I wouldn't pressure wash it if it's a mineral surfaced, (rool roofing) covering but rather take a garden hose with a pressure nozzle along with a roofers stiff broom and scrub/rinse away any areas where dirt is stuck, let dry, and coat twice with an elasomeric roof coating. Make sure before coating that you have all loose debris picked up, mastic around any protrusions such as vents if needed, and tighten up any nailed areas if loose. You also want it to be at least 50 degrees for 24 hours cure time on most coatings and of course the warmer it is, the easier it is to work the product.