The addition of tiles would likely be covered by s108 SSMA as an alteration for the purpose of improving or enhancing the common property. A rooftop covered with tiles is a different thing to a rooftop with a bare membrane only, in a similar way that a rendered wall with pipework chased into it is a different thing to bare brickwork with pipes attached, as discussed in Qlenquarry Park Investments Pty Ltd v Hegyesi.
This would require a special resolution passed at a general meeting.