If the entire deck is “Common Property”, a term that does not necessarily imply common or shared use, then the responsibility for fixing the leak rests with your Owners Corporation, who may in this instance be able to recover costs from the current owner of the floorboards; although that will be “messy” as it's a previous Proprietor who placed them there, and it may not be apparent that the floorboards damaged the membrane, or when.
On the other had, if your Plan has registered a Special By-Law covering the Exclusive Use of part of that deck by the Proprietor of one (1) Unit, then it's usual for that Special By-Law to include a Condition that the Proprietor from time-to-time of that Unit is responsible for all on-going maintenance and repairs to the area over which they have that exclusive use.
I'd suggest that you speak with your Strata Manager about whether or not a Special By-Law of the type referred to (above) has been Registered.
Alternatively if you're in NSW, you can do an on-line search of your property’'s Strata Plan using the Land & Property Management Authority's “on-line shop” where all “Dealings” (e.g. Special By-Laws) are listed by Dealing number at javascript:mctmp(0); and then at the following link, where the wording of the individual Dealings can be found: javascript:mctmp(0);