Friends, I'm sharing my test results of different roof-stain removal efforts and products, including this one, which I now recommend because it works and is worth ten bucks.
Some Background: I have a 12-year old, grey colored, GAF 30-year asphalt roof that I let go too long re: black mold, blended with copper-colored and green-algae stains. Some areas have all three, others just black mold, or that strange copper tone that weirdly glistens in direct sun.
I first tried a pool-shock based, DIY mixture. I mixed a cup of powdered pool shock with 1-gal of water and spayed it on my roof. I waited 10 minutes and then re-sprayed that same area with this stuff:
20 minutes later, I garden-hose-sprayed the treated area off. Some green and black mold was removed, but plenty of staining remained. A repeat treatment did little better. I lost patience.
So I went back to Home Depot to try "M-1 Roof and Deck Cleaner":
It said mix 13 oz in a gallon of water and spray it on. I probably used 20 oz. because, well hey, my stains were really dark. I sprayed the mixture using a $16 hand-pump sprayer, something like this:
I came back in about 15-20 minutes and still there was no change -- UNTIL I sprayed, close-up (4” away) each shingle with my garden hose (narrowly focused spray using a standard nozzle). I didn’t want to use my pressure washer, it was too much of a hassle to get it out of the garage.
It worked! All the dark mold and gunk was stripped out of each shingle.
Mind you, I had to work some. I had to methodically spray each shingle up close, so that’s a lot of back-and-forth, tiring my arm.
To be clear: You must stand over a patch of roof and bend over and direct-spray each shingle, from about 3-4" away, at full blast (guessing my hose was outputting at 200-300 psi or so).
Yes, you’ll get wet.
But all the gunk came out of each shingle, and it was satisfying to see my roof shingles restored to what they once were. And honestly, the mold I had looked like it would never come out. So, I was impressed by M-1, and pleased at the price ($10 bucks a gallon).
Bottom line: From spray on to spray-off, it takes about a half hour for each 60 square feet of affected roof. I had about 180 square feet of afflicted roof area.
Of course, you need to do this when the sun is brightly shining on your roof so you can see the dark mold shingles from the wet ones.
I suppose the job would go faster with a pressure washer and wide enough spray head. But I was worried I'd use too much pressure and prematurely wear out my shingles. A garden hose, with standard output, will do the trick. It just takes longer.
I also invested in a (more expensive) "Spray & Forget" solution:
That product is designed for the lazy. You spray, then walk away. No immediate results, though. It says it takes weeks if not months to work (relies on rainfall).
I don’t know if that’s true (just tried it today), but after I finished my “M-1” job, I sprayed the just-cleaned area with it. I figure it's a good way to prevent recurrence of the mold and algae that constantly afflicts my Savannah, GA roof (Savannah is a high-humidity environment and thus one is constantly battling mold and other forms of "ick" on one's home).
Easy Instructions, Easy to Apply, Easy to Use