Door Sill Pans: Stopping Leaks Under Exterior Door Thresholds

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Water that gets past an exterior door threshold rarely announces itself. It soaks quietly into the subfloor, the rim joist and the bottom of the jamb, and the first visible sign is often a soft spot in the floor or paint that will not stay on the brick mould. A door sill pan is the inexpensive layer that stops that slow damage. It sits under the threshold, catches any water that gets past the door and its seals, and sends that water back outside instead of into the framing. This guide explains what a sill pan is, how pre-formed and site-built pans differ, why back dams, end dams and slope matter, how a pan ties into the house wrap, and what retrofitting one during a door replacement typically involves. Code details vary by jurisdiction and door manufacturer, so treat the specifics here as general guidance rather than a substitute for the installation instructions or a local building official.

Part of our Structure & Exterior guide — start there for the full picture →

What a Door Sill Pan Is and What It Does

A sill pan is a waterproof tray installed in the rough opening of an exterior door, beneath the door unit’s sill. Its job is simple: assume water will get past the door eventually, and give that water somewhere harmless to go. No door is perfectly watertight forever. Wind-driven rain gets pushed under thresholds, sweeps wear out, sill caulk cracks, and snow piles against the bottom of the door and melts. Without a pan, whatever water gets past those defenses lands on the subfloor and the top of the rim joist, which are typically wood products that do not tolerate repeated wetting.

A properly built pan has three features that matter:

• A back dam. An upturned lip along the interior edge of the opening. It stops water from running inward toward the finished floor.
• End dams. Upturned corners at each side of the opening, so water cannot run off the ends of the pan and down into the wall cavity beside the door.
• Slope or drainage to the exterior. The pan, or the surface beneath it, should direct water outward. Some pre-formed pans have built-in slope, and some site-built pans rely on a beveled shim or tapered sill block underneath.

The outside edge of the pan is the one side left open, or lapped over the exterior cladding or flashing below, so collected water can drain out. A pan that is sealed shut on all four sides is just a bathtub under the door.

Pans are not the same as thresholds

The threshold, sometimes called the sill or saddle, is the visible part of the door unit that people step over. The pan is hidden beneath it. Replacing a worn threshold does not add a pan, and a new door unit set directly on bare subfloor still has no pan unless someone installs one. Our guide to door threshold replacement covers the visible part; the pan is the layer underneath that does the real waterproofing.

Pre-Formed Versus Site-Built Pans

There are two broad approaches, and both can work when they are installed correctly.

Pre-formed pans

Manufactured pans are molded plastic, PVC, or formed metal trays sized for common door widths, or adjustable two-piece and three-piece units that slide together to fit the opening. Many have the back dam, end dams and a sloped floor built in. The advantage is consistency: the corners are factory-formed, so there is no folded or cut membrane at the most vulnerable spot. The disadvantages are fit and availability. An odd-width opening, a door with sidelites or a sill that sits on an uneven slab may need a custom solution. Some pre-formed pans also need to be set in sealant per the maker’s instructions, and some need support underneath so they do not flex when someone stands on the threshold.

Site-built pans with flexible flashing

The other common method uses self-adhered flexible flashing membrane, folded and stretched to form the pan in place. The installer typically adds a back dam (often a strip of wood or a tapered piece of trim along the interior edge), then lays flashing across the sill, up the back dam and up each side of the opening several inches. Corners are the tricky part, which is why many flexible flashing products are designed to stretch into a three-dimensional corner without cuts. Separate corner boots or pre-formed end dam pieces are sometimes added for extra protection.

The quality of a site-built pan depends heavily on the membrane, the primer if one is required, the temperature at installation and the care taken at the corners. Our guide to flashing tape explains the differences between butyl and asphalt-based membranes, why some need primer on certain substrates, and how cold weather affects adhesion. That last point matters on the Front Range, where many door replacements happen in shoulder seasons with cold mornings.

Which approach is better?

Neither is universally better. A pre-formed pan reduces the risk of a botched corner. A carefully built flexible pan adapts to unusual openings. What matters most is that the finished pan has a continuous back dam, sealed end dams, a path for water to exit, and correct laps with the surrounding weather barrier. Check the door manufacturer’s installation instructions, because some specify a particular pan method and the warranty may depend on following it.

Why Missing Pans Cause Rot at Doors

Doors without pans are a common source of hidden water damage, and the damage pattern is consistent enough that inspectors and remodelers learn to look for it. Water gets under the threshold, migrates sideways and inward along the subfloor, and wicks into the end grain of the jamb legs and the edge of the subfloor sheathing. Because the area is covered by the threshold, flooring and trim, it can stay wet for long periods without anyone seeing it.

EPA’s mold guidance makes the underlying principle plain: moisture control is the key to mold control, and if wet or damp materials are dried within 24 to 48 hours after a leak or spill, in most cases mold will not grow. Framing under a door that gets wet with every storm never gets that chance to dry. Over seasons, the likely results include:

• Soft or delaminated subfloor just inside the door, often felt as a spongy spot or seen as flooring that has buckled or lifted. Our guide to subfloor repair covers what fixing that involves.
• Rotted rim joist or band board below the door, which may only be visible from a basement or crawl space. This is the expensive one, because the rim joist carries wall loads.
• Decayed jamb bottoms and brick mould. Paint peels, wood softens and the door may start to sag or bind. Our door jamb guide explains the parts involved, and our door frame repair guide covers when a frame can be patched and when it needs replacement.
• Mold growth on the underside of the subfloor or inside the wall cavity beside the door.

Wood decay generally needs sustained moisture, so the slow, repeated wetting from a door without a pan is exactly the condition that allows it. For a broader look at how damaged framing is assessed and repaired, see our guide to wood rot repair.

Doors on the weather side of the house, doors with no overhang above them, long patio and sliding door sills, and doors that open onto a nearly level deck or stoop where snow piles against the sill are the usual trouble spots on the Front Range.

How the Pan Ties Into House Wrap and Wall Flashing

A sill pan does not work alone. It is one part of a drainage system that depends on every layer lapping correctly, like shingles: upper layers overlap lower layers so water always flows outward and down.

In a typical new-construction or full-tear-out sequence, the weather-resistive barrier is installed on the wall, the rough opening is prepared, the sill pan goes in first at the bottom, the door is set, side flashing goes on over the door’s nailing fin or against the jamb and laps over the pan’s end dams, and head flashing goes on last, with the house wrap above lapping over it. The key rule is that the pan sits under everything above it, and its exterior edge laps over whatever is below it, so water exits on the outside of the wall. Our guide to house wrap explains how the weather barrier is layered and why reverse laps are a common defect.

Where pans fail, the cause is often a lapping mistake rather than the pan itself:

• The house wrap or side flashing tucked behind the pan instead of over its end dams.
• The pan’s exterior edge trapped behind siding or trim so water cannot get out.
• Sealant run continuously along the exterior edge of the sill, closing the drainage path.
• Fasteners driven through the bottom of the pan to secure the sill, creating holes at the lowest point.
• The back dam missing or too low, so water can flow inward.

Air sealing is a separate job

A sill pan handles liquid water. Air leakage around the door is a related but different problem. ENERGY STAR’s guidance on sealing behind window and door trim lists drafts around a closed door, visible gaps around the trim and peeling paint around the frame among the signs of air leaks. Its project steps involve removing interior trim and sealing the gap between the jamb and the wall framing with non-expanding foam sealant made for windows and doors, backer rod or caulk, while warning not to force backer rod in so hard that the frame deflects. That interior air seal complements the exterior water management. It should not block the pan’s drainage path to the outside.

Retrofitting a Pan During Door Replacement

Adding a pan to an existing door is rarely practical without removing the door. Sliding a membrane under a set threshold does not create a back dam, end dams or proper laps. The realistic moment to add one is when the door is being replaced, or when water damage has already forced the door out for framing repairs.

A typical retrofit sequence, described in general terms, looks like this:

1. Remove interior trim, exterior trim or brick mould, and the old door unit.
2. Inspect the subfloor, rim joist and jamb area for decay. Probe suspect wood with an awl or screwdriver. Replace rotted material before going further, because a pan over damaged framing just hides the problem.
3. Confirm the sill area is level side to side, and prepare any slope to the exterior with a beveled shim or tapered sill block if the pan does not provide it.
4. Install the pan, with its back dam, end dams and exterior lap, following the pan and door manufacturers’ instructions.
5. Set the new door in sealant as specified by the door maker, typically in a way that does not seal off the pan’s drainage at the exterior edge.
6. Integrate side and head flashing with the existing house wrap, cutting and taping the wrap as needed so laps are correct.
7. Seal the interior gap between jamb and framing, reinstall trim and add or adjust sweeps and weatherstripping.

Our guide to exterior door replacement cost covers how door type, framing condition and finish work affect the total, and a pan is one of the line items worth asking about specifically.

Sweeps, Weatherstripping and the First Line of Defense

The pan is the last line of defense. The first line is the door’s own seals: the sweep or bottom seal, the threshold’s adjustable cap, and the weatherstripping around the jamb. Keeping those in good condition reduces how much water ever reaches the pan.

• Door bottoms and sweeps. A worn or torn sweep lets wind-driven rain and air through. Our guide to the door sweep explains the main types and when to replace them.
• Jamb weatherstripping. Compression or kerf-in weatherstripping seals the sides and head. Gaps at the bottom corners, where weatherstripping meets the threshold, are a common leak point. See our guide to weatherstripping doors for options.
• Adjustable threshold caps. Many thresholds have a cap that can be raised or lowered with screws to meet the door bottom.

What an Inspector Can and Cannot See

A sill pan is almost always concealed once a door is installed. The ASHI Standard of Practice lists exterior doors among the items a home inspector inspects, along with wall coverings, flashing and trim. In practice, that means looking at the door’s condition and operation, the threshold, the seals and the visible flashing and trim. Inspectors generally do not remove thresholds or trim to confirm whether a pan exists.

What an inspector can do is look for the symptoms of water getting past the door:

• Soft, stained or lifted flooring inside the door.
• Peeling paint, swelling or decay at the bottom of the jamb or brick mould.
• Stains, decay or mold on the rim joist or subfloor underside, seen from the basement or crawl space below the door.
• Elevated moisture meter readings at the floor inside the threshold, if the inspector uses a meter.
• Sill caulk that is cracked or missing, or a sill sealed in a way that traps water.
• Grade, stoops or decks sloping toward the door.

If any of those show up, a recommendation for further evaluation by a qualified contractor is typical. Our structure and exterior section covers the rest of the exterior inspection, and the main guide to hiring a home inspector explains how to choose an inspector and what a standard inspection includes.

Typical Costs

Prices vary with the region, door type, framing condition and who does the work. As a rough 2026 guide:

• Pan materials alone: flexible flashing for a site-built pan is often in the tens of dollars per door. Pre-formed pans commonly run from the tens of dollars to somewhat over a hundred for a standard entry door, with wider or multi-piece pans for patio doors costing more.
• Pan as part of a door replacement: when the door is already out, adding a pan usually adds a modest amount of labor, and many installers include it in their standard method.
• Retrofit on an existing door: because the door has to come out and be reinstalled, the cost is closer to a reinstall, often several hundred dollars or more before any repairs.
• Rot repair: replacing damaged subfloor, rim joist or jamb sections can easily cost more than the door and pan combined, depending on how far the decay has spread.

Get written quotes that state whether a pan is included, what type, and how hidden damage will be priced if it turns up. Building permits for door replacement vary by jurisdiction, so check with the local building department.

References

• A Brief Guide to Mold, Moisture and Your Home — U.S. Environmental Protection Agency
• Sealing Behind Window and Door Trim Project — ENERGY STAR
• ASHI Standard of Practice for Home Inspection — American Society of Home Inspectors

Buying a home with soft floors or stained trim by an exterior door? Reach out to schedule a Front Range inspector who checks doors, thresholds and the framing beneath them.