Not sure why someone would use plywood on a balcony exposed to weather, because it is not waterproof, will "blow" with water absorption, and may well trap water where it stays damp and causes issues in the house itself. An inspection by a qualified builder could start you in the right direction, but don't be surprised if you are opening a can of worms. Someone may, at some point, have merely covered up a roof with plywood "deck" and never put in proper drainage, flashing, etc.