A plastics sheet material comprises a pair of opposed faces (12, 14) between which run a series of elongate voids (20), the voids (20) being arranged so that there are two series of uninterrupted interconnections (14, 18) between the two opposite faces (12, 14) characterised in that the interconnections (14, 18) of each series meet at or near the respective faces. The voids (20) will in practice be parallel to one another and run the length of the sheet material, thus the material can be produced in one pass by extrusion from a die. This means that the material can be produced potentially much cheaper than, for example, an equivalent laminated product. Furthermore, being produced in one piece, the structural integrity is greater and the variety of configurations which may be employed is very high. The opposed faces may both be external faces, or one or more may be an internal face if the sheet is of a more involved cross-sectional profile. The sheet (10) of the invention allows individual sections to be made thicker for a given gross sheet thickness and weight per unit area. The number of intersections (22) in sheet of the invention can be reduced which also reduces the weight per unit area significantly while the sheet (10) as a whole and the individual sections maintain their resistance to buckling. In the preferred form of the invention, the interconnections (16, 18) meet at the surface of the opposed faces.