In my dealings with potential customers who want their black roof stains removed, I'll sometimes run into someone who thinks the roof should be scrubbed clean with a hard-bristle brush in conjunction with some sort of cleaner. At first this "method" might seem to make sense, but it's a definite no-no and I'll tell you why.
I can pretty much guarantee that if you try to clean your roof stains with aggressive scrubbing or agitation, you're going to lose a lot of shingle granules in the process. With enough force these will pop right off of asphalt shingles, which is why I also do not recommend roof cleaning with a power washer. Granules do more than just add the color to your shingles. They are vital to your roof's health because they reflect sunlight, thus keeping your attic cooler and your AC bills lower. If you think your attic is warm now just climb up into one that is covered by a roof that is missing a lot of granules or is covered in black algae stains - you'll probably notice the difference in temperature. In general, the quicker your roof loses granules the quicker it will lose its integrity, and the quicker you'll need to get a new roof. So hopefully you can see how important this so-called "shingle grit" actually is.
The other reason you shouldn't be scrubbing or pressure washing your roof stains is that in most cases it's simply not necessary. I go into this in a lot more detail throughout this site, but the bottom line is that if your contractor is using the correct chemicals then no surface agitation should be required. You simply spray on the chemical with the help of a non-pressure pump, allow several minutes for it to kill the algae, and then rinse gently with a garden hose. The only rare instance where a power washer might be useful is with very thick, green moss, but if all you have are black algae roof stains then you should have no need for scrubbing or power washing. Make the mistake of cleaning your roof with high-pressure or aggressive agitation and you will probably only shorten its lifespan. Use the right roof stain remover without pressure and you will extend your roof's lifespan.
Roof stain removal doesn't have to be complicated, but it is so important that you do it the correct way or hire someone who knows how to do it properly. It's very possible that you've had some so-called "expert" try to convince you that chemical roof cleaning is "bad" and will destroy your roof, gutters, and plants but it just isn't true. I've performed so many roof cleanings with chemicals alone that I could practically do it in my sleep, and I've never once had a situation where we caused damage to a home. All we've ever done is ADD years to a roof's life, not subtract them. I can't say the same for someone who relies on pressure or scrubbing, however. Either you can take my advice or toss it aside, but don't blame me when you decide to hit your shingle stains with a power washer and notice afterward that there are literally piles of shingle granules sitting in your gutter and on the ground.
