Why the Pan Under the Threshold Is the Step That Gets Skipped
This article is for commercial contractors, facility managers, and architects who have watched a threshold fail within a year of installation -- not because the threshold itself was wrong, but because the pan flashing detail underneath it was never done correctly. Getting the threshold selected and delivered is only half the job. The sill condition beneath it determines whether water stays out of the building for a decade or migrates into the subfloor before the first winter is over.
What Is a Threshold Pan, and Why Does It Matter?
A threshold pan (also called a sill pan or sill pan flashing) is a formed channel -- typically sheet metal or a pre-manufactured pan unit -- installed beneath a door threshold at the rough sill. Its purpose is to capture any water that penetrates past the exterior threshold surface and redirect it outward through weep holes or a sloped face, rather than allowing it to travel into the rough opening or subfloor assembly.
Even a properly selected threshold -- saddle, interlocking, or thermal break -- sits on top of the sill. If the sill below is not flashed correctly, the threshold is doing its job while water routes around it through the framing. In commercial applications such as schools, healthcare facilities, retail storefronts, and industrial service entries, this failure mode shows up as rot, corrosion at the door frame base, and staining inside the building -- typically discovered long after the general contractor has left the job.
The Three Details That Are Most Often Wrong
1. The Pan Is Missing Entirely
On many commercial projects, particularly hollow metal frames set in CMU or poured concrete sills, no pan is installed at all. The assumption is that the concrete sill is inherently waterproof. In practice, cracks at the frame-to-sill interface and fastener penetrations create reliable water paths. Any exterior threshold installation on a slab-on-grade or elevated sill without a pan flashing is a future callback waiting to happen.
2. Weep Holes Are Blocked or Absent
Interlocking threshold assemblies and some pan systems include provisions for weep holes at the front face of the sill pan. These must remain clear after installation. Common errors include:
- Applying sealant or caulk over the weep hole face in an attempt to "seal everything"
- Installing the threshold over bedding compound that fills the pan cavity and blocks drainage
- Cutting the threshold length short so the pan lip is exposed but unprotected, creating a dam rather than a drain
3. End Dams Are Not Formed or Installed
A sill pan that is open at both jamb ends allows water to exit laterally into the wall cavity -- exactly the wrong direction. End dams, formed by turning the pan material up at each jamb or using a pre-manufactured end dam component, are required to ensure water exits only through the front weep face. This detail is frequently omitted on field-fabricated pans and is a common source of wall cavity moisture in schools and healthcare buildings where the exterior envelope is under sustained weather exposure.
How the Threshold Type Affects the Pan Requirement
Not every threshold type responds the same way to a flawed pan detail, and understanding the relationship helps during specification.
- Saddle thresholds -- the most common exterior type, with heights ranging from roughly 1/4" to 3/4" -- sit flat on the sill and offer no inherent drainage path. They rely entirely on the pan below to handle water that gets past the sweep seal.
- Interlocking thresholds, used primarily on inswinging doors, include a hook strip that engages a door bottom sweep. Some interlocking assemblies can be furnished with an aluminum pan for waterproofing the sill area, and that pan may include weep holes and a formed lip. This built-in pan provision is only effective if end dams are properly addressed and the pan is not blocked during installation.
- Thermal break thresholds add insulation between the exterior and interior extrusions. The break does not provide waterproofing -- the pan detail beneath is equally critical here, often more so, because the dual-extrusion profile can trap water between components if the sill is not sloped to drain.
- Panic thresholds (latching type, used with vertical rod exit devices) introduce a strike surface at the sill. Debris, water intrusion, and corrosion at the pan-to-strike interface can affect latch engagement. A clean, properly drained sill condition directly protects device function over time.
Sill Slope and Bedding: Two Details That Interact
The rough sill beneath the threshold should slope toward the exterior -- typically 1/8" per foot minimum -- to encourage drainage rather than ponding. When a threshold is set in a bed of sealant or threshold mastic, that bedding must not create a dam at the front edge. Apply bedding at the back and sides of the threshold only, leaving the front face clear so any trapped water has an exit path. On concrete sills, confirm the sill is sloped before the threshold goes down. Correcting sill slope after the frame is set and the threshold is in place is a much larger problem.
Fastener Penetrations Through the Pan
Threshold anchoring fasteners that pass through a pan flashing are each a potential water intrusion point. Best practice:
- Use appropriate fasteners for the floor construction type (concrete anchor, wood screw, or machine screw depending on substrate)
- Apply a compatible sealant at each fastener penetration before driving the fastener home
- Do not over-torque fasteners into aluminum thresholds -- distortion of the extrusion can compromise the sweep-to-threshold seal
Getting the Threshold and Pan Detail Right From the Start
The threshold itself -- material, profile, height, and ADA compliance -- is only one part of the opening condition. Pemko and Hager both offer threshold lines with pan-compatible profiles and accessory components suited to a range of commercial sill conditions. Selecting the right threshold profile for the frame detail, sill construction, and weather exposure is a conversation worth having before the door frame is set -- not after the certificate of occupancy walk.
DoorwaysPlus.com carries commercial thresholds, threshold pans, sill seals, and complementary weatherstripping from lines well-suited to schools, healthcare, retail, and industrial applications. If you are looking at a sill condition that does not fit a standard catalog selection, our team can help identify the right combination of components before the installation date arrives.
Questions about your sill detail or threshold selection? Contact DoorwaysPlus.com for application guidance and competitive pricing on commercial threshold systems.