Roof forming element (1; 100) for a roof (3; 103) of a building, which roof closes an opening of the building, wherein the roof (3; 103) is at least partly formed by a plurality of such roof forming elements (1; 100) which each close a part of said opening, the roof forming element (1; 100) comprising - an elongate, preferably extruded, polymer roof plate having a top plate and a bottom plate parallel to the top plate, which top and bottom plate (14; 16) are integrally interconnected via a plurality of ribs (21, 22) extending in a length direction of the roof plate, - a beam which is manufactured, preferably extruded, as one integral part together with the roof plate, wherein the beam extends in the length direction of the roof plate, is located at a first longitudinal side of the roof plate, and protrudes downwards from the bottom plate of the roof plate, wherein the other, second longitudinal side is free from such a beam protruding downwards from the bottom plate, so that the integral combination of the roof plate and the beam has an L-shaped cross-section, - a cover (40; 140) for at least substantially completely covering the top plate at a distance therefrom, so as to define an interior space between the cover (40; 140) and the top plate, wherein the cover (40; 140) comprises at least one cover element which extends over the entire width of the roof plate and over at least part of the length of the roof plate, and - coupling means (30, 32) for coupling the roof forming element (1; 100) to a further (1'; 100'), neighbouring roof forming element of said plurality of roof forming elements, wherein the roof forming element (1; 100), at least in a mounted condition of a plurality of such roof forming elements (1; 100) forming the roof (3; 103), is configured such that the second longitudinal side of the roof forming element (1; 100) is arranged to be supported by the first longitudinal side of a further (1'; 100'), neighbouring one of said plurality of roof forming elements, so that the beam of the further roof forming element also supports the roof plate of the roof forming element (1; 100), and so that the respective roof plates of said roof forming element (1; 100) and further roof forming element (1'; 100') are flush with respect to each other. - an elongate, preferably extruded, polymer roof plate having a top plate and a bottom plate parallel to the top plate, which top and bottom plate (14; 16) are integrally interconnected via a plurality of ribs (21, 22) extending in a length direction of the roof plate, - a beam which is manufactured, preferably extruded, as one integral part together with the roof plate, wherein the beam extends in the length direction of the roof plate, is located at a first longitudinal side of the roof plate, and protrudes downwards from the bottom plate of the roof plate, wherein the other, second longitudinal side is free from such a beam protruding downwards from the bottom plate, so that the integral combination of the roof plate and the beam has an L-shaped cross-section, - a cover (40; 140) for at least substantially completely covering the top plate at a distance therefrom, so as to define an interior space between the cover (40; 140) and the top plate, wherein the cover (40; 140) comprises at least one cover element which extends over the entire width of the roof plate and over at least part of the length of the roof plate, and - coupling means (30, 32) for coupling the roof forming element (1; 100) to a further (1'; 100'), neighbouring roof forming element of said plurality of roof forming elements,