, owner of highly rated DR Roof in Bellingham, Ma. "It's an organism that's attracted to the limestone in the shingles," he says.

As the shade-and-moisture loving algae decays, it turns black, causing the unsightly stains. "Most homeowners are driven to clean their roof by aesthetic reasons," says Joan Crowe, technical services director for the National Roofing Contractors Association, which takes no position on whether roof cleaning is necessary.

Besides being an eyesore, Hamer says the algae can reduce the lifespan of the shingles. "The algae feasts on the shingles and causes them to decompose," he says.

Mark Negron, a representative for highly rated O'Lyn Roofing Contractors in Norwood, Ma., says the best way to remove the stains is by applying a solution that kills the algae. "The solution we use ensures the roof will remain algae-free for up to two years," he says. Crowe recommends using a soft brush or sponge and a mild bleach-water solution, and advises against cleaning with pressure washers that can damage shingles.

Hamer and Negron suggest hiring an insured professional, rather than cleaning the roof yourself. "Roof work is dangerous," Negron says, adding that the size, shape and pitch of the roof influences the cost, which ranges from $600 to $1,200 for his company.

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