All floor, wall, and ceiling joints must be smooth and designed to prevent accumulation of moisture or dirt. This may involve more expensive materials than otherwise might be used. For example, it is not good practice to use acoustic tile for the ceiling in a food plant. In fact, it is not recommended to have any ceiling in a food plant, if possible. The hidden side of the ceiling tends to become dirty and is a potential source of contamination. Good food plant design uses a precast roof structure in which case a ceiling is not needed.