Thursday 2 August 2018

Mosses on your roof? Here's what you should know...

As an ecologist and bryologist, I've spent a long time studying the little green things we call mosses. If you've never taken a minute to appreciate the beauty and intricacy of these tiny plants, I suggest you do so. The first time I looked at a fresh clump of moss under a micrcoscope I was hooked. Unseen to most, mosses form entire forests on a microscopic level, all within a centimeter or two of the soil or organic material upon which they're growing. These micro-forests contain complete food chains, with primary consumers (herbivores), several different tiers of secondary consumers (carnivores), and of course, the decomposers (mostly fungi). It's a fascinating world!



One of the most common questions people ask me, however, when they discover that I study mosses, is "how do I keep them from growing on my roof?". If you even Google this the phrase "mosses on roof shingles", the entire first page is packed with recommendations from various home-renovation and landscaping contractors on how to get rid of them - there's even a Wikipedia entry for it!

There is an assumption underlying this question though, that the mosses are somehow bad for the roof shingles, when in truth we simply do not know whether this is the case in the general sense - simply put, nobody has really studied the question.

To be sure, mosses may regularly be found growing in places where the shingles are somewhat degraded. But correlation does not equal causation. In other words, the mosses could be responding to the conditions that caused the shingle degradation, rather than causing the degradation themselves. And in some cases, mosses grow in places where there is little to no degradation whatsoever. My working hypothesis is that, while there are likely cases where the mosses contribute (slightly) to the degradation of shingles, most of the impact of mosses will be neutral to slightly beneficial, because (1) degradation is caused by other things, and the mosses are simply responding to those same things, and (2) mosses have several traits that might actually slow such degradation. Allow me to explain further.

Mosses are what biologists refer to as "poikilohydric"; this means they absorb all their water passively from the atmosphere through their leaves and stem. They do not have roots like the large herbaceous plants we're more familiar with. Instead, they have tiny little hair-like appendages called "rhizoids", which help them to stick on their chosen surface. These rhizoids are highly unlikely to cause damage to your shingles, because they don't usually penetrate into the ground. And even if they do find a nice opening, they're physically incapable of growing more than a couple of milimeters into it, because these rhizoids are the size of the fuzz on a piece of synthetic fleece or felt fabric. Simply put, mosses are pretty much physically incapable of doing physical damage to your rooftop in the way that a herbaceous plant does.

Before I go further with discussion of mosses, let's take a step back and address the primary causes of shingle degradation: erosion and fungal digestion.

The primary cause of shingle degradation is almost certainly water and wind-driven erosion for most rooftops. Think about it, where do you first see shingles with the corners and edges turning up? In places where water is channeled (e.g., joints between angled roofs), and where the shingles are more exposed to winds from multiple directions (the apex of the roof). Both of these actions will knock those little sandy particles loose, and these particles will then bounce down over other shingles, knocking other particles loose as they go. So let's start by acknowledging that erosion is the primary source of physical shingle degradation.

Next, we must acknowledge the primary chemical means by which shingles are degraded. Most asphalt shingles are made of 3 main things: recycled cellulose fiber (i.e., old newspapers), tar, and coarse sand. Cellulose fiber is undigestible for most animals (humans included), but fungi LOVE the stuff. There are thousands of species of fungi dedicated to digesting wood fiber. Some prefer certain tree species, others prefer particular parts of the tree. Some are more generalist in their choices, and will go for just about anything that contains cellulose (like paper or shingles). These fungi actually grow vast microscopic webs through the cellulose fibres, slowly breaking down the cell walls and incorporating the carbon from the cellulose into their own bodies. Because cellulose fibres are essentially hollow tubes that have been sealed off, breaking down this cellulose means that something formerly more like rigid PVC pipe is now more like empty paper-towel tubes; the more digested it becomes, the more the mat of cellulose fibre begins to work like a sponge, capturing and holding water tightly. If you've ever had to deal with mould in your house, you know that the continual presence of water can exacerbate the problem, because moisture brings on a whole-other set of fungal colonizers.

Recap: shingles are physically broken down by erosion, and chemically broken down by fungi. 


Now, let's bring the mosses back in - I'll make three points that relate to what mosses may be doing in this rooftop ecosystem: two that suggest the mosses are responding to conditions, and one that suggests they are contributing to the conditions.

The first point in favour of mosses responding to rooftop conditions, but not causing them, is that some mosses produce antimicrobial compounds. These compounds can inhibit the growth of certain bacteria, fungi, and parasites. In fact, certain species (e.g., Sphagnum fuscum) are so good at this that they were extensively used as absorbent wadding for bandages during the world wars. Ecologists have also shown that fallen trees decay more slowly when covered by certain moss species, particularly in peat bogs. To my knowledge, nobody has studied whether the moss species that commonly grow on rooftops will inhibit the growth of shingle-decay fungi, but let's keep our minds open to the possibility at this point. If they do, they would actually be reducing the rate of shingle decay, not contributing to it.

The second point in favour of mosses responding to rooftop conditions, but not causing them, is that by growing over your shingles, these mosses are likely shielding the shingles from erosion. Several scientists are actively studying their role as a critical component of something called a biological soil crust (BSC), which reduces soil erosion in sandy ecosystems (like pine barrens and deserts). By covering up loose soil (or shingles), the mosses block the actions of water and wind. Their presence also leads to the creation of a thin layer of organic compounds, which can act as a type of glue to hold particles in place. So even though mosses may not colonize until shingles are a bit eroded, their presence probably inhibits further erosion.

Now a point in favour of mosses contributing to shingle degradation. (Note that I'm saying "contributing to" instead of "causing", because the mosses wouldn't be there if the roof wasn't already regularly experiencing high moisture levels & erosion, combined with a bit of fungal degradation.)  Since they absorb their water passively from the atmosphere, mosses like it to be wet. They tend to colonize wet spots on your roof before dry spots because of this. But they also enhance their own habitat as they grow. The tendency to form dense, spongey mats, allows them to hold onto rainwater a little bit longer than if they were isolated shoots. So where you have mosses, your shingles may be wet for a little longer than other places on your roof. If an antimicrobial-compound-tolerant species of fungus can colonize your shingles, it will have a habitat that is slightly more wet than other parts of your roof, and may therefore grow a little more quickly through those cellulose fibres than it otherwise would.

So between the first two points, mosses may be reducing shingle degradation on both the physical and chemical front. From the last, they might be slightly enhancing the growing environment for decay-fungi, which would contribute to shingle decay.

Now to answer the question that I'm so frequently (not) asked: Should you attempt to get rid of the mosses on your roof by the use of moss-killing strips or sprays?

Probably not.

Given the arguments I've made above, getting rid of the mosses will be unlikely to increase the lifespan of your shingles, and may actually reduce it instead. Moreover, if your roof shingles are really degraded (e.g., leading to a leaky roof), the problem is not the mosses, it's the degradation of the shingles by cellulose-digesting fungi.

So if you're concerned about mosses degrading your rooftop, but the roof itself seems to be okay (not leaking or obviously coming apart), you can probably rest easy and enjoy the nice green carpet you got for free. You may even enjoy some additional benefits from having mosses on your roof, like increased insulative value and moderated stormwater runoff. This is actually an active area of research for me, so stay tuned to hear more about the benefits of having a mossy-green roof.

If your roof is obviously no longer doing its job (e.g., leaking), you don't have a moss problem, you have a shingle-rot problem: killing or removing the mosses will not solve this, and may even make it worse. In this case, it's probably time to re-shingle and re-seal the roof.

1 comment:

  1. Fascinating and well-written piece on this subject! I will bookmark this post to potentially share with those folks I encounter that are overly fixated on killing off their roof's population. Mosses are so lovely, I truly can't comprehend the lack of appreciation some have. Thanks for the time and energy you put into writing this piece.

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