What is cantilever slab? A cantilever is a rigid structural element which extends horizontally and is supported at only one end. Typically it extends from a flat, vertical surface such as a wall, to which it must be firmly attached. Like other structural elements, a cantilever can be formed as a beam, plate, truss or slab. how long can it be cantilevered? Also maximum size of cantilever provided should be 10m i.e ;length of the cantilever should not be more than 10 meters.