What you need to do before you start

An exterior window sill is the horizontal piece at the bottom of the window frame that sheds water away from the wall. When it cracks, rots, or pulls away from the frame, water gets behind it and damages the wall and interior framing. Replacing it means removing the old sill, checking the frame underneath for rot, and installing a new one that slopes slightly outward so rain runs off instead of pooling.

Before you buy materials or tools, inspect what you have. Look at the sill from outside and inside. If the wood is soft when you press it with a screwdriver, or if you see dark stains or mold on the wall below, the damage goes deeper than the sill itself. You may need to replace part of the frame or the wall sheathing underneath. That work is more involved and may require a contractor, especially if the rot has spread into the header or jambs.

If the sill is the only damaged piece and the frame around it is solid, you can do this job yourself with basic carpentry tools. The work takes a full day for one window, longer if you are learning as you go.

Key Takeaways

  • Remove the old sill by cutting it into sections with a reciprocating saw, then prying out the pieces and the fasteners holding them.
  • Inspect the frame and sill pan underneath for rot; if wood is soft or discolored, that area must be repaired before the new sill goes in.
  • Cut the new sill from pressure-treated lumber or composite material, slope it about 1/8 inch per foot toward the outside, and fasten it with corrosion-resistant screws or nails.
  • Seal the joint between the new sill and the frame with exterior caulk rated for wood movement, and paint or stain the sill before water gets to it.

Removing the old sill without damaging the frame

Start by removing any caulk or sealant around the sill with a utility knife or caulk removal tool. This takes time but prevents you from tearing the frame when you pry. Once the caulk is gone, use a reciprocating saw with a metal-cutting blade to cut the sill into two or three sections. Make cuts near the ends and in the middle, but stay clear of the jambs (the vertical sides of the frame). Cut only the sill itself, not the frame underneath.

Pry out each section with a flat pry bar, working from underneath if you can. The sill is usually fastened with nails or screws driven through the frame into the sill, so you may feel resistance. Once the sill is out, remove any remaining fasteners and scrape away old caulk, paint, or debris from the sill pan and frame. The sill pan is the metal or membrane that sits under the sill and directs water to the outside. Do not damage it during cleanup.

Checking the frame and sill pan for hidden damage

With the old sill out, look closely at the sill pan, the frame members underneath, and the sides of the opening. Press the wood with a screwdriver tip. Solid wood will resist; rotted wood will feel spongy or the tip will sink in. Check the corners and the joint where the sill pan meets the jambs, because water collects there.

If you find soft wood, you have two choices. Small areas of rot (less than 1 inch deep and smaller than a few square inches) can be cut out and filled with epoxy wood filler, which hardens and can be shaped. Larger rot means replacing that section of frame, which usually requires removing the window unit itself and is beyond a sill-only job. If you are unsure, take a photo and show it to a carpenter or contractor before you continue.

If the sill pan is damaged, dented, or has holes, it should be replaced. This is more complex because the pan is often sealed to the frame and sheathing. If the pan is intact, clean it and leave it in place; it will support the new sill.

Measuring and cutting the new sill

Measure the length of the opening from the inside of one jamb to the inside of the other. Add 1 to 1.5 inches so the new sill extends slightly into the jambs on both sides. This overlap helps shed water and anchors the sill. Measure the depth from the inside of the frame to the outside edge of the wall sheathing, then add about 1/4 inch for the slope.

Cut the new sill from pressure-treated lumber (2 by 4 or 2 by 6, depending on the depth you need) or composite material like Azek or Trex. Pressure-treated wood resists rot and is the standard choice. Composite materials do not rot and do not need paint, but they cost more and are heavier to handle. Cut the sill to length and width, then use a table saw or hand plane to cut a slope on the top surface. The slope should drop about 1/8 inch per foot of depth, so water runs toward the outside edge. Round the outside edge slightly with sandpaper or a router so water sheds cleanly instead of running back underneath.

Installing the new sill and sealing the joints

Set the new sill in place and check that it sits flat and level along its length. Shim underneath with cedar shims if needed. The sill should slope toward the outside; use a level to confirm the slope is even. Once it is positioned correctly, fasten it with corrosion-resistant fasteners. Use either stainless steel or hot-dipped galvanized screws (2.5 to 3 inches long) or ring-shank nails. Drive fasteners through the sill into the frame at 12-inch intervals, at least two on each side. Avoid fastening directly into the sill pan if you can; if you must, seal the hole afterward with caulk.

Once the sill is fastened, caulk the joints where it meets the jambs and the frame. Use exterior-grade caulk rated for wood movement, such as polyurethane or acrylic latex. Do not use silicone; it does not hold paint. Run a continuous bead along both sides and the back edge where the sill meets the frame. Smooth it with a wet finger or caulk tool. Let the caulk cure per the manufacturer's instructions before painting.

Finishing the sill so it lasts

Paint or stain the new sill within a few weeks of installation. Bare wood absorbs water and will begin to gray and check. If you used pressure-treated lumber, let it weather for a few months before staining, or use a stain rated for pressure-treated wood. If you used composite material, follow the manufacturer's finishing instructions; many composites do not need paint but benefit from a UV-protective coating.

explore at least two coats of exterior paint or stain, sanding lightly between coats. Pay special attention to the end grain at the sides and the slope on top, where water is most likely to soak in. A good finish is the difference between a sill that lasts 20 years and one that rots in five.

Common mistakes that lead to sill failure

The most common mistake is not sloping the sill enough or sloping it the wrong way. A flat sill or one that slopes toward the house will collect water and rot quickly. Check your slope with a level before you fasten it down. Another mistake is using fasteners that rust, which stains the sill and eventually lets water in around the fastener holes. Use stainless steel or galvanized fasteners only.

Skipping the caulk or using the wrong caulk is also common. Gaps between the sill and frame are highways for water. Caulk those joints and keep them sealed. Finally, do not paint over bare wood or skip the finish. The sill is exposed to sun and rain year-round; it needs protection from the start.

Frequently Asked Questions

Can I replace just the sill without removing the window?

Yes. The sill sits below the window frame and can be removed and replaced while the window stays in place. You do need to remove the interior trim and any caulk around the sill, and you need access from outside to pry it out. If the window is on a second story or difficult to reach, you may want a contractor to handle it.

What material lasts longest for an exterior sill?

Composite materials like Azek or Trex do not rot and last 30+ years with minimal maintenance. Pressure-treated lumber lasts 15 to 20 years if painted and maintained well. Cedar or other untreated wood rots much faster and is not recommended for exterior sills. Composite costs two to three times more than pressure-treated wood but requires no painting.

How long does a new sill take to install?

One window takes a full day if you are removing the old sill, checking for rot, cutting and fitting the new one, and finishing it. If the old sill comes out easily and there is no hidden damage, you may finish in six to eight hours. Caulk and paint add time and must cure before the window is fully weatherproof.

What if I find rot in the frame under the sill?

Small areas can be cut out and filled with epoxy wood filler. Larger rot means replacing that section of frame, which usually requires removing the window unit. This is a bigger job and may need a contractor. Take a photo and show it to a professional before deciding whether to proceed on your own.

Do I need to remove the window to replace the sill?

Not usually. The sill sits below the window frame and can be removed from outside without disturbing the window. You do need to remove interior trim and caulk, and you need good access from outside. If the window is difficult to reach or the sill is heavily fastened, removing the window may be faster, but it is not required.