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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.

Requirement	Model Code Standard	Section
Where flashing is required	Approved corrosion-resistant flashing is required at exterior window and door openings, and it has to drain water either to the exterior wall finish or out onto the water-resistive barrier so the wall can shed it	IRC R703.4
Which method you may use	Flashing at openings must follow the fenestration manufacturer’s instructions, the flashing manufacturer’s instructions, details from a registered design professional, or an approved flashing standard — the code lets you pick, but you have to actually follow one of them	IRC R703.4
Pan flashing at sills	A pan flashing is required at the sill of exterior window and door openings, and it must be sealed or sloped so water is directed out to the wall finish or the WRB rather than pooling in the rough opening	IRC R703.4.1
Head and jamb protection	An opening using pan flashing must also have flashing or equivalent protection at the head and the jambs — a sill pan alone is not a complete detail	IRC R703.4.1
Manufacturer-instruction exception	Pan flashing is not required where the window or door is installed per manufacturer instructions that include their own approved flashing method — keep those instructions on site, because that exception is only as good as the paperwork the inspector can see	IRC R703.4.1 Exception
Membrane materials	Self-adhered flashing membranes must comply with AAMA 711, fluid-applied flashing with AAMA 714, and mechanically attached flexible flashing with AAMA 712 — housewrap tape and generic duct-style tapes are not substitutes	IRC R703.4
Head flashing and drip caps	Flashing is required continuously above projecting exterior trim, which is what puts a drip cap over a head casing or band board; the cap must be lapped by the WRB above so water runs over it, not behind it	IRC R703.4

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.

Sections cite the 2021 International Residential Code and 2023 National Electrical Code as model standards. This is BuildSpec’s own summary — not ICC or NFPA text; read the authoritative language at codes.iccsafe.org. Your state may be on a different edition with amendments — check your jurisdiction page or get an Address Report.

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