13.  Why are window sill flashings
      amongst the most important
      flashings on the house?

Under windows are some of the areas of most frequent water penetration into the envelope. The windows concentrate water which then runs onto the stucco.  Under the windows is one of the most sensitive locations for this concentrated water for a couple of reasons.  First the thermal expansion/contraction of the windows, being of dissimilar materials, causes separation between the stucco and the window frame and a crack develops.   Therefore if the water is concentrated in this area….it will stress this joint.  If the tarpaper is done properly….it should last for the lifespan of the cladding which is said to be 25-30 years.  However, often there is a serious installation error at this point.  When the stucco applicator arrives on the site….he usually finds the windows already in place…with the tarpaper wrap inside the window sill and going around the outside of the window sill and down below the window sill.  However, the stucco applicator installs his tarpaper from the bottom of the wall…so when he gets to the bottom of the window his tarpaper does not line up with this strip under the window.  What does he do?  He usually just laps over top of the tarpaper that is there.  He might put a bit of red tape under the window to help….but if he does not put his tarpaper under the 4 inch strip left under the window sill……he has now created a reverse lap.  This means that the tarpaper is lapped the wrong way.  Water running off the window will impact the section below the window and, in time, the crack around the window will allow more and more water to penetrate the envelope.  If this water then encounters the tarpaper lapped in reverse …then the water goes behind the concealed barrier and quickly passes the 4 inch strip of tarpaper that is below the window and runs onto the natural wood.  This causes deterioration of the wood in that section.  It is simply the most common cause of wooden substrate damage on a building.  If it is the cement- type stucco then it will take a long time to cause serious damage because it is so breathable.  However, if it is 'plastic' stucco or a painted finish, it will cause rot.  If it is acrylic stucco then you can almost be sure that the weather wall will have rot under each window that has a reverse lap.
That makes the sill flashing the most valuable flashing on the building.  Nevertheless, there are other areas of concern on buildings as well….for example balconies and compression joints.