My home is for sale, and I want my roof to look good.  The roof is only about 10 years old, but looks weathered beyond it's years due to tannin from trees, I have a heavily wooded property.  My pressure washer is one of those low-strength electric models, more for washing cars than anything else, you know, the $100 Karcher model.  Thusly, I don't think I'd have a problem with it ripping shingles apart like a gas model.  Have any of you done this?  I see ads for pressure washing companies listing roofs as a thing they do, so I bet I could do this.  Any input?

I wouldn't.  The little grainy stuff is supposed to stay on the shingles.  

Scott

I have a gas powered pressure washer.  I used it to get the tree mold off. It worked fine with no problems.  Just be careful not to get it too close. I held it at a distance that would barely remove the mold with a couple of passes.