Pressure can be used but, I don't recommend it.
Clean them as you would normally like a shingle roof but use a mix that is slightly stronger.
Good Luck!

Ken J. Schwarz
K.J. Pressure Washing, LLC Pressure Washing & Non Pressure Roof Cleaning
Pinellas, Pasco, Hillsborough & Hernando Counties, FL. www.KJPWLLC.com www.ROOFCLEANINGTAMPA.com
727-688-0687