Window and Door Sill Flashing

By Stephen McDermott

Exterior windows and doors are replaced for variety of reasons including, inadequate performance (drafts and water infiltration), aesthetics and operational problems.  Replacement windows and exterior doors are available to the public in many forms and levels of quality.

The installation of sill flashing is often not incorporated into the replacement of the door or window due to the added cost or simply because the Owner is unaware of the benefit.  The benefit and purpose of sill flashing is to redirect water to the exterior envelope, away from window perimeters and exterior doors.

By itself, a quality window and door will perform well against the demands of the environment, but the sills can leak over time from weathering or poor construction.  Weak links during the replacement of windows and doors happen at the perimeter conditions which vary depending of the exterior wall construction.  A suitable seal against water and air infiltration is dependent on the exterior wall construction.  The installation of sill flashing during a replacement project has the benefit of resisting water migration through the wall elements and sealants that if left unchecked can create wall distress and deterioration below the windows and doors over time.

The installation of sill flashing offers long term benefits in that as a window or door ages its ability to resist air and water infiltration is reduced.  The sill flashing intercepts water leakage and redirects the water to the building exterior.  The installation of sill flashing only adds a small percentage to the construction cost on a window or door replacement project and is typically able to outlast the performance life of the windows and exterior doors.