Abstract: A window frame assembly including a weather resistant cladding structure having a telescopic expansion joint. The cladding, preferably made of PVC, has side jamb members which are hollow and which form telescoping members in conjunction with corner pieces or end caps which engage with the nose of the sill cladding. The window frame assembly is made up of wooden side jambs, a wooden upper jamb and a wooden sub sill to which the cladding structure is attached. The telescopic members having the expansion joint permit thermal contraction and expansion of the cladding structure relative to the wooden jamb and sill members.