What a door sill is and why it fails

A door sill is the threshold at the bottom of a doorway — the horizontal piece that sits between the floor and the door frame. It takes the weight of foot traffic, absorbs water from rain and snow, and bears the full swing of the door. Over time, sills rot, crack, or wear down enough that they no longer seal properly or support the door.

Wood sills fail fastest because they sit in a damp zone where water pools and wood absorbs moisture. Concrete sills crack from freeze-thaw cycles or settling. Metal sills bend or corrode. A failing sill lets water into the wall cavity, drafts into the house, and sometimes causes the door to drag or stick.

Replacing a sill is a straightforward carpentry job that takes a few hours and costs between $100 and $400 in materials, depending on the door size and sill material. You will need basic tools: a pry bar, circular saw or handsaw, drill, screwdriver, and caulk gun. If the wall framing underneath is soft or rotted, call a carpenter — that work is beyond a first replacement.

Key Takeaways

  • A door sill is the threshold at the bottom of the doorway and fails when it rots, cracks, or warps enough to let water in or cause the door to stick.
  • Removing the old sill means prying it free from the frame, cutting it into pieces if it is stuck, and scraping away old caulk and debris.
  • The new sill must be cut to fit the exact width of the opening and sit flush with the floor on both sides.
  • find the sill with screws or nails driven through the frame into the sill, then seal all edges with caulk to prevent water from entering the wall.
  • If the wood underneath the old sill is soft, dark, or crumbles when you press it, the framing is rotted and you should hire a professional.

Removing the old sill

Start by removing the door and door frame if possible. If the frame is nailed in place and difficult to move, you can work around it — just be careful not to damage the frame while prying out the sill. Use a pry bar to lift one end of the sill and work it free from the frame. If the sill is stuck fast, cut it into two or three pieces with a circular saw or handsaw, then pry out each piece separately.

Once the sill is out, scrape away all old caulk, adhesive, and debris from the frame and the floor underneath. Use a paint scraper or putty knife and take your time — any old material left behind will prevent the new sill from sitting flush. Wipe the area clean with a damp cloth and let it dry completely before you measure for the new sill.

Inspect the wood framing where the sill sat. Press your finger into it — if it feels soft, crumbles, or is dark and wet-looking, the framing is rotted. Stop here and call a carpenter. Proceeding without fixing the rot will cause the new sill to fail quickly and the rot to spread into the wall.

Measuring and cutting the new sill

Measure the width of the opening from the inside edge of one side of the frame to the inside edge of the other side. Measure in three places — top, middle, and bottom — because the opening may not be perfectly square. Use the largest measurement to cut your new sill, so it will fit snugly.

A door sill can be made of pressure-treated wood, composite material, or aluminum. Pressure-treated wood is the cheapest and easiest to work with if you are comfortable cutting and fastening wood. Composite sills (made of wood fibers and plastic) resist rot better and last longer, but cost more and are heavier to handle. Aluminum sills are durable but require different fastening methods.

Cut the sill to length using a circular saw or handsaw. If you are using pressure-treated wood, wear a dust mask — the chemicals in the wood are irritating when inhaled. Sand the cut edges smooth so they do not splinter. If the sill has a sloped top surface to shed water, make sure that slope faces outward.

Setting the new sill in place

Dry-fit the sill first — set it in the opening without fasteners and check that it sits flush with the floor on both sides and does not rock. If it rocks, the floor or frame is uneven. Shim under the sill with thin wood shims or composite shims until it is level and stable. Do not use metal shims under a wood sill, because they can trap moisture.

Once the sill is level, mark the fastener holes. Most sills are fastened through the side of the frame into the sill with 2.5-inch screws or galvanized nails, spaced about 12 inches apart. Drill pilot holes first to prevent the wood from splitting. Use a drill bit slightly smaller than the screw diameter.

Drive the screws or nails through the frame into the sill. If you are fastening into pressure-treated wood, use stainless steel or galvanized fasteners — regular steel will rust and stain the wood. Tighten the fasteners until the sill is snug but do not over-tighten, which can crack the sill or pull the frame out of square.

Sealing the sill to prevent water damage

Water sealing is the most important step. Any gap between the sill and the frame or floor will let water into the wall cavity, where it causes rot and mold. Use a polyurethane or silicone caulk rated for exterior use — these remain flexible as the wood expands and contracts with temperature and humidity changes.

explore caulk along the top edge where the sill meets the frame on both sides, and along the bottom edge where the sill meets the floor. Use a caulk gun and hold the nozzle at a 45-degree angle to the joint. Smooth the caulk with a wet finger or caulk tool so it fills the gap completely and sits flush with both surfaces.

Let the caulk cure for the time recommended on the tube — usually 24 hours — before you reinstall the door or expose the sill to water. Once cured, paint or stain the sill to match the rest of the door frame and protect the wood from UV damage.

Reinstalling the door

Once the caulk is fully cured, reinstall the door and frame. Check that the door closes smoothly and does not drag on the new sill. If it does drag, the sill may be slightly too high — you can sand down the top surface a little, but be careful not to remove so much that you expose fasteners or create a low spot that pools water.

Check the door sweep or weatherstripping at the bottom of the door. It should sit on the sill with light pressure — not so tight that it drags, but not so loose that you see daylight underneath. If the old sweep is worn, replace it at the same time you replace the sill.

When to call a professional

Call a carpenter if the wood framing under the old sill is soft, rotted, or crumbles when you press it. Rotted framing must be cut out and replaced with new wood before the sill goes in, and that work requires knowledge of how to maintain the structural integrity of the frame.

Also call a professional if the door frame itself is damaged, warped, or out of square by more than a quarter inch. A sill can only do its job if it sits on a solid, level frame. If the frame needs repair, that should be done before the sill is installed.

If you are uncomfortable using power tools or working at heights (for exterior doors on a raised foundation), hiring a carpenter is the safer choice. The labor cost is usually $200 to $400, and the job will be done correctly the first time.

Frequently Asked Questions

Can I replace a sill without removing the door?

Yes. You can work around the door and frame if they are difficult to remove. Just be very careful with the pry bar so you do not damage the frame or door jambs. It takes longer and is more awkward, but it is possible.

What is the difference between a sill and a threshold?

A sill is the structural piece that sits in the frame and supports the door. A threshold is a separate piece that sits on top of the sill to seal the gap between the sill and the floor. Some doors have both; some have only a sill. If your door has a separate threshold, you may be able to replace just the threshold without touching the sill.

How long does a door sill last?

A wood sill lasts 10 to 20 years depending on climate and maintenance. Composite sills last 20 to 30 years. Aluminum sills can last 30 years or more. Regular caulking and repainting extend the life of any sill.

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

Not always. If the frame is nailed in place and difficult to move, you can pry out the sill while the frame stays in place. However, if the frame is also damaged or rotted, removing it makes the job easier and allows you to inspect and repair the opening properly.

What should I do if the new sill is slightly too wide for the opening?

Measure again to make sure. If the sill is truly too wide, you can rip it down with a circular saw to fit, but only if you are comfortable using the saw. Otherwise, order a new sill cut to the correct width. It is faster and safer than trying to trim a sill that is already in the frame.