Why do people install flashings beneath masonry sills? Are they required if a single-piece stone sill is used?

Sill flashing controls water that penetrates through the masonry sill and helps control water penetrating the perimeter sealant joint around the windows. The sill flashing will prevent water from saturating the top of the masonry beneath the sill. These flashings are especially important when using rowlock brick sills or other sills that have many joints. Multiple joints increase the risk of developing excessive water penetration into the masonry below.

In hot humid climates, sill flashings may also be helpful in preventing hot humid cavity air from condensing on the underside and perimeter of the window. In this case, the sill flashings should be integrated with a cavity seal at the window head and jambs.

When using a single-piece stone or precast concrete at a window, the flashing is less important. Very little water will penetrate a single-piece stone or concrete sill. Flashings, however, are still useful in controlling water penetration at perimeter sealant joints or preventing hot humid cavity air from reaching the perimeter of the window. Where flashing is not used, the stone or precast concrete sill should project beyond the face of the masonry and contain a drip. This will prevent water penetration at the joint beneath the sill. This joint will often crack due to differential movement between the stone or concrete sill and the brick masonry.