What are the requirements for drop ceiling tiles that touch a kitchen hood? I have heard that a 3″ gap between the ceiling tiles and the hood is acceptable. However, since this is a ceiling with sprinklers in it, would a trough around the hood not provide a means for hot smoke and gases to fill the void above the ceiling tiles thereby delaying the sprinkler activation? What non-combustible material would be permitted around the hood? cement board with no Styrofoam balls? Steel? Type X drywall (which is not non-combustible)? What is everyone else accepting?