The vapor barrier is intended to stop moisture from condensing in the roofing layers. Vapor generally flows from an area of high to low vapor pressure. The vapor pressure depends on the temperature and the amount of relative humidity. Certain recreational and industrial activities result in a high interior relative humidity (pools, fruit storage, processes with lots of open liquids, etc.). If your building generates a high relative humidity then a vapor barrier should be considered upon re-roofing. The best way to prevent vapor transmission through the roofing system (and other building components) is through adequate ventilation. Many times the practicality and cost of doing this ventilation is prohibitive and a roofing vapor barrier is the next choice. If a vapor barrier is installed its effectiveness is lessened as screws that hold down roofing material over the vapor barrier puncture it. Roof penetrations also must be sealed to prevent vapor from escaping around them. If you have high humidity, the roof deck itself should have some protection from underneath. We have seen steel deck rust from underneath and eventually require replacement. For a more technical discourse on vapor control please refer to “The Manual of Low-Slope Roof Systems” by C.W. Griffin and Richard Fricklas.