Home Tips: How To Replace A Windowsill?

Condensation, warping, sun damage and several other things can affect your windowsill, causing it to pull away from the window and exposing your home to moisture and dust. When this happens, you know it’s time to get them replaced. You don’t have to hire a professional to get the job done; you can do it yourself with the right tools and materials.

What Is A Windowsill?

A windowsill is that flat piece at the bottom of the window that extends outside the house. It forms the down part of the window frame. It has both decorative and functional purposes. Aside from being a decorative covering that provides additional support for the window, it also performs the important function of preventing water from entering into the house or walls through the window track and damaging the walls, floors, and furniture.

Without a windowsill in place, your home isn’t fully protected from dust and moisture because once it rains, water can easily get into the house through any little opening in the window.

Why Do Windowsills Rot?

Windowsills sit horizontally, and when it rains, the water usually falls right into it. Since wood is the most common material used to construct Windowsills, they’re usually affected by rot because wood isn’t rot-resistant. You should check your Windowsills regularly for signs of rot. You can do this using a screwdriver to poke the sill. If it pushes in easily, it means your sill is rotted out.

Materials And Tools Needed To Replace A Windowsill

If you want to replace your windowsill yourself, you need to prepare for the task adequately. The first step is to get all the necessary tools and materials to make your job easier. These materials aren’t hard to find; you can get them at the Skirting Board Shop. Below is a list of the materials and tools you’ll need:

  • Utility Knife
  • Table Saw
  • Circular Saw or Power
  • Large, slip joint pliers
  • Hammer
  • A Measuring Tape
  • Shim
  • Pry bar
  • Drill
  • A pair of safety glasses
  • Compound meter saw
  • Galvanized nails or screws
  • Paint

Step-By-Step Guide For Replacing A Windowsill

If you’re planning to replace your windowsill soon, here’s a step-by-step guide for you to follow:

Step 1: Take The Sill Out

After gathering your materials together, the first thing to do is to take out the sill. You do this by using your utility knife and harmer to break off the caulk around the windowsill. Free the sill from the trim board by using your knife to cut underneath the window sill. Once you’ve removed the sill from the trim, you have to pry it from the wall.

Place your utility knife in-between the sill and the wall, then pull the sill off the wall. You can also use your pry bar or chisel to complete this task. Once you’ve successfully pulled the sill from the window frame, set it aside on a safe workspace because you’ll still need it to serve as a sample for the new one you’re about to make.

Remove all the finishing nails with your plier, and take off any other decorative features that secure the sill. If your sill is the sloppy type, you’ll need to take off the wooden support that holds it up.

Step 2: Get Rid Of The Debris And Caulking

Caulks and debris prevent the sill from sitting firmly on the frame and makes it easy for moisture to enter and ruin all your efforts to dress up your windows.

Use the knife to scrub off all the debris accumulated in the window frame. The knife alone may not do a very neat job. You also need a solvent and sandpaper to thoroughly clean any leftover caulk or debris on the frame so that it doesn’t obstruct the new sill. Get rid of nails or wood that’s ridden with rot.

If you notice that the rot in the wood is only limited to a small area and it’s not widespread, you can fill up the affected spots with a wood filler instead of having to remove the whole wood.

Step 3: Measure The Windowsill

Take accurate measurements of the old sill. Use your measuring tape to measure the length and breadth of the old sill, then make a cardboard portrait of it.

Alternatively, you can place the old sill on the piece of wood you intend to use for the new one and trace the outline of the old sill on the wood. Your carpenter’s pencil should work for this.

Step 4: Cut The New Sill

Use the measurements taken to cut a new windowsill. The new sill should be a bit bigger than the former one. You can add a few inches to the measurements before you cut so that you’ll be able to make the necessary adjustments easily. It’s always better to chisel your wood to fit into the frame than having a sill that’s too small.

Step 5: Fix The Sill In Place

Once you’re done cutting the new sill, place the windowsill on the window frame to see if it fits. If it doesn’t, make the necessary arrangements. When you’re satisfied that the sill will fit, sand the edges with your sandpaper, then paint it in line with the surrounding woodwork.

Push the new sill into the jamb on each side of the window frame, once this is done, secure the sill. You do this by driving screws up into the jamb. Nail the sill into the framing below, make sure that you place the nails every 8 inches along the back of the sill. Caulk the joints around the edges to seal cracks. Let it settle in for 24 hours then fix the trim.


By following the simple steps above, you’ll successfully replace your windowsill all by yourself!


Tags: home, window, windowsill

Author: Maja Markovski

Maja Markovski

A 35-year-old female architect with a passion for innovative, sustainable design. I blend creativity and functionality to transform spaces into beautiful, practical environments.


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