The Reverse-Lap: Water Intrusion Through Windows and Doors

I receive a lot of calls from homeowners, apartment managers, and business owners for potential construction claims.  Some complaints are mild and don’t warrant hiring a lawyer.  But certain claims make me concerned.  One of the biggest “red flags” is when a potential client complains of water intrusion through windows or doors.

To explore the cause of water intrusion in these areas, I often engage an expert witness to perform limited destructive testing.  Typically, this involves removal of siding to examine the installation underneath.  And fairly frequently we find that water intrusion is the result of improper installation of windows and doors, often due to a “reverse lap.”

A building is designed using various materials to keep water outside of the structure.  And how those materials are installed is very important.  In short, the materials should be installed much in the same way that shingles are installed on a roof – lapped on top of one another so as to shed water.  In short, water that somehow enters the system by getting past the siding material should encounter water-resistant materials and flow down the vertical surface of the building so that it exits the system at the bottom (or at other exit points).

The materials that make up the exterior consist primarily of the siding, weather-resistant barrier (often referred to as “WRB” or “Tyvek”), and flashing.  Flashing is particularly important at windows and doors because these are openings in the exterior of the house – penetration points.  They are therefore opportunities for water to get past the siding, particularly at the tops or “heads” of the windows and doors.  It is therefore important that water which hits these points encounters materials that are properly installed.

A common cause of water intrusion is where materials are reverse-lapped.  In short, this means that a material higher-up on the structure ends, or terminates, behind another material lower down on the structure.  A typical reverse-lap condition at a window is where the WRB is installed so that it ends behind a flashing piece on the window.  The problem here is that water which rolls down the WRB then rolls behind the flashing, rather than outside of it.  This water then usually makes its way into the home.

If you are seeing water intrusion at windows or doors or other places in the home, it is often worth investigating.  An improperly-installed window or door can allow water to enter the structure, including the framing.  And most any construction expert will tell you that the most destructive force in the world is the force of water over a long period of time.