a ballast is any heavy material used to hold down another material.

Most commonly, ballasts are used on roofs. A ballasted roof is one where the roof membrane is not mechanically attached, or adhered. It is simply laid on top of the [[insulation]] or [[substrate]], and the ballast (most commonly gravel or smooth rocks) are laid on top. The ballast allows for drainage, protects the membrane, and reduces [[SolarGain solar gain]], but adds a significant [[load]] which must be supported by the [[superstructure]] below.