So I started repairing a stairway in a 10 story building. A pipe on the roof froze and bursted causing a flood. The sheetrock on the upper floors feels solid. My question is at what point do I replace the sheetrock? Should I replace some of the ceilings and not worry so much about the walls? I scraped away damaged areas, primed everything with binz shellac based primer and then plastered. Any comments or suggestions would be helpful.