Being roof is not important so much. Killer was at roof. But zoro can easily beat him before. We should follow path written for every character.
Zoro was at roof to protect his captain. And has been mvp of the roof .
At the same time , sanji is leader under roof. And has been mvp.
Zoro saved several times his captain and did his best to knock down Kaido. He accept sacrifice by taking side affect drug.
Sanji saved Tama , nami , usopp , momo , prevent arrow and saved alliance . He sent perorine out of fight . Also he carried and gave zoro to doctor. Also, after luffy declaration, sanji and zoro don't care and said keep going alliance people .
I could forgot something both also.
If you look what happened so far in the onishgama, zoro and sanji have already made lots of important thing. And they keep going to make still.
For me , luffy will beat kaido at end of the day sure.
But zoro and sanji made lots of thing until so far , and right now , sanji took a true monster , apparently zoro will take a monster.
So , being roof is not important. Important thing is what they have done till now in the arc.
Even though I hate wano arc writing style, left and right hand if future pirate kings reflected very good