My Introduction To Roof Algae (No, It's Not On A Metal Roof)
By Shawn Zuver, editorial content director
It was the summer of 2010 when I was personally introduced to roof algae. It started with a rather innocent question one evening as my wife asked about the dark stains on the rear-facing (Northwest) side of our home’s asphalt shingle roof. I’d picked up a lot of things about construction over the past 25-plus years, but this discoloration was something that I’d never encountered with metal roofing.
I had noticed dark stains like these, which are most visible on lighter colored residential or commercial roof shingles, in a growing number over the past several years. They seem to be on every block in our northern Ohio town and throughout many parts of the country that I’ve traveled, yet I’d never given them much thought.
A quick internet search informed me that we were the proud owners of roof algae. Depending on which opinion could be believed, the shingles could be left alone, removed immediately and transported to an outer region of the solar system, or the roof could be cleaned. I chose to believe the third opinion, which seemed to be shared by the majority of sources. One leading asphalt shingle supplier noted that it was purely an aesthetic issue which has no adverse affect on the shingles’ performance. Based on my experiences with metal roofing, which I’ve never seen affected in this manner, I was disappointed about the stained appearance but at least took solace in the fact that the roof’s integrity was apparently not compromised.
At this point, there are surely some readers who will wonder why someone who has worked in the metal construction industry since the mid-1980s wouldn’t have a metal roof on his home. It’s a reasonable question and one that I’ve asked myself several times – particularly when the roof algae emerged. I love metal construction products: my basement has metal “R” panels that cover the foam-insulated concrete walls that were used to construct that space, I put up a nice metal-sided and -roofed shop at our previous home, and I’m always making crude home plan sketches that incorporate metal framing and panels outside and inside. However, when we bought our current house, it was only two years old and replacing the shingled roof seemed far too wasteful until it has run its useful lifespan. So, unfortunately, it’s not metal that covers our heads.
Back to those roof stains. The methods of cleaning roof algae that I uncovered over the next week or two ran the gamut of ideas, such as scrubbing with a stiff brush/broom while using soap and water, washing the roof with a chlorine solution, or using zinc to counter-act the algae. Like most things, the proponent of each method was dead-certain that their method was the most beneficial, though I was having a hard time understanding how it could be a good idea to scrub the shingles which would surely cause the granules to rub off at an alarming rate.
Leaving the roof to eventually become completely stained by the algae just wasn’t an acceptable option to us, yet the diversity of the cleansing methods was confusing at best. I kicked around the ideas for another week or two and paid attention to every roof that I could find – even more than usual – and was shocked to find that the overwhelming majority of shingled roofs in our town were affected in some manner. I could only guess that those that were untouched were either new or covered by specially-formulated shingles that had been introduced, relatively-recently, in an attempt to address the problem.
My moment of epiphany came one evening while I scanned a commercial roof that was covered by dark stains from eave to ridge, except for one section approximately 15” wide. As my eyes followed the thin path of clean shingles from the rain gutter upward, I found that it led to a galvanized vent pipe that extended through the roof; above the pipe, the algae continued unabated to the building’s ridge. In that instant, I became convinced that the zinc run-off from the galvanized coating had to be the reason for the algae-free area below it. Zinc’s performance in metal construction is well-documented so I felt confident that this was the right method to attack the problem.
Later that day I ordered a liquid zinc solution designed to clean roof algae and applied it a week later after mixing it with the required amount of water in a backpack sprayer. (Of course, there was a considerable amount of preparation work that had to be done first, along with great care to make sure that plants, trees and other items weren’t hit with any overspray.) During the past 12 months the product has done a pretty good job of containing the spread of the roof algae, and has gradually decreased some of the stains that were already on the roof. I’ll be applying it again soon to try to get rid of the remainder of those stains, while also spraying it on other sections of our roof where the algae started to appear this spring and summer. I have a sneaking suspicion that this could become an annual event for the life of this roof.
My experience with roof algae has been an eye-opener, to say the least, and despite my modest success in dealing with it, I don’t feel qualified to endorse this or any other method. The one thing I can say - with a great amount of certainty - is that when it does come time for a new roof, I’ll be choosing metal.
Shawn Zuver is editorial/content director for DesignandBuildwithMetal.com. He has been covering the metal construction industry, including residential and non-residential construction, since 1985. To contact Shawn, call (419) 581-2051 or email shawnz@designandbuildwithmetal.com.
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