I've been asked to clean the roofs of two homes with cedar shakes; one is new, one is 5yrs. old w/green mold in the shaded areas. Both want a clear preservative/sealer. The client with the new roof is aware it needs cleaned first. I'd use oxalic acid based cleaner to start with, but I'm unsure what clear preservative/sealer would be best, how long it would last, what type of maintenance(1,2,3 year intervals?) I usually do decks but I realize this is a different game. Also, do you charge by sq.ft.+ degree of dif.? TIA