A method for manufacturing a compound construction element, in an extrusion process wherein a composite material is pressed through an extrusion die, which composite material includes a matrix of thermoplastic synthetic material including wood particles or other, cellulose containing particles in a content on the order of 50% by weight or more, wherein also one or more elongated additional elements are passed through the extrusion die, which additional elements are brought into tight engagement with the composite material, after which the compound element is shaped and preferably cooled in a die.