A single or double hung window assembly has a rectangular main frame supporting at least one rectangular sash frame for vertical sliding movement, and each frame has members formed from extrusions including rigid plastics material. A horizontal upper sash rail or member is reinforced by an elongated and continuous metal stiffening channel having a width substantially greater than its height and defining a longitudinally extending and upwardly facing undercut cavity. End portions of the stiffening channel support retractable sliding tilt latch bolts each having a snap-fit connection to a corresponding actuator slidably mounted on the upper sash rail. A sash lock member is also mounted on the upper sash rail and is secured to the stiffening channel. A pre-assembly of the stiffening channel and latch bolts is inserted through an opening in a vertical sash frame member, and stiffening channels are also inserted into other sash frame members.