The problem with your floors isn't the rain on the subfloor. Depending upon the span 2x8 construction is a minimal floor, and frankly I wouldn't have ever gone that cheaply. If you have even a 10' span, the 2x8 construction probably is short on code, but for sure short on sense. If the basement isn't finished below, you can install cross and solid bridging to improve the floor and reduce flex. Give us a little more information about the size of the span and what's underneath for better answers.