A system for covering fluid containment tanks to contain vapors and minimize evaporation of the vapors into the surrounding atmosphere. The system includes a flexible cover and restraint elements, guiding rails which support the cover over a tank and provide guidance for extending and retracting the flexible cover, a belt drive extends the leading end of the flexible cover and a drivable drum provides a way to retract the flexible cover and upon which the flexible cover will be rolled and stored, and a method to locate tank elements beneath the flexible cover system to prevent operating interferences.