Q: How do I know whether or not to replace gypsum board after it has been submerged in a flood?
A: You will want to reference
GA-231-15 Assessing Water Damage To Gypsum Board. Most notably, gypsum board that has been exposed to sewage or flood waters must be replaced. Levels of bacteria, such as E. coli, can be hundreds of times above safe levels in flood water. Also, hydrocarbons from underground gas storage tanks and fuel leaks from submerged vehicles can be present. Often, the gypsum board will need to be removed anyway to assess the underlying substructure.