My carpenter put up a covered porch...stairs going out on either's about 3ft off the ground with half rise stairs. Built over limestone gravel. The stairs and supports on the east side of the porch are all rotten and need to be replaced. They seem to have this white thick mold growing on them. Any ideas as to why this is happening? This is fairly a new build...maybe 3yrs old. It's all pressure treated lumber....I'm hoping this doesn't happen to the whole structure.