webs 3a 1 3a 2 connected at the apices 3a 3 of the vees each such stiffening member is moulded as a single unitary structure at the apices 3a 3 are flanges 10 seen in fig5 which will be described in more detail below although the preferred form of elongate stiffening member 3a is as shown and as described above other elongate members of moulded reinforced plastics material could be used for example the i section stiffening beams referred to above since the sheet members 2a 2b are of steel and the elongate stiffening members 3a are of moulded glass fibre reinforced plastics material it will be understood that they are of different materials and have a different young 39 s modulus that of the plastics members 3a being very much less than that of the steel members 2a and 2b other materials can be used but the material of the sheet members must be different from that of the stiffening members and the young 39 s modulus of the stiffening members must be considerably less than that of the sheet members and preferably at least 50 less fig5 is a diagrammatic detail section of a flange 10 the flanges 10 are integral with the strips 3a 1 and 3a 2 of the stiffening member at each apex 3a.