When exterior trim becomes a water collection device–sooner or later–bad things are going to happen.
It is all too common to see wood trim components assembled from multiple pieces in such a way that when they are painted and caulked, the installer figures the assembly will shed water adequately. In some climates and in some locations, even in adverse climates, these installations can be fairly forgiving. I find it completely baffling that anyone constructing the trim details pictured below, could ever think this assembly was going to be OK long term.
This installation is more of a water collection device than a water shedding installation.
In the rainy Pacific NW these assemblies generally do not fair very well and sooner or later—whether due to lack of maintenance or simply because water has a knack for finding its way into places we do not want it.
It is generally considered “best practice” to have all horizontal surfaces flashed with metal and properly counter-flashed by the vertical surfaces that sit on top of it–not to mention that is also logical.
The highlighted areas in the next picture are the horizontal surfaces that should be properly flashed with metal and these metal flashings should then be counter-flashed by the vertical surfaces that are above them.
Such an installation can last indefinitely and will keep water from getting where we don’t want it.
This is a good kind of flasher to be.