Abstract: An integrally formed plastic strip for use as a nailing fin for installing a window assembly in a building has a first stiff plastic member with nail holes through which the strip is attached to the supporting structure, a second narrower stiff plastic member for insertion into a kerf running along the outside of the window assembly for attaching the nailng fin to the window assembly, and a further narrow section of flexible plastic which hingedly connects together the stiff plastic sections so that the nailing fin can be attached to the window assembly and folded back to reduce space requirement during shipment of the window assembly.