Is there actual evidence that moss damages roofs?

December 21, 2021 9:41 AM

I'm reading Gathering Moss by Robin Wall Kimmerer, and she says there's no hard evidence that moss damages roofs. Is she right?

I thought it was a well-known fact that moss destroys roof shingles and had to be diligently removed to avoid serious roof damage - but the author says:

Roofing professionals have led homeowners to believe that mosses lead to degradation of the shingles and eventually to leaks. For an annual fee, they will gladly remove them. Allegedly, the moss rhizoids penetrate tiny cracks in the shingles and accelerate their deterioration. However, there is no scientific evidence to support or refute this claim. It seems unlikely that microscopic rhizoids could pose a serious threat to a well-built roof. One technical representative for a shingle company acknowledges that he's never seen any damage by mosses. Why not just let them be?

When I read this, I did some searching, but couldn't find any actual studies or evidence that moss does, in fact, damage roofs.

What objective, scientifically supported findings do we have about the effect of moss on roofs?

Thanks!

posted by kristi to Home & Garden (6 answers total)

Oregon State University Extension points to a different reason:

The growth of moss doesn’t damage surfaces, said Pscheidt, co-author of the Extension new publication The Care and Maintenance of Wood Shingle and Shake Roofs. The moss traps soil and debris, which retain water and keep the roof wet for a longer period of time. That encourages the growth of fungi and hastens decomposition of wood roofs.

Moss clings to asphalt and metal roofs, as well, but doesn’t degrade those materials. However, moss will lift the shingles, allowing water through to damage underlying materials.

posted by zamboni at 10:08 AM on December 21, 2021 [13 favorites]

Bryophyte Ecology: Vol 5, Ch. 5, part of a charming ebook by MTU emeritus professor Janice Glime, has a (somewhat anecdotal) section on the pros and cons of eliminating moss.

Bryonetters seem to be in agreement that the bryophytes do no harm on roofs (e.g. Rod Seppelt, 4 October 2010; Bill McKnight, 28 June 2017; Michael Richardson, 28 June 2017). Annie Martin (Bryonet 28 June 2017), who spends lots of time crawling around on roofs to rescue mosses, reports that "Ironically, rather than damaging the roof, I have observed that shingles underneath moss colonies are not degraded or in a state of deterioration. Instead, the shingles are almost pristinely new. The surrounding asphalt shingles show evidence of degradation due to UV, wind, rain, snow, etc. It is my opinion that mosses protect the shingles. Tiny rhizoids hold tightly to the surface but do not compromise the integrity of the roof."

But roofers and urbanites like to convince us otherwise. And even a Bryonetter explains reasons why bryophytes might need to be removed (Mark Smits, Bryonet 28 June 2017). Smits explains that at one part of his house he has to remove bryophytes because they block the water flow, causing the roof to leak. Ken Kellman (Bryonet 28 June 2017) agreed; damming can force water under the tiles. And bryophytes can build up enough soil that tracheophytes can take root. Johannes Enroth (Bryonet 28 June 2017) added his experience to this. On a roof with tiles made of concrete, the mosses get wet, freeze, and thaw repeatedly. This causes damage to the tiles, especially in eroding the tile edges. Vinegar (50%) eliminated the mosses, but the lichens remained unharmed.

posted by zamboni at 10:17 AM on December 21, 2021 [6 favorites]

However, moss will lift the shingles, allowing water through to damage underlying materials.

This is visible in Seattle and points northwest of us, certainly. It could technically be the captured duff and the vascular plants that root in it, but that will happen with big healthy moss cushions, so.
posted by clew at 10:28 AM on December 21, 2021

I live in the San Juans, Washington State. Moss grows on our cars, here. We have several big trees on the property, creating shade for the house. There are a few parts of the roof that tend to get mossy. Originally, the roof was cedar shakes, and yeah, it was perpetually damp and rotted. Now, it's asphalt shingles, and I still have to pay attention to remediation and prevention. If I just let the moss go, it turns into a big, thick blanket, and pushes the shingles up. It's at least an annual chore.

Does it actually damage the roof? I don't know, but I just spent thousands of dollars to have the roof replaced, and I'm not about to just shrug and see if it turns into leaks. The roofers recommended certain products for remediation and prevention, but also suggested I just go pick them up at the local hardware store. They weren't trying to push an add-on or an ongoing care plan, so I have no reason to believe they were trying to sell me snake oil, because they weren't trying to sell me anything at all.
posted by xedrik at 2:27 PM on December 21, 2021 [1 favorite]

My neighbor had a hilariously mossy asphalt shingle roof replaced a few weeks ago and I saw a lot of sheathing being replaced.
posted by Geckwoistmeinauto at 8:25 PM on December 21, 2021

Pretty much anything that traps sitting moisture against/inside/on top of your house is bad for it. Moss traps water to maintain the moist environment that it thrives in.
posted by Summers at 8:08 AM on December 22, 2021