Half-module for lost formwork element with a flat surface, comprising - a single-piece body which comprises a flat rectangular plate, and a spacer having a generally frustoconical shape, connected to the plate at a distance from the outer edges of the plate on the side opposite to the flat surface of the plate, and opening on the side of its largest section through an opening in the rectangular plate, the body comprising - first complementary connecting devices (24, 25) arranged on two opposite first sides, and intended to connect first sides of two adjacent half-modules in a position in which second sides of these half-modules are aligned, and - second complementary connecting devices (30, 32) arranged on the two opposite second sides, and intended to connect second sides of two adjacent half-modules, and the spacer comprising - third complementary connecting devices (34, 36, 38, 40) arranged on the part of the spacer furthest from the plate of the body and intended to connect the parts of the spacer the most distant from the plate of two half-modules arranged back to back, - the conicity of the spacer and the thickness of the body being such that two half-modules can fit together by inserting the spacer of a half-module into the spacer of the other half-module through the opening in the plate, - said half-module further comprising a cover for closing the opening, - said cover being mounted on a central stud from a bottom of said spacer for ensuring a centering of said cover when mounting it on said body, characterized in that - said cover is loosely mounted on said central stud, - and in that said cover comprises a set of snapping members for cooperating with housings of corresponding shape in said body of said half-module, - said snapping members projecting from an external periphery of said cover. - a single-piece body which comprises a flat rectangular plate, and a spacer having a generally frustoconical shape, connected to the plate at a distance from the outer edges of the plate on the side opposite to the flat surface of the plate, and opening on the side of its largest section through an opening in the rectangular plate, the body comprising - first complementary connecting devices (24, 25) arranged on two opposite first sides, and intended to connect first sides of two adjacent half-modules in a position in which second sides of these half-modules are aligned, and - second complementary connecting devices (30, 32) arranged on the two opposite second sides, and intended to connect second sides of two adjacent half-modules, and - third complementary connecting devices (34, 36, 38, 40) arranged on the part of the spacer furthest from the plate of the body and intended to connect the parts of the spacer the most distant from the plate of two half-modules arranged back to back, - the conicity of the spacer and the thickness of the body being such that two half-modules can fit together by inserting the spacer of a half-module into the spacer of the other half-module through the opening in the plate, - said half-module further comprising a cover for closing the opening, - said cover being mounted on a central stud from a bottom of said spacer for ensuring a centering of said cover when mounting it on said body, - said cover is loosely mounted on said central stud, - and in that said cover comprises a set of snapping members for cooperating with housings of corresponding shape in said body of said half-module, - said snapping members projecting from an external periphery of said cover.