Sill flashing is the flashing installed at the bottom of a window or door opening, typically formed into a sloped pan shape that catches any water reaching this location and directs it outward, away from the rough opening framing and any window or door unit installed above it.
This component addresses a particularly vulnerable point, since the sill sits at the bottom of the opening where water naturally collects after running down the sides and back of the window or door frame during rain.
Modern sill pan flashing, often a preformed product rather than simply bent flat metal, has become increasingly standard practice, offering a more reliable, consistent shape than field-fabricated flashing typically achieves.
What Does Sill Flashing Involve?
- A sloped pan shape directing water outward
- Integration with jamb flashing at the sides
- Placement beneath the window or door unit itself
- End dams preventing water from escaping the pan’s sides
Why Is Sill Flashing Important?
Water that collects at a window or door’s base needs a reliable path away from the rough opening framing, and a properly sloped sill pan provides exactly that, preventing water from sitting against the wood framing or working its way into the wall or subfloor beneath the opening. This protective function addresses one of the more common sources of hidden rot around windows and doors.
End dams, small upturned sections at each end of the sill pan, are a critical detail often overlooked in simpler flashing approaches, since without them water can simply flow off the sides of the pan rather than being contained and directed properly out toward the exterior.
Sill Flashing vs. Head Flashing
Sill flashing protects the bottom of an opening with a sloped, water-catching pan design. Head flashing protects the top edge, directing water down and around the opening rather than catching and redirecting water that has already collected.