Use pan flashing under all windows and doors

Pan flashing protects the rough sill beneath windows and doors from water damage.
It can be made from plastic sheeting, or EPDM membrane, or something more durable such as lead-coated copper, or even stainless steel.

All flashings beneath windows and doors are important first, because these represent the lion’s share of wall penetrations and second, because they are on the bottom they collect whatever leaks in one spot. Advisable nearly everywhere, flashings are especially important beneath windows exposed to severe weather.

Nearly as important as the flashing is how it’s tied into the wall behind the siding. Leaking water should roll down to the pan, lip over to a drainage planePath that water would take over the building envelope. Concealed drainage-plane materials, such as building paper or housewrap, are designed to shed water that penetrates the building’s cladding. Drainage planes are installed to overlap in shingle fashion (weatherlap) so that water flows downward and away from the building envelope. and out the bottom. Water that leaks into wall cavities causes rot and mold, which can lead to premature building failure and occupant health problems.

Green Points

NGBSNational Green Building Standard Based on the NAHB Model Green Home Building Guidelines and passed through ANSI. This standard can be applied to both new homes, remodeling projects, and additions. /ICC-700: 6 pts. For flashing installed around all exterior fenestrations, skylights, and doors.

Learn more in the Green Building Encyclopedia:

Video Series: Leak-Free Windows and Doors

Further Resources

Building Science Corp.
Pan Flashings for Exterior Wall Openings
Common Flashing Details