Improper or missing sill flashing is one of the most common causes of window leakage. This article outlines best practices for integrating sill pan flashing, back dams, and drainage slopes to preserve durability and waterproofing performance.
Purpose of Sill Flashing
Sill pan flashing collects and redirects water that may infiltrate around or through a window. It acts as a final barrier to keep water from reaching wall framing, insulation, or interior finishes.
Sill Flashing Methods
- Use preformed sill pans or create site-fabricated sill flashing using membrane and corner patches
- Include a back dam to prevent interior water migration
- Slope the sill to the exterior with shims or beveled lumber
- Wrap sill membrane up the jamb sides at least 6″ for continuity
Drainage and Air Barrier Integration
Install self-adhered flashing tape over the pan and lap the WRB over the sill membrane. Avoid reverse laps by sequencing tape and membranes from bottom up and shingling layers. Apply sealant only where necessary—never fully seal the pan base to allow drainage.
Installation Best Practices
- Use corner patches to reinforce transitions at jamb and sill intersections
- Pre-wrap the sill before window installation for better adhesion and weather resistance
- Use mechanical fastening only above the sill—not through the pan base
- Include a water-resistant shim pad or drainage mesh below the window if applicable
Mockups should be reviewed and tested prior to siding installation, especially in multifamily or commercial projects subject to warranty and QA inspection.
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