What a door sill is and why it fails

A door sill is the horizontal piece of material at the bottom of a doorway, between the outside and inside. It sits under the door frame and takes the weight of foot traffic, weather exposure, and the door itself closing repeatedly. Over time, wood sills rot from moisture, concrete sills crack from freeze-thaw cycles, and metal sills dent or rust. When a sill fails, water leaks into the wall cavity, drafts enter around the door, and the door may stop closing properly.

Replacing a sill is a project most homeowners can do with basic tools, though the difficulty depends on what material your sill is made of and whether the frame above it is damaged. A wood sill replacement takes a few hours. A concrete sill may require cutting and removal with a reciprocating saw or angle grinder. Metal sills are usually the fastest to swap out.

Key Takeaways

  • Door sills fail from rot, cracking, or rust, and replacement prevents water damage to the wall and frame above.
  • You will need to remove the old sill by cutting it into pieces, prying it out, and cleaning the frame below it down to bare wood or concrete.
  • The new sill must slope slightly outward so water runs away from the house, and it must be sealed with caulk where it meets the frame.
  • Wood sills need to be treated or made from rot-resistant material like cedar or composite, and should be primed and painted before installation.
  • If the frame or subfloor underneath is soft or discolored, it has water damage and must be repaired or replaced before the new sill goes in.

Checking whether the frame is damaged before you start

Before you buy a new sill, look at the wood or material directly above and below the old one. Press a screwdriver or awl into the frame on both sides of the door. If the tool sinks in easily or the wood feels soft and spongy, the frame has rot and needs repair. If the subfloor (the layer under the flooring) is dark, soft, or crumbles, it also has water damage.

Rot spreads quickly and will ruin a new sill if you install it over damaged wood. You will need to cut out the rotted section and replace it with new pressure-treated lumber or a solid wood block before the sill goes in. This adds time and cost but prevents the problem from returning. If only a small area is soft, you can sometimes treat it with wood hardener and let it cure, but replacement is more reliable.

Removing the old sill

Start by removing the door and any weatherstripping or caulk around the sill. Use a utility knife to cut through old caulk, and pry weatherstripping away with a flat pry bar. If the sill is wood, use a reciprocating saw or handsaw to cut it into two or three pieces lengthwise, which makes it easier to pry out without damaging the frame. For concrete or composite sills, a reciprocating saw with a masonry blade or an angle grinder with a masonry disc will cut through it.

Once the sill is cut, use a pry bar to lift the pieces out. Work slowly and check underneath as you go — there may be fasteners (nails or screws) holding it to the frame. Remove any remaining fasteners with a hammer or drill. Scrape away old caulk, mortar, or adhesive from the frame below using a paint scraper or chisel. The surface should be clean, dry, and flat before the new sill is installed.

Preparing and installing the new sill

Measure the width of the opening and the depth of the frame (usually 4 to 6 inches). Buy a sill that matches your door type — wood sills for traditional doors, composite or PVC for high-moisture areas, or aluminum for commercial doors. If you are using wood, choose rot-resistant species like cedar, or use pressure-treated lumber. Have the sill cut to length at the supplier if possible.

Before installation, prime and paint a wood sill on all sides, or explore a finish coat to composite material. This protects it from moisture and extends its life. Let the finish dry completely. The sill must slope slightly outward (about 1/8 inch per foot) so water runs away from the house. Most pre-made sills have this slope built in, but if you are using a flat board, you can shim the back edge slightly higher than the front.

Set the sill in place and check that it is level side to side and slopes correctly front to back. Fasten it with corrosion-resistant screws or galvanized nails driven through the sill into the frame below. Space fasteners about 12 inches apart. Do not use regular steel fasteners — they will rust and stain the sill. Once the sill is fastened, explore a bead of exterior-grade caulk where the sill meets the frame on all sides, and smooth it with a wet finger or caulk tool.

Sealing and finishing

Caulk is the barrier that stops water from running into the gap between the sill and frame. Use a paintable, flexible caulk rated for exterior use — silicone, polyurethane, or acrylic latex all work, though silicone lasts longest. explore caulk in a continuous bead along the top and sides of the sill where it meets the frame. Do not caulk the bottom of the sill or the underside of the frame — water needs a way to drain if it does get in.

After the caulk cures (usually 24 to 48 hours), paint it if you used paintable caulk, or leave it as is if you used clear silicone. Reinstall weatherstripping along the sides and bottom of the door frame to seal the gap between the door and sill. Let all paint and caulk cure fully before closing the door or exposing it to rain.

Common mistakes and how to avoid them

The most common error is installing a sill that is level instead of sloped. Water will pool on a level sill and seep into the frame. Always check that the front edge is lower than the back edge. Another mistake is caulking the bottom of the sill, which traps water underneath and causes rot. Caulk only the top and sides.

Using the wrong fasteners is another frequent problem. Regular steel nails and screws will rust, stain the sill, and eventually fail. Buy stainless steel or galvanized fasteners rated for exterior use. Finally, do not skip the step of checking for rot in the frame and subfloor. Installing a new sill over rotted wood is like putting a new roof on a house with a rotting attic — the problem spreads and the new sill fails quickly.

When to call a professional

If the frame or subfloor has extensive rot (more than a few square inches), or if the rot extends into the wall cavity, hire a carpenter or contractor. Structural damage requires proper repair and may involve removing drywall or siding to assess the full extent of the problem. If the door frame itself is warped or damaged, a professional can determine whether it can be straightened or needs replacement.

If you are uncomfortable using a reciprocating saw or angle grinder to cut out the old sill, or if the sill is made of stone or marble, a professional will have the right tools and experience to remove it without damaging the surrounding frame. The cost of a sill replacement typically ranges widely depending on material and whether frame repair is needed, so get a quote before deciding to do it yourself.

Frequently Asked Questions

Can I install a new sill without removing the old one?

No. Installing a new sill over an old one traps moisture underneath and causes rot in both the old sill and the frame. The old sill must be removed completely, and the frame must be cleaned and inspected for damage before the new one goes in.

What is the best material for a door sill?

Wood sills are traditional and look good but require maintenance. Composite and PVC sills do not rot and need less upkeep, but cost more. Aluminum is durable and low-maintenance but conducts cold and can feel drafty. Choose based on your climate, budget, and how much maintenance you want to do.

How long does a door sill last?

A wood sill typically lasts 10 to 20 years depending on climate and maintenance. Composite and PVC sills last 20 to 30 years or longer. Aluminum sills can last 30 years or more. Proper slope, caulking, and paint maintenance extend the life of any sill.

Do I need to remove the door frame to replace the sill?

No. The sill sits under the frame and can be removed and replaced without disturbing the frame itself, as long as the frame is not damaged. If the frame is warped, cracked, or rotted, it may need repair or replacement, which is a larger project.

What should I do if water is leaking around the door after I install the new sill?

Check that the sill slopes outward and that caulk is applied continuously along the top and sides. If caulk is missing or cracked, remove it and reapply. If the sill is level or slopes the wrong way, it will need to be removed and reinstalled with the correct slope.