The bottom of a door is one of the hardest spots on a building to keep dry — low to the ground, walked on, and often stripped of the pan flashing that would catch the water that gets past the threshold.
The threshold at the bottom of an exterior door sits low, takes foot traffic, and faces water driven right at it — and it is one of the most common places a building leaks. A sill pan under the door is supposed to catch any water that gets past the threshold and drain it back out, but pans are frequently missing, too short, or lacking the dams that make them work. Accessibility requirements that lower the threshold make the detail harder still.
The failures are predictable. The sections below cover the four that ACE sees most — missing or short pan flashing, back-dam and end dam gaps, accessible threshold detailing, and interior floor and drainage coordination — and how to keep the base of the door dry.
These challenges rarely stay in one silo: the same discipline runs through our field notes on cladding attachment failures, the same playbook behind slab-edge thermal bridging, and lessons from deck ledger flashing failures, and it all ladders up to ACE’s broader remedial waterproofing and repair.
Missing or short pan flashing is the core door threshold and sill pan failure, because the pan is the last line of defense that catches water getting past the threshold and drains it back out, and a door set directly on the substrate without a pan — or on a pan too short to reach the drainage plane — sends that water into the floor and wall. A continuous pan sized to collect and drain the water is essential.
The pan has to integrate with the wall’s water-resistive barrier and flashing so the water it collects is directed outward, not simply moved to a different hidden spot.
Because the pan is concealed under the door once installed, verifying it before the door is set is the practical way to confirm it is there and correct.
Representative of the door-base failures ACE finds in opening investigation and peer-review work. Illustrative distribution, not a published statistic.
Back-dam and end dam gaps let a pan leak even when it is present, because without a turned-up back dam at the interior edge and end dams at the sides, the water the pan collects simply runs off the back or ends into the building. A pan with an integral back dam and end dams contains the water until it drains forward and out.
The back dam is what keeps water from running under the threshold to the interior floor, and its absence is a frequent reason a pan is present but the door still leaks.
Because the dams are what make the pan a functioning tray rather than a flat sheet, they are the detail most worth confirming.
A door set without a proper sill pan, or a pan lacking dams, sends water past the threshold into the floor and wall.
Install a continuous pan with back dam and end dams, integrated with the barrier, and coordinate the threshold with interior and exterior drainage.
Water that gets past the threshold is caught and drained back out, and the chronic leak at the base of the door stops.
Accessible threshold detailing complicates the waterproofing, because accessibility requirements call for low thresholds with minimal height change, which reduces the room available for a back dam and a drainage path at exactly the doors that need them. The detail has to reconcile a low, walkable threshold with a functioning pan and drainage.
Solutions such as trench drains or recessed drainage just outside the door, coordinated with the pan, allow a low threshold while still managing the water, but they have to be designed together rather than in conflict.
Because this is where code accessibility and water management pull against each other, it deserves deliberate detailing rather than a default threshold dropped onto a standard wall.
Get the sill pan, dams, and threshold drainage reviewed before the door and floor are finished.
Schedule a consultationCall (866) 389-8883Interior floor and drainage coordination determines whether the whole detail works, because the pan and threshold have to relate correctly to the interior finished floor and to exterior drainage, and a floor that sits above the pan’s drainage point, or exterior grade that drains toward the door, defeats the pan. The levels and drainage have to be coordinated across the threshold.
Exterior surfaces should slope away and drain, and interior floors should sit so the pan can discharge, which is a coordination issue among several trades rather than a single detail.
The most valuable move overall is treating the door base as a coordinated system — pan, dams, threshold, and interior and exterior drainage together — and verifying it before the door and floor finishes hide it.
In ACE’s field work, most door threshold and sill pan failures problems trace back to a few recurring locations — Missing or short pan, No back dam / end dams, and Accessible-threshold conflicts — rather than the open field of the wall or roof. The cost of resolving them climbs by roughly an order of magnitude at each stage: a detail corrected during design costs a fraction of the same fix during construction, which is itself far cheaper than a repair once the facility is operating. That economics is why we push envelope decisions as early in the project as possible.
Specify a continuous sill pan with back dam and end dams, integrated with the barrier, and coordinate the threshold with interior and exterior drainage.
Confirm the pan, back dam, and end dams are present and integrated before the door is set and finishes cover them.
Where a door leaks at the floor, check for a pan and dams and correct the drainage across the threshold.
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