The Brick Institute of America recommends that flashing be installed under rowlock brick window sills, but this creates two levels of flashing, one below the window sill and one at the wall's base 2 feet below. Any water that leaks through the brick sill will be handled by the through-wall flashing anyway. Why do I need flashing in both locations?
Flashing is installed in masonry walls for two reasons: to direct any water that enters masonry walls back to the exterior and to prevent water from entering directly into the outer wythe along the top. The main purpose of flashing at window sills is to protect the top of masonry below the window. Although the through-wall flashing 2 feet lower will handle the water once it enters the wall, water entering at the window sill may cause deterioration. Sill flashing should be installed whenever the sill material is not impervious.