Many architects specify flashing beneath masonry sills below windows. Why is this flashing needed?

If water penetrates the masonry in this location, it simply flows into the cavity and down to the flashing below, which in most cases is only a few feet. Couldn’t this lower flashing serve both purposes?

Flashing beneath the masonry rowlock or other masonry sills is installed for many reasons. Perhaps the most common one is to prevent water from penetrating the sill and saturating the top of the masonry below it.

There are often many joints in the masonry sill, especially if a rowlock sill is used. Water readily penetrates these joints and saturates the course of brick directly below the sill. This water can cause premature deterioration of the masonry, especially in freeze/thaw environments.

It also may result in excessive efflorescence, which can cause deterioration of the masonry due to salt crystallization within the brick units. By placing the flashing directly below the masonry sill, water that penetrates this sill does not reach the top of the masonry below, but instead flows out of the wall.

Flashing in this location also reduces the volume of water entering the cavity from penetrating the sill and, depending on how it is installed, from penetrating the perimeter sealant joint for the window.

Finally, in some cases this flashing, in conjunction with a cavity seal at the jambs and head of the window, also function to prevent moist cavity air from reaching and condensing on the window frame. This condensation can be a problem, especially in hot humid climates.

In these climates, masonry can stay wet for a good portion of the year. During hot weather or when sunlight bathes the masonry wall, the cavity air becomes very hot and humid. Water from the moist cavity air condenses on the window frame that spans this cavity because the frame is colder than the dew point temperature of the cavity air, especially in locations where the interior is air conditioned.