Abstract: A drainage system for horizontally sliding closures, especially horizontally sliding closures having a solid sill such as is typical with patio doors and the like. The drainage system includes a valve assembly having a top wall, opposed end walls and a side wall that defines a drainage aperture. The drainage aperture is normally closed by a flap which permits water to drain through the valve assembly but inhibits the infiltration of outside air. The valve assembly is mounted over a drainage slot cut through the fixed rail. To ensure most efficient drainage performance, the fixed rail must be vented. This is preferably accomplished by replacing the usual end caps for the fixed rail with slotted end caps which maintain the internal air pressure in the fixed rail at substantially atmospheric pressure. The advantage is a simple, inexpensive very effective drainage system which is suitable for new product and the retrofit of installed doors and/or windows.