Should flashing be used beneath rowlock brick window sills? If through-wall flashing is installed beneath these rowlocks, how are the sills anchored? If the anchors penetrate the flashing, does this lead to deterioration of the masonry below in freeze-thaw environments?

Rowlock brick sills contain many joints and are particularly susceptible to water penetration. For this reason, I do not recommend their use in freeze-thaw environments.

When they are used in freeze-thaw environments, however, the flashing beneath them is critical to prevent water from saturating the top of the masonry wall below. Snow and ice collects on the rowlock sills. When they melt, water can penetrate the sills and refreeze, resulting in deterioration. This scenario is a particular problem with the anchorage of these sills because they break apart.

When rowlock sills are used, they can be anchored using galvanized corrugated wall ties in the head joint between every fourth to sixth brick unit. When possible, I prefer that these anchors be attached to the backup wall on the vertical face, behind the sill.

It also is possible to extend flat strap anchors through the flashing. However, any penetrations through the flashing must be carefully sealed.