How I removed lichen from my rubber roof

At some point in our house’s history, a three-season room was added to the back of the garage. It’s an unusual space in some ways, but we’re not going inside for this post. Instead we’re going up to the roof to see how I removed the lichen that was growing there.

The roof over the addition has a low slope, and in that case many people recommend a rubber covering, formally an EPDM roof. That’s ethylene propylene diene monomer, which is as far as we’re going to go right now (but feel free to read more here). This large rubber membrane is tough and waterproof, except sometimes at the seams — that’s another post.
When I moved into the house, the roof was covered with a greenish gray lichen.

No doubt the large maple tree right over the house provided the cool, damp conditions for this growth to flourish. It wasn’t urgent so I put it off for several months. But I didn’t want the house to go the way of this medieval dwelling I saw when I visited the Norwegian Museum of Cultural History in Oslo.

Time for some rooftop rejuvenation. How to separate these mossy invaders from their home? I don’t know how they attached so tightly to the smooth rubber but they did. I read online that a bleach solution would wash them away, and bleach is cheap, but I wondered: was I really sure that bleach would be safe on the black roof, the gutters, the vinyl siding, the vegetation below, and anywhere else that this splashy job might take it? I was not. And so I was willing to let the guy at Menards talk me into SealBest Sprayable Roof Cleaner, which is supposed to be a more environmentally friendly product. I also bought this Libman scrub brush.

Up on the roof, I brushed off as much dry lichen as I could, then broke out the roof cleaner. The bottle itself has a hose attachment, another advantage over the DIY bleach solution. You screw in your hose and turn the diverter to spray a mix of cleaner and water. Basically I sprayed the green stuff and then hit it hard with the brush. This produced something along the lines of a lichen smoothie (yum!). Then I adjusted the bottle attachment to rinse with plain water, washing the smoothie stuff into the gutter.

That lichen hangs on, though! I did the wash-scrub-rinse procedure three or four times before I got it all (more or less). Here’s the roof partway through.

And at the end.

Now I have a smooth EPDM roof again, as nature intended. I don’t know if the lichen will come back, or how to prevent it, but hopefully I can remove it the next time before it goes full Little Shop of Horrors.