On October 20, 2011; I wrote a bit on this blog about moss eating away at roof surfaces. Since then I’ve done more research. Moss does not “eat away” at your roof. Moss is a bryophyte. It does not have roots and gets its food from water and sunlight – not the substrate (the stuff it grows on). It eats away at nothing. Check the science. Its the stuff that often is present with moss (and often hidden by the moss) that does the damage; along with sunlight, below freezing weather, moisture and power washing. Sunlight, and high pressure power washing do way more damage to a roof than moss can ever do.
The original post was a result of observing how the moss interacts with the rain water runoff from the roof, and I got carried away with the details. Since then, I’ve done more research. One important observation I have made here is that where the moss was most prevalent, the water was of the best quality. A new filter was added receiving water from an area on the roof with very little moss. Subsequently, that new filter was overwhelmed with debris and excessive organic contaminants. Also, I’ve actually gone up on the roof here to take a very close look at the moss growing there. The roofing material does “appear” to be slowly breaking down where the moss is growing; however it is also breaking down where the moss is minimal and the sunlight is maximum. So what’s up with this? Are the contractor’s who insist that moss eats away at your roof correct? Possibly. Moisture definitely does lots of damage – over time – and moss can hold moisture. I’ve had a roofing expert out here to inspect the roof. He said nothing about the moss being a big problem. He explained that the insulation under the roof in the attic, the ridge vent functionality and moisture present in the air are the biggest problems. Moss is an indicator of other problems, not a problem in itself.
Consider the links below:
The last link is the most interesting from the point of discovering where the idea that “moss eats away at your roof” comes from. The person who has this website is an experienced contractor. At the top of the page that the link takes you to, you will discover his explanation of how “moss” destroys composition roofing. If you read it carefully, you will see that he already has seen and knows from experience what we have discovered from our academic research on this touchy topic. Now, read all three of these links and the previous post on this blog here for numerous other (scholarly) links and the truth of the matter will become obvious. The moss is not what actually breaks down the roofing material; its what is present (usually) with the moss. Lichens, bacteria, moisture, organic debris, and sunlight are the issues that are most destructive to composition roofing material. The only thing that is obvious from a visual inspection however, is that when there is moss on a roof, the material breaks down quickly. The science described in the links above, including the links in the previous article on this blog, show that its not the moss that breaks down the roofing, and actually sunlight is more destructive to roofing material than moss or, moisture, or lichens. Also, if you read the contractor’s website closely, you will see that he accurately describes the tremendous damage “power washing” does to a roof – it literally rips the sand off of the shingles exposing the petroleum based subsurface to the sunlight, which will break down that petroleum base in a few years. So, the scenario is typically this: A homeowner gets moss on their roof, calls a contractor to power wash the moss off. Then, 3 or 4 years later their roof fails. They blame the moss. Totally wrong. Prevarication of facts and big misunderstanding.
Now, from all this people get the idea that moss is really bad stuff. Moss killer is all over the place in the hardware stores. People put chemicals on their lawn to kill that nasty moss. Some people are smart, however. They have a moss instead of a lawn. Weeds seldom penetrate the moss and it does not really need watering here in the Pacific Northwest. Low maintenance, soft and pretty. Hmmmm.
If you keep the organic debris off of the roof, and sweep the moss with a stiff broom while the moss is wet and before it gets established, it is possible to keep the roof basically moss – free and poison – free, with minimum damage to the roof. This will also help to prevent the stuff (lichens and bacteria) that really does “eat away at your roof” from ever getting started.
If anyone has information that shows the science here is wrong, or that my interpretation is wrong, please post it with a link to the source and we will change this post accordingly. No link, no change. Your are entitled to your opinion; but it will change nothing here.