A three-dimensional constructional element 10 is shown having opposing and substantially planar panels 30, 40 mounted on opposing surfaces of a flexible planar member in the form of a flexible mesh material layer 52 therefore making a composite sheet of three layers. Panel 30 includes transverse grooves 12, 14 formed therein and panel 40 includes transverse grooves 16, 18 formed therein. The structure is deformable about each groove 12, 14, 16, 18. In a preferred embodiment the panels 30, 40 are divided into many segments defined by the transverse grooves to allow the constructional element to be deformed so that one of the exterior surfaces of panel 30 is concave while the opposing exterior surface of panel 40 is convex. Also disclosed herein is a method of forming a curved surface structure including the steps of deforming the constructional element 10 into a desired position and securing the element 10 into that position by introducing a grouting material 74, 94 thereonto so that the grouting material can enter the grooves 12, 14, 16, 18 to rigidly affix the element 10.