Window and Door Flashing Requirements — Pan, Head & WRB Integration

Flashing is the part of the wall that assumes water will get in. The code doesn’t ask you to make the opening watertight — it asks you to build a path that collects whatever leaks and sends it back outside, which is why the order of the laps matters more than the amount of sealant.

Worth knowing

• Shingle fashion is the whole idea: the sill pan goes on first and laps out over the WRB below, the jamb flashing laps over the pan, the head flashing laps over the jambs, and the WRB above laps over the head flashing. Any lap installed backwards turns the assembly into a funnel.
• A sill pan needs somewhere for water to go. Site-built pans usually get a back dam at the interior edge and end dams at the jambs, and the sill is left unsealed at the bottom so the pan can drain — sealing all four sides of a window is a common and expensive mistake.
• The most frequent failure points inspectors and forensic investigators find are unflashed sills, head flashing tucked under the trim instead of over it, and WRB slit above the opening and taped rather than lapped.
• Self-adhered membranes are temperature- and substrate-sensitive. Cold-weather versions, primers, and roller pressure are all specified by the manufacturer, and a membrane applied outside its listed conditions may not be considered installed per instructions.
• Flashing only works if the drainage plane behind it is continuous — see the housewrap and water-resistive barrier guide (/guides/code/house-wrap) for how the WRB laps, seams, and terminations tie into these details.

Common questions