Q If the sill beneath the window does not extend into the masonry at the jambs, should the flashing stop at the end of the sill at the masonry's rough opening, or should it extend beyond the masonry sill and into the jambs?
A Water can easily penetrate the joint between the masonry jamb and stone sill if the sill does not extend into the jambs. This action can result in efflorescence or deterioration of the masonry beneath the sill. The Brick Industry Association's Technical Note 26 recommends that even when the sill stops at the jamb of the window, the flashings extend to the next brick head joint beyond the sill. An end dam should be provided at the termination of this flashing to prevent water from flowing off the end and remaining in the wall.
Although this recommendation is practical, the appearance may present a problem to most architects. For this reason, I typically recommend using sills that extend beyond the jamb, which are often referred to as “lug” sills. In this case, the flashing extends to the end of the sill and would not be as visible.
In many cases, the flashing is terminated with end dams against the sides of the rough openings. This installation performs reasonably well if the flashing end dams are fully bonded to the brick on either side. The arrangement reduces the risk that water penetrating at the joint between the sill and jamb will result in water bypassing the flashing.
Norbert V. Krogstad is a consultant at Wiss, Janney, Elstner Associates Inc., Northbrook, Ill.