Abstract: The present invention relates to floating roof storage tanks, having a tank shell which is supported on a fixed base, and a floating roof supported by the liquid stored in the tank. Included in the base or roof is a displacement means to substantially displace the stored liquid when the floating roof is in its lowest position. In this way the volume of the heel of liquid unable to be removed from the tank is reduced. The displacement means can take the form of a container which protrudes downwardly from the roof, or a layer of ballast on the base of the tank. The ballast layer can be a liquid heavier than the stored liquid which can be retained in a dam if desired.