LESLIE: Well, as the saying goes, a rolling stone gathers no moss. But a roof, especially one that doesn’t get much sun, sure can. Moss on the roof isn’t going to cause much harm but it can look pretty ugly.
TOM: Definitely. And there are ways to clean the roof and things you can do to keep that moss from coming back. Here with some ideas on how to do just that is This Old House general contractor, Tom Silva.
TOM SILVA: Well, thank you. It’s nice to be here.
TOM: You know, we get a lot of calls and everyone calls that stuff on the roof that’s green and black and ugly "mold." But it’s not always mold; in some cases, it’s moss or other things, right?
TOM SILVA: Right. It’s also – lichen is another word for it, too.
TOM: Now, that attaches to the shingles. Why does – why is a shingle surface so attractive to the moss and the lichen and the algae?
TOM SILVA: Since we started using fire-rated shingles, there’s actually a ceramic crystal in the shingle that the moss, the mold or the lichen attach themselves to.
TOM: Oh, interesting. So it’s not really interested in the asphalt qualities of it; it’s really that ceramic coating that it’s going after.
TOM SILVA: Exactly, exactly.
TOM: Now, is that why this seems to be more of a problem in the last 10 to 20 years than ever before?
TOM SILVA: You hit it right on. It started about 20 years ago, when we started using what they call "Class A shingles."
LESLIE: So now, since this mold or moss or whatever it is is attaching itself to this fire coating – fireproof coating – when you do remove it, are you damaging or lessening the effect of the fireproof coating?
TOM SILVA: Not generally but it really depends on how you remove it.
TOM: So what’s your secret solution?
TOM SILVA: Well, my secret solution is bleach and water.
TOM SILVA: But you’ve got to remember, bleach is going to destroy your clothes and anything else it touches, so you want to protect yourself and the surroundings, like your plants. You want to wet them down first before you do anything and even cover them with plastic.
And the solution that I like is about 50/50.
TOM SILVA: So it’s strong.
LESLIE: And you have to be really careful because it’s super-slippery on that surface.
TOM SILVA: The moss up on that roof can be like ice, so you don’t want to step on it. And when you wet it down with the bleach-and-water, it’s really slippery.
TOM: Now, if you don’t want to use bleach, are there any less-damaging potential solutions that you can use?
TOM SILVA: Yeah, there’s some biodegradable chemicals out there that do a great job.
LESLIE: Now, it always seems like it comes down to a maintenance issue. Is there anything or any preventative steps that you can take to sort of lesson the work you’ll have to do down the road?
TOM SILVA: Sure. There are – right above the area that you get the moss, the lichen or the mold, you can actually attach – to the underside of the tab of the shingle, exposing about 2 inches to the weather – they call "zinc strips."
TOM: OK. So this kind of looks like a piece of flashing and you slip it up underneath the shingle, let a little bit hang down and it’s made of zinc?
TOM SILVA: Right. Right. It comes in rolls or strips.
TOM: Now, why is zinc the important metal here? What does it do?
TOM SILVA: Well, basically, when it rains, the rain collects the particulates off of the zinc and it runs down the roof and it kills the moss or mold or mildew. It doesn’t like it; it won’t grow there. It says, "I don’t want to be here." So, your roof stays clean.
TOM: So it’s sort of a natural mildicide, in a way.
TOM SILVA: Yeah, absolutely.
TOM: Now, what about if you have – we always find that this is worse on homes that have a lot of shade. Is it a good idea to kind of look at the tree-scape around the house, to see if you can get a little more sunlight there, if it’s possible?
TOM SILVA: If you can prune out the trees and get a little bit of light on that roof, it definitely helps. You notice that you always get it on one side of the roof and not on the other side.
LESLIE: All of this work that we’re doing to remove the algae or the moss or the mildew, it’s not going to hurt the roof shingles in any way, shape or form?
TOM SILVA: Well, not by removing it with the water or chemicals. You don’t want to get up there and scrape it because the scraping will definitely damage the shingles.
TOM: Now, Tom, the moss on the roof, it looks horrible but does it ever get to a point where it really damages the shingles?
TOM SILVA: Absolutely. It can not only damage the shingles, it shortens the lifetime of the shingles. But it can get into the roof because it’s holding the shingles and keeping them wet so much, it’ll damage the substrate below. And eventually, it could rot the roof.
TOM: So definitely an important home maintenance project. Tom Silva, the general contractor from TV’s This Old House, thanks so much for stopping by The Money Pit.
TOM SILVA: It’s my pleasure.
TOM: And for more great tips just like that, including a video that will teach you how to keep your roof clean, visit ThisOldHouse.com.
LESLIE: And make sure you watch Tommy and the entire This Old House team on This Old House and Ask This Old House on your local PBS station.
TOM: And This Old House is brought to you by Trane. Nothing stops a Trane.