There is a difference between the membrane and anything that may be laid over the membrane to render the rooftop trafficable. In the latter case, something like decking doesn’t form an essential part of the rooftop.
Note that section 106(3) provides that the OC can determine not to repair, maintain or replace common property, if this will not affect the safety of the building or detract from the appearance of any property. (assuming the decking is common property, which is arguable)
I don’t know what the by-law granting exclusive use says, although you mention that the owner must not damage the rooftop – that would be the membrane. That seems to contemplate allowing the owner to install something on the roof over the membrane.
You might want to go through the records to see if there is anything about previous installation of decking, which I imagine would have been paid for by the person who had exclusive use. Look at it this way, if you are the only one who benefits, why should the other owners pay?