Surface tension causes water to defy gravity and flow around and underneath surfaces. Of concern to builders, this means that rainwater running down the side of a building will flow underneath architectural features that interrupt its path, eventually finding its way into the cladding or building's interior unless there is a conscious effort to block its path.
As Joe Lstiburek explains in his recent Building Science Corporation article Mind the Drip, when an old building in a cold climate is retrofitted with insulation, the absence of kerfs and drip edges can cause water to be absorbed by the building instead of running off of it, leading to possible freeze/thaw damage. That's why there are - or should be - drip edges along the eave of a roof and kerfs along the bottom edges of window sills.
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