[GregNH]

All high rise buildings have water tanks on the roof tops or very top floor to hold water for sprinkler systems. They were gravity fed when activated.

[woodbutcher1963]

I understand that all buildings have tanks on the roof for fire suppression.

I do not BELIEVE(I think I do not know for sure) that these tanks held enough water to extinguish the fire. Also, I believe they thought that the pipes carrying the water may have been severed in the initial impact of the airplane.

It would take 10’s of thousands of gallons of water to extinguish the fires created by the jet fuel from these aircraft. Probably, significantly more than any tank on the roof could ever hold.

The remaining water would have to come from municipal pipes.
The volume of water in those pipes was not sufficient enough to put out fires in multiple buildings all burning at the same time. Hence the temperatures eventually reached the point that the steel lost its compression and tensile strength and buckled.