A sign-off sheet?
Is there is already a by-law in place that says that the owners of the roof-access apartments are responsible for the repair and maintenance of the the roof? If not, no sign-up sheet will override the principal that all owners (not just the roof-access apartment owners) are responsible for maintaining common property.
If there is already a by-law in place that says that the owners of the roof-access apartments are responsible for maintenance of the roof, then the sign-up sheet might just be clarifying that you all understand this; a kind of preparation for the work and associated cost that is coming your way.
The only reason I can think of that the owners of the roof-access apartments might be responsible for maintenance of the roof is that those owners (current or past) might have made modifications or done something on the roof, and when that happened a by-law was created saying that the owners of those apartments would be responsible for the roof maintenance going forward. So check the existing by-laws and if there is nothing there relating to the roof, don’t take on that responsibility.