Rooftop gardens have been touted as a type of cool roof that can help reduce energy use and costs. But one plant life you don't want to see on your roof is Gloeocapsa magma, a blackish algae that thrives on moisture seems to have an affinity for material used in some roofing products. When left unchecked, it can damage shingles.

This algae creeps its way upward on roofs, gradually turning shingles dark brown or black. It's easy to spot along some east-west streets with lots of trees, and sometimes it's present house after house.

My wife and I recently noticed Gloeocapsa magma algae on our 10-plus-year-old roof (shown), as well as lichens, which look like greenish-white splotches made with a paintbrush. Fortunately, we didn't have any other growth, such as moss, which can dig in beneath the shingles and upgrade the buildup from two- to three-dimensional.

I looked into some DIY solutions to eliminate the algae, such as oxygen bleach. But the safer, more-prudent move—especially with bursitis in one of my shoulders—was to hire a pro who'd do the job at a reasonable price and not use the chlorine-bleach solution the Asphalt Roofing Manufacturers Association recommends (PDF), which can damage vegetation around the house. (Note: If plants and grass are watered before applying the bleach solution and if the bleach solution is applied correctly, vegetation shouldn't be harmed by the cleaning.)

Pricing for a good roof cleaning, according to Baltimore's My Clean Roof, is based on many factors, including roof height, roof pitch and angles (not all roofs are walkable), and the algae species involved. We got a wide range of cleaning quotes, from a few hundred dollars up to $1,200.

We also spoke with one fellow who wanted to pressure-wash the roof (not recommended), and another who wanted to install copper or zinc strips without washing first. While using the strips would provide a long-lasting solution to the algae problem, the $1,500 price seemed way too high.

In the end, we found a roof-shampooing franchise operation, which quoted us a price of $350. We agreed to the price this past Monday evening, and the work was done two days later. (The cleaned roof is shown, right.) The company even threw in cleaning the siding of the two front dormers and gave a one-year warranty on the work.—Ed Perratore | e-mail | Twitter | Forums | Facebook

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