I have not done that kind of work, mostly decks and house washing here, but Im assuming that it is plastic or vinyl and not painted and you only need to clean under neath it. I would think about charging $.15-.20 sq./ft. I would probably think about how long I think it would take me and calculate making at least $50/hr. Have you been in business for a while? I started two years ago and I am located in Marietta (Lancaster County). If you have any questions you can call me some evening at 684-5699.