Abstract: The extruded metal sill assembly of a window frame is partitioned into longitudinally-extending inner and outer chambers by a vertical dividing wall. Water draining from the inside of the window and through a weep opening in an inside top wall portion of the sill collects within the inside chamber for drainage to the outside through valved openings in the dividing wall and a discharge opening in a lower wall portion of the outer sill chamber. The valved openings are controlled for water discharge by means of a flap valve swingably mounted at the outside of the dividing wall between the two chambers, the flap valve carrying a horizontal actuating vane extending into the inner chamber below the weep opening and being adapted for activation by the weight of water draining thereupon through the sill weep opening.