Quick answer. The symptom does not on its own establish the correct replacement weatherstrip or a single proven failure. Observe the exact joint and normal closing behavior before replacing a part; if there is a building-water path, alignment problem or safety-rated assembly, obtain model-specific service guidance.
Water beneath the door has more than one possible route
A door-bottom sweep is the moving leaf's contact with a threshold; the sill and its junction with side framing are a separate part of the building's water-management path. DOE's Building Science Education resources identify the sill as vulnerable and emphasize flashing and drainage toward the exterior. Those sources do not diagnose an individual door by photo. [DOE_FLASH] [DOE_FLASH2]
Record where the first moisture appeared
Before attempting another repair, make a dated set of photos after the area is dry. During an ordinary rain event, from a safe indoor position, record the first wet location: directly below the moving leaf, at one jamb/sill corner, across the whole threshold, or emerging from the trim/wall. Note wind-driven rain versus calm rain and whether the opening was normally closed and latched. Water appearing later may have traveled; a puddle's final position alone is not a definitive origin.
Use an observation matrix, not a guessed part
Center of door bottom: record visible sweep damage and contact, but do not assume a longer sweep fixes concealed leakage. One sill-jamb corner: document whether water appears at a junction rather than at midspan. Below fixed trim: request assessment of sill flashing and frame-to-wall joints. No water but cold draught: switch to air-leak testing; the two symptoms are not interchangeable. For continuous wetting, visible rot or concealed-wall staining, stop and obtain a professional moisture investigation.
Avoid blocking escape routes for water
Do not caulk over factory weep/drainage openings or pour sealant into the sill channel without the manufacturer's drawing. A doorway assembly may be designed to shed incidental water outward. That design has to be identified before altering it. DOE notes that correct pan/sill flashing is integrated with the drainage plane, not simply replaced by a thick compressible strip. [DOE_FLASH2]
What to photograph and ask
Collect the door brand/series, swing direction, threshold cross-section from any official service diagram, sill-to-jamb corner photographs, status of existing sweep/door shoe, and the date/weather of the event. Ask: Is this model's lower corner supposed to have a pad or end dam? Which drains must remain open? Is field water-testing or flashing inspection required? Which exact part, if any, is manufacturer-authorized?
What this guide cannot establish
It cannot confirm a failed sweep, an approved sill retrofit, a rated assembly modification or a dry concealed wall. If repeated water ingress affects flooring, accessibility/egress, electrics or structural fabric, stop DIY experimentation and arrange a qualified inspection.
Bounded source notes (checked publisher web pages; 2026-10-12)
- [DOE_FLASH] U.S. Department of Energy, Building Science Education, Fully Flashed Window and Door Openings: Sill Flashing, BSE web education page, retrieved 2026-10-12; locator: What/Why sections: sill vulnerability, directed drainage, back dam. https://bsesc.energy.gov/energy-basics/fully-flashed-window-and-door-openings-sill-flashing (Scope: Not a diagnosis of a specific leaking doorway; no opening-disassembly instruction)
- [DOE_FLASH2] U.S. Department of Energy, Building Science Education, Windows and Doors are Installed: Fully Flashed Window and Door Openings, BSE web education page, retrieved 2026-10-12; locator: What/Why/How: integrate flashing with drainage plane. https://bsesc.energy.gov/energy-basics/windows-and-doors-are-installed-fully-flashed-window-and-door-openings (Scope: Not a seal-only repair prescription)
- [NPS_AIR] U.S. National Park Service, Weatherization of Historic Buildings: Eliminate Air Infiltration, Updated 2022-04-11; reviewed 2026-10-12; locator: Air infiltration; caulking frame-to-wall versus weatherstripping moving sash and jamb. https://www.nps.gov/articles/000/weatherization-of-historic-buildings-air-infiltration.htm (Scope: General distinction, not permission to seal a particular window or approved installation)