I have been recommending treatments for home owners after I pressure wash on a surface.  The pressure washer gets the job nice and clean but it does not kill the root of mold, algae and moss.  If you keep your walks treated regularly you should never have to pressure wash again which is the best thing for your structure.  Every time someone pressure washes, it is a little bit abrasive.  I use safe and effective products, Wet and Forget Moss and Algae Killer.  Safe to walk on after it drys no problem.  Safe around pets and plants. 

Use regular treatments of Wet and Forget for all of your roof cleaning and pressure washing in Portland Oregon needs.