How do I replace a damaged exterior door threshold to prevent drafts and water intrusion?

Short answer

Replace the damaged threshold with a properly sized, exterior-rated adjustable threshold set over a sealed sill pan or flashing. Remove the old threshold, repair any rot, install pan flashing with a slight outward slope, bed the new threshold in polyurethane sealant, fasten securely, and adjust the cap and door sweep to create a tight seal that sheds water.

Why this works

Exterior thresholds are your door’s first line of defense against drafts and water. The right replacement—installed over a waterproof sill pan or flashing with proper slope—prevents water from entering under the door and creates consistent compression against the door sweep to stop air leaks. Skipping the pan or using the wrong sealants is a common cause of future leaks and rot.

Tools and materials

• Replacement exterior threshold (preferably adjustable aluminum/composite; match jamb depth: 4-9/16" or 6-9/16"; length: 32" or 36")
• Pre-formed sill pan, PVC pan, or butyl flashing tape system
• Polyurethane or hybrid exterior sealant (not latex)
• Stainless or exterior-coated screws; concrete anchors if on slab
• Shims (composite or cedar)
• Low-expansion window/door spray foam or backer rod as needed
• Door sweep (replace if worn)
• Tools: oscillating multi-tool with flush-cut blade, pry bar, utility knife, drill/driver, masonry bit (if slab), chisel, torpedo level, tape measure, square, caulk gun, vacuum, shop rags
• Optional: wood hardener/epoxy and copper naphthenate preservative for minor rot repair

Time: 2–4 hours for an intermediate DIYer. Cost: $80–$250 depending on threshold type and flashing.

Prep and assessment

1. Inspect the door system. If your threshold is integral to a prehung sill with severe rot into the jambs/subfloor, you may need a full sill replacement or even a new prehung unit. If rot is contained to the threshold and top layer of subfloor, a standalone threshold replacement is fine.
2. Measure carefully:
   • Door width (commonly 32" or 36")
   • Jamb depth (4-9/16" for 2x4 walls, 6-9/16" for 2x6 walls)
   • Desired profile: standard adjustable, low-profile/ADA, or outswing-compatible sill nose
3. Check for door sweep condition and adjustability. Plan to replace or re-adjust it after install.

Step-by-step replacement

1) Remove the old threshold

• Pull the door slab for easier access: tap out hinge pins and set the door aside on padded sawhorses.
• Score interior/exterior caulk lines with a utility knife.
• Remove any threshold screws. Use a pry bar to lift the threshold. If jamb legs sit on the threshold, use an oscillating tool to cut any nails or shims under the jambs so the threshold can slide out. Be patient to avoid splitting jambs.

2) Repair and prep the sill

• Scrape and vacuum debris. Probe the subfloor and jamb bottoms.
• Treat minor rot with copper naphthenate; reinforce with epoxy if needed. If subfloor is soft or crumbling, cut it back to sound wood and patch. On concrete, clean thoroughly.
• Verify slope: you want the sill to slope slightly outward to shed water.

Target specs
- Slope: ~1/8" fall to exterior across the threshold depth
- Door-to-threshold contact: uniform compression at the sweep
- Gap under door (before sweep): ~1/4"–3/8"

3) Install sill pan or flashing

• Best practice: set a pre-formed pan or build one with PVC and butyl flashing tape. Form a back dam at the interior edge (~1/4" ridge) to stop water from migrating inside.
• Run butyl flashing onto the subfloor/sill and up the jambs at least 3–4". Lap pieces shingle-style so water flows out. Do not use asphalt with vinyl components.

4) Dry-fit and trim the new threshold

• Test-fit the threshold between jamb legs. Many units tuck slightly under the jambs—undercut the jamb bottoms with a flush-cut saw if needed for a clean fit.
• Scribe and trim ends for tight fits against casing or sidelights. Keep the factory drip edge at the exterior intact.

5) Set and fasten

• Lay two generous beads of polyurethane sealant on the pan/flashing: one near the interior back dam, one near the exterior nose. Add a few dollops in the field.
• Bed the threshold into the sealant. Check for outward slope and level side-to-side; shim as needed (use composite shims).
• Fasten per manufacturer instructions:
  • Wood subfloor: stainless screws into solid framing/subfloor.
  • Concrete slab: pre-drill and use Tapcons or sleeve anchors.
• Wipe squeeze-out on the interior. Leave exterior weep paths—don’t seal every gap that would trap water.

6) Adjust and seal

• Rehang the door. Adjust the threshold’s cap screws to achieve even compression against the door sweep. Typically, turning screws clockwise raises the cap; tweak a little at a time across the width.
• Replace or adjust the door sweep to lightly compress against the cap without binding.
• Caulk the exterior joint where the threshold meets the stoop/deck with polyurethane sealant. Use backer rod for gaps >1/4". Lightly tool for a smooth shed profile. Avoid sealing across any designed weep notches.

7) Test for drafts and water

• Close the door on a strip of paper at several points; mild resistance is good and uniform.
• Hose test: a gentle spray from the exterior for a minute while someone checks inside. No seepage should appear behind the back dam.

Tips for best results

• Choose composite/rot-proof thresholds with replaceable vinyl inserts for longevity.
• Pre-finish cut ends with sealant or paint.
• Keep the adjustable screws accessible; seasonal tweaks can improve performance.
• If you have a storm door, ensure the threshold nose and sweep don’t conflict.
• In snow-prone areas, a taller exterior nose and well-sealed back dam provide extra protection.

Common mistakes

• Skipping the sill pan/flashing—this is the root cause of repeat rot.
• Using latex caulk—use polyurethane or hybrid exterior sealants.
• Over-raising the adjustable cap so the door binds or the sweep deforms.
• Blocking weep paths with caulk, trapping water.
• Fastening into punky wood—repair or replace damaged subfloor first.

When to call a pro

• Extensive rot in jambs or subfloor, or evidence of structural damage.
• Masonry sills with complex profiles that require custom flashing.
• Out-of-square openings where the door won’t seal even after adjustment.
• Historic doors or ADA thresholds that must meet specific clearance and accessibility constraints.

With the right threshold, proper flashing, and careful adjustments, you’ll stop drafts and keep water out for the long haul.