Window Flashing

Flashings at the top of windows and exterior walls referred to as "head flashings" will not protect the window from water intrusion without counterflashing. If the window is leaking at the head, such condition is generally indicative of the failure to provide a flashing - counterflashing relationship over the window installed "in such a manner as to make it weatherproof" as required by the building code.

It is nevertheless not uncommon for windows to be installed with head flashings installed backwards, so that they collect water and conduct it to the interior as a window leak, rather than to the exterior. 
Proper installation for head flashings over doors and windows includes a base flashing component, with a vertical leg and a horizontal leg, the vertical to keep water from getting behind the water resistive barrier to the interior construction, and the horizontal leg to arrest the downward flow and redirect it to the outside of the window.
The vertical leg of the base flashing has to be "counterflashed" by the water-resistive barrier at the top or other appropriate counterflashing means such that any water above the flashing is directed to the outside face of the vertical leg of the base flashing rather than behind the flashing. Water that gets behind the vertical leg of the head flashing over windows and doors shows up on the inside as leaks over windows. To contain water on the top of the flashing until it can be drained to the exterior, end dams should be used.

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