Rotten window sills sneak up on people. One day, the wood looks a little soft, and before you know it, the corner crumbles under your fingertip. The good news is you can replace a damaged sill on your own without needing heavy tools or a contractor. This guide keeps things clear and practical so you can tackle the job with confidence.
Why Rotten Window Sills Happen
Rot is usually a moisture problem. Rain sneaks in through cracks, paint chips away, and the wood stays damp until it breaks down. Catching it early keeps the damage from spreading to the frame and wall.
Here are common signs your sill is done for:
- Soft or spongy spots
- Flaking paint that reveals dark wood
- Musty smell around the window
- Visible holes or crumbling edges
If you spot any of this, it is time to replace the sill.
What You Need Before You Start
You do not need fancy equipment. Most of these tools are already in the average toolbox.
- Pry bar
- Hammer
- Wood screws
- Exterior wood
- Saw (hand saw or electric, either works)
- Exterior caulk
- Paint or wood sealant
- Sandpaper
- Measuring tape
Keep everything within reach. It makes the job smoother and reduces mistakes.
Step 1: Remove the Rotten Sill
Start by gently prying up the old sill. Work slowly so you do not damage the trim or the frame around it. If the wood is extremely soft, it may lift out in pieces. That is fine. The goal is to expose a clean, solid area to place the new sill.
Step 2: Clean and Prep the Area
Once the sill is out, sweep away all debris. Make sure no rot has spread into the surrounding frame. If you find small soft spots, sand them down and seal them with wood hardener. If the damage is large, you may need to replace a bit more wood than expected, but most small cases do not go beyond the sill.
Step 3: Measure and Cut Your New Sill
Measure twice. Cut once.
That rule saves headaches.
That rule saves headaches.
Transfer your measurements to your new piece of wood and cut it to size. Place it into the opening to check the fit. If it is slightly snug, that is perfect. You can sand the edges to smooth things out.
Step 4: Install the New Sill
Set the sill in place and secure it with screws. Do not overtighten. You want it firm and level, not warped. Once it sits cleanly, run a bead of exterior caulk along the edges to keep water out.
Step 5: Paint or Seal for Protection
Bare wood cannot withstand rain. Make sure you paint or seal the sill with a weather-resistant product. Two coats is ideal. This step is what keeps you from having to repeat this project in a few years.
Extra Tips for a Longer Lasting Window Sill
- Keep exterior paint in good shape.
- Clear debris from the sill so water does not pool
- Check for cracks every spring and fall.
- Maintain a clean seal around the window.
Small habits prevent big repairs.
Replacing a rotten window sill is one of those projects that looks harder than it is. With a little patience and the right steps, you can handle it on your own and save a good chunk of money. Plus, it feels good to fix something that protects your home.