How To Replace Rotted Window Sill: Do It Yourself

How to replace a rotted window sill? Over time, wood expands and contracts. When this happens, the wood on one side of a window sill will sag outward, making it look like an old house or fence siding.

The process feeds on itself; eventually, all the clear material that was once above the original surface is gone. This can create a large crack in your window.

If left alone, your windowsill can become detached from its frame, permanently damaging your home.

To get a window sill to fit properly, it’s important first to measure what size your windowsill is. This will tell you if you need an adjustable or fixed sill. A good rule of thumb is to buy a window sill that is one size larger than the window itself.

There are different choices when it comes to types of wood. Some are more durable than others. The best option for your home is something made out of pressure-treated wood because it is resistant to mold, mildew, and insects.

How To Replace Rotted Window Sill In 9 Easy Steps

Now that you have a clear idea of why a sill gets decayed and which replacement options are for you, let’s get into how to replace a rotted window sill all by yourself.

StepsEstimated Required Time
Remove The Side Casing10-15 minutes
Remove The Old Window Sill10-20 minutes
Clean Off The Debris5-10 minutes
Apply The Adhesive10-15 minutes
Measure The Old Window Sill10-15 minutes
Cut The New Window Sill15-30 minutes
Install The New Window Sill15-30 minutes
Secure With Caulk10-15 minutes
Install The Side Casing10-15 minutes

Step 1: Remove The Side Casing

Required Tools

  • Safety gloves.
  • Safety goggles.
  • Pry bar.
  • Utility knife.


  • The side casing needs to be removed to make room for removing the old sill.
  • Replace them as well if the side casings are decaying as well.
  • Carefully pry off each piece between the siding and the casing by cutting through the caulk bead.
  • Keep the trim so you may use it as a model for the new ones.
  • Verify that the flashing is intact and put correctly beneath the siding and casing.

Step 2: Remove The Old Window Sill

Required Tools

  • Safety gloves.
  • Safety goggles.
  • Utility knife.
  • Hammer.
  • Prybar or chisel.


  • Using a utility knife and a hammer, remove the caulk or sealant covering the window sill.
  • Apply the pry bar or chisel to the window frame gently and pull it away from the glass.
  • When it is out, put it safely somewhere so it can be used as a model for the new window sill.
  • Additionally, try to remove the wooden support holding any sloped sills up.
  • Recent homes might have beveled wood slivers that extend.
  • Eliminate any additional objects or ornamental elements that keep the sill in place.

Step 3: Clean Off The Debris

Required Tools

  • Safety gloves.
  • Safety goggles.
  • Rag.
  • Utility knife.
  • Epoxy mixture.
  • Wood filler.


  • Scrub the dirt on the window frame using the rag and the utility knife.
  • Any remaining sill or caulk should be completely cleaned using the provided solvent and sandpaper.
  • At the first sign of deterioration, remove any remaining screws or nails or pull out the wood.
  • Apply the epoxy mixture if the rot is localized to a single frame area and is not widespread.
  • The wood filler is a plastic material that fills gaps and hardens as soon as it touches wood.
  • They connect with the material and adequately seal it when they fill in any holes that are already there.

Step 4: Apply The Adhesive

Required Tools

  • Safety gloves.
  • Safety goggles.
  • Epoxy.
  • Hand drill.
  • Marine-grade waterproof glue.


  • Epoxy can be used to patch up any rotten areas on the old sill.
  • If necessary, trim the new sill’s back edge to fit it perfectly against the old wood and its bottom edge tightly against the siding.
  • Drill pilot holes through the front and back edges of the new sill approximately every 16 inches.
  • Apply a bead of marine-grade waterproof glue on the old sill.

Step 5: Measure The Old Window Sill

Required Tools

  • Safety gloves.
  • Safety goggles.
  • Pencil.
  • Ruler.


  • To determine whether you require a 2-by-4 wooden board or anything larger, measure the size of the window sill.
  • Don’t forget to make the board wider than the area it takes up.
  • Put the old sill on the new board and draw the outline using a carpenter’s pencil.
  • Measure the sill’s size if it is distorted and damaged.
  • It is usually a good idea to purchase an extra board in case one is insufficient for the job.

Step 6: Cut The New Window Sill

Required Tools

  • Safety gloves.
  • Safety goggles.
  • Jigsaw.


  • Use your jigsaw to cut the board to the new window sill’s shape.
  • Cut it in a bit larger size because you can trim it later.
  • Making the cut larger than the previous size will ensure that the area is filled, preventing water leakage.
  • Create a slope for your new window sill if your old one had one.
  • To create the right angle for the sill, you can use the slope of the original side casing.

Step 7: Install The New Window Sill

Required Tools

  • Safety gloves.
  • Safety goggles.
  • Hand drill.
  • Screws.
  • Caulk gun.


  • Before putting it in place, look at the new sill to see if any other changes are required.
  • Place the sill over the adhesive tightly.
  • Grab your drill and carefully drill holes so it doesn’t cause the new window sill to crack.
  • Make sure the screws are positioned exactly where the old sill was on the outside section.
  • When the installation is finally finished, cover the screw heads with caulk.

Step 8: Secure With Caulk

Required Tools

  • Safety gloves.
  • Safety goggles.
  • Caulk gun.


  • Using a caulk gun, caulk the window sill’s edges completely.
  • The best caulk to use outside is a waterproof one.
  • Before installing the casing, give the caulk about 24 hours to settle.

Step 9: Install The Side Casing

Required Tools

  • Safety gloves.
  • Safety goggles.
  • 8d stainless steel ring-shank nails.
  • PVC trim boards.


  • Place the side casing in place with nails after applying a bead of caulk next to the siding’s ends.
  • You can use 8d stainless steel ring-shank nails and cellular PVC trim boards.
  • Place the nailheads just below the surface and apply a dab of the two-part adhesive to each one.
  • Sand the dabs level once they have hardened.

That’s it. You have successfully replaced your rotten window sill, and your window now looks like new.

Now the question may arise: why does someone need to replace their rotten sill? Well, take a look at what happens when you don’t get rid of one.

What If I Don’t Replace My Rotten Exterior Window Sill?

A rotten exterior window sill has significant impacts if you don’t replace them in time. Let’s learn about them.

  • It is possible for moisture and condensation to keep spreading and damage the wall.
  • Mold and mildew can gather in the home and cause health issues.
  • Extreme dampness may have an impact on the wall’s electrical system.
  • Cosmetic items with decay, such as your trim or sill, may encourage further rot.
  • Your window frame may be impacted by this, requiring a full replacement in the future.
  • Additionally, adjacent bricks and shingles may become moldy.

Bottom Line

So there you have it! How to replace a rotted window sill. Hopefully, this article has given you some new ideas for replacing your rotten window sill.

Replacing a rotten window sill is a great way to revive any home. It’s also an easy DIY project, which means you can do it yourself and save money. All you need is some wood, screws and nails, a drill, and a hammer.

Happy Decoring!


How much is a sill?

A window sill installation costs $1.86 per sill, with prices ranging from $1.49 to $2.24. Each sill costs $74.25 in labor and materials, which ranges from $57.80 to $90.70. Costs for a typical 6-sill project range from $346.81 to $544.21, or $445.51 on average.

Is it easy to replace a window sill?

Old window sills can be easily repaired or replaced with simple equipment and only a few hours of labor.