Q: Is it necessary to put through-wall flashing beneath a masonry sill, if there is another one a few feet below the sill? It seems that this flashing is unnecessary, especially in the case of a single-piece stone sill that extends into the jamb.

Are there cases when the through-wall flashing beneath the sill can be omitted?

What is the purpose of this flashing?

A: There are many reasons why flashing beneath masonry sills is often recommended. The most common is to stop water that penetrates the sill from saturating the masonry wall beneath. This problem is primarily associated with sills that have multiple joints.

Since the top surface of most window sills has a very low slope, it is relatively easy for water to penetrate through vertical joints between sections of the sill, particularly when snow and ice collect there. Snow and ice melt during the day under bright sun, even if the temperatures do not exceed 32º F. This melted water can seep through joints between the sill's units and refreeze at night, resulting in damage.

In many cases, flashing beneath the sill also can serve as a seal to prevent cavity air from reaching the underside of the window. The flashing bridges the gap between the back of the masonry and face of the backup.

Air within a brick wall cavity might be very humid, and extended periods of rain can saturate the masonry. As the moisture within the masonry evaporates, it creates a very humid environment in the cavity. Without a cavity seal, hot moist air can enter the interior of the building around the window, which also can result in condensation problems. Flashing beneath the sill can be integrated with cavity seals on the jambs and head of the windows to prevent moist cavity air from entering the interior in this location.

At window openings with “lug” sills (those that extend into the jamb), flashing is typically not necessary unless it is required to form a cavity seal. The stone or precast sill should extend beyond the exterior face of the masonry and contain a drip cut beneath the sill. Since virtually no water penetrates through the thickness of the stone or precast concrete, the sill itself functions as its own flashing and prevents water from saturating the top of the masonry wall beneath.

A Principal at Wiss, Janney, Elstner Associates Inc., Northbrook, Ill.