Water management is a top priority in the building industry. Often named the No.1 ruin of buildings, water demands our attention. If not for water, designing and constructing buildings would be relatively easy. But our walls have windows, which means interrupted continuity—and opportunity for water movement through an assembly. We cut holes in walls, insert windows into the holes, and then are left to re-create the original level of continuity.
One of the most important aspects of a window installation is managing water at the rough sill. Our water management strategy is twofold. First, stop water from migrating to the interior face of the exterior wall. Second, address any accumulated moisture. While there are many ways to accomplish both, we typically use the following four options.
Option 1: Flanged window inside a rough opening with a back dam. The sill membrane flashing is installed on the outside face of the rough sill, across the top of the rough sill, and up and over the back dam. The benefit of this detail is that the back dam provides a vertical surface to stop interior migration of moisture; and the membrane flashing keeps the moisture from being absorbed into the wall framing. The downside of this detail is that it doesn’t promote drainage of moisture to the exterior. One solution is to install the rough sill at a slight angle to the exterior to direct moisture outward.
Option 2: Sloping back dam. Like Option 1, a sloping back dam prevents inward moisture migration. The benefit is that it provides positive sloping drainage to the exterior to get rid of any water that gets into the rough opening. It is also a cost-effective approach that is fairly easy to install. One slight but notable drawback is the need for a reverse-slope shim…