to be honest this 0.5mm is ok, even if you got a wood that is '5mm' there is some tolerance in sheet measurements, even humidity might affect by this much. since its an exterior panel I wouldn't worry to much about itLooks like the only 5mm wood panel I can get is poplar, would that be strong enough? I don't know too much about wood, but I do know oak is supposed to be much stronger. If I went with oak though, I'd have to shave roughly 0.5mm off the entire thing...