Over-Flashed (Watershed) Back Flashings

Watershed back flashings are easy to install and to weatherproof, particularly if the roof is already in place. The drawbacks are their limits in width and, sometimes, noise or condensation issues. Long lengths of watershed flashings may require multiple end laps which are vulnerable to leakage. Where there are end laps or foot traffic is expected on the watershed flashing, the flashing must be supported in the pan or the profile by rigid closed cell foam or similar.

In many residential cases where the flashing is visible, the aesthetic values of watershed flashings may render them inappropriate for this application, unless the penetration is situated close to the apex.

The maximum width of a watershed flashing is controlled by the coil width of 1.2 m The practice of making wider watershed flashings by running flashings horizontally with laps at 1.1 m is not acceptable, as the numerous joins are prone to leakage. Wider watershed flashings can be fabricated using longitudinal standing-seam techniques on suitable support.