A method of manufacturing a structural beam comprising an elongate base part comprising a polymer. The base part has a length, a width and a height. The beam also comprises an elongate reinforcement part comprising a strip comprising a unidirectional polymer, bonded to the base part at an outer surface of the base part and extending in the length direction along the length of the base part. The strip has a higher Young's modulus than the base part. The method also relates to a beam and to a building comprising such a beam.