Pan flashings under window sills must slope to drain to the exterior to prevent standing water and subsequent leaks.

For reliable performance, slope pan flashings at between one eighth inch and one quarter inch per foot toward the exterior sill; this allows capillary flow and gravity to move water out rather than pooling under the sill where rot and corrosion start. Install a back dam at the interior edge of the pan flashing a quarter inch to three eighths inch higher than the drain plane to stop water from running back into the wall cavity while still allowing the exterior slope to shed moisture. Extend side flashings up behind the cladding and over the pan flashing by at least three inches to create continuous drainage paths, and create a positive lap pattern so water flows over layers rather than into seams. Use waterproof membranes that are chemically compatible with adjacent sealants and flashings, and mate them to the window’s integral flange or to a sill pan adapter with a continuous bead of compatible sealant to maintain a watertight transition. Ensure the pan flashing ties into the wall water resistive barrier with a shingle-style step flashing approach so any infiltrating water sheds outward; test slopes by pouring a small stream of water to confirm unobstructed flow before finishing the cladding. Keep fasteners clear of the drainage path or use corrosion resistant fasteners and seal them; continuous cleats or clips are preferable to face fastened returns that can puncture the membrane or hold water.

The three most common mistakes are insufficient slope that allows standing water, missing or inadequate back dam that lets water run into the interior, and poor integration with side flashings or the water resistive barrier that creates bypass paths. Fix shallow slopes by removing and re-setting the pan with the recommended one eighth inch to one quarter inch per foot fall so the water flows freely toward the exterior. Add or rebuild a back dam at the interior edge at a quarter inch to three eighths inch height to block return flow while not impeding exterior drainage. Rework the flashing sequence so side flashings extend over the pan and tie into the water resistive barrier in a shingle pattern and replace incompatible membranes or sealants to restore continuous, compatible barriers.

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