Flashing and Sill Pans on Stucco Walls

Spacious sunroom with large glass windows offering an outdoor view.

Flashing does one job: it sends any water that gets behind the stucco back out of the wall. Every teardown riverside window installation grevillea park has logged shows the same thing about window installation in Riverside, CA, that the window itself rarely leaks and the rough opening around it often does.

The sill pan comes first

Beautiful pink blossoms adorn a house exterior with white windows and black shutters.

A sill pan is a waterproof tray at the bottom of the rough opening. It catches water that gets past the frame and turns it outward. It can be built from flashing tape, a pre-formed plastic pan or a flexible membrane, and it should slope slightly to the exterior with the back edge turned up. Without one, the bottom of the wall is the first place to rot.

Tape in the right order

Low angle shot of a modern residential building facade against a clear blue sky.

Flashing tape is applied bottom to top, like shingles. The sill goes first, then the jambs, then the head, so every upper piece laps over the lower one and water always runs over a seam instead of into it. A common mistake is taping the head first, which sends water behind the jamb tape.

The nailing fin and the stucco return

The nailing fin is the flange around a new construction window. It is fastened to the sheathing and covered with tape at the sides and top, while the bottom fin stays open to drain. Stucco then returns over the fin edge. The joint where stucco meets the frame needs a backer rod and sealant, since stucco cracks there first as the wall moves.

Why dry weather misleads

Riverside gets little rain, so a poor flashing job can sit unnoticed for years. The first heavy storm of the winter, often after a dry fall, finds every shortcut. Wind-driven rain on a west wall is the worst case. Water also enters at a hose, a sprinkler or a pressure wash aimed at a seal, so dry months do not make it safe.

What to see on a finished job

A finished opening shows a continuous sealant line, no gaps at the corners of a vinyl window or aluminum window frame, and a sill that sheds water outward. A bay window or bow window needs extra attention at its roof and seat, because the horizontal surfaces collect water. If the installer can describe the flashing sequence in order, the job is likely sound.