Decking bearers, Hardwood V Treated Pine V steel purlins

G'Day Mates,
What is more long term cost effective for bearers, Local Hardwood, Treated Pine, Steel Perlins?
I need to replace an existing deck or maybe just some of it.
The area is 7.5m long by 2.5 wide. The span between posts is also 2.5M
The deck is off my 2nd floor so is about 3m above ground.
The original bearers were Oregan, as specified by a supplier, everyone tells me that was my first mistake.
The balcony is on the Eastern side of my house & shaded form the North & East by trees so although I am on a ridge it does develop a fine coating of grey mould, maybe scrubbing it more often be the go there.
Whats the go with Treated pine decking, is the corigated side supposed to be the top or bottom surface?


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