How To Replace A Windowsill: Home Improvement Tips

The windowsill is a relatively small piece of wood running from the inside to the outside of the window. This piece of décor isn’t impervious to the elements or from wear and tear. Hence, you may need to replace it over time.

Continue reading to know the steps necessary to replace a windowsill.

window sill repair

Procure the Tools

Like many do-it-yourself (DIY) projects, you can make the task of replacing the windowsill easier with the correct tools.

Some of the pieces of equipment you need are listed below:

  • Hammer
  • Tape measure
  • Flat pry bar
  • Handsaw
  • Large pliers
  • Drill
  • Shims
  • Primer and paint
  • Decking screws or galvanized nails

Aside from the items listed above, you also need to purchase lumber that matches the sill you need to replace. If you’re unsure about the material, you can bring the damaged, broken, or rotten windowsill to the store for the experts to give you a new sill that matches the old unit in thickness and width.

You can also use an online product calculator like the tool found on the Skirting Board Shop in the resource area of the website. Use this app to help you measure the total area required for the new windowsill and for other DIY projects you may have in mind.

Remove the Sill

Now that you have your materials, it’s time to remove the old windowsill.

Start this process by breaking the caulk (sealant) surrounding the sill. Use a hammer and utility knife for this step. If you need extra help, you can use a chisel or pry bar on the frame to break the sealant. But be careful when removing the sill so as not to damage the other areas of the window.

If you’re not careful, you might have to do more than replacing the windowsill. Exert extra force without practicing caution, and you may have to repair the window glass as well.

After removing the old sill, don’t forget to use a rag and utility knife to scrub the remaining debris and caulking from the window. Use a solvent and sandpaper to clean some difficult-to-remove portions of the sealant.

Measure and Cut the New Sill

When buying lumber for the new windowsill, a good rule of thumb to follow is to buy wood with 10% more dimensions than the old unit. Having an excess is better than having a lack of material in this regard, as the last thing you’d want is to waste money buying an incorrectly sized piece of wood.

Use your measuring tapes, rulers, and saws to measure and cut the excess lumber from the new windowsill. Note that you should create a similar slope to the new windowsill if the old unit has a sloped design.


Install the Sill

After cutting the new windowsill with the correct dimensions, you can do a test fit by sliding the piece of lumber into the window frame. Take the time to check each location of the fitting. Ensure that the new windowsill stays in place even if you haven’t applied sealant yet.

If you find the fit to be more-than-satisfactory, get your drill and boreholes in the lumber. These holes will help avoid cracks from forming on the new sill.

Next, place screws on the outer part of the sill. Ensure that all screw locations are in the exact place as the old sill. Otherwise, you might need to modify the window frame or start creating a new windowsill from a new piece of lumber.

Moreover, check to see that the new screws are at least 6 inches apart from each other. This distance should secure the sill firmly in place.

Failure to secure the windowsill in place might cause problems for the long-term. For example, you might want to start a windowsill garden, but the extra weight from the plants or vases may cause the wood to bend or warp.

Finish the installation by adding a layer of caulk around the edges of the seal. Use a caulk gun to apply the sealant in place. Also, use caulk with water-prevention features to ensure that water can’t get in from your windows during heavy rains. Don’t forget to add caulk to the screw heads to hide these components from view.

Paint the Windowsill

You might need to paint the windowsill to match the home’s design. Painting the windowsill might be a necessity to maintain consistency in design. If so, you might need to buy extra items, such as primers, paints, and paintbrushes.

Start by ensuring that the caulk is dry. Clean the windowsill of any debris left behind from the installation. Coat the lumber with primer and leave it to dry. Once dried, use an appropriate paintbrush to paint the sill using long, smooth strokes. Apply a second coat if needed.


Replace an old windowsill to maintain the beauty of your home’s design. Remember to use the correct steps in replacing the sill to avoid problems like water leaks. Follow this guide, and you’ll be able to install a windowsill that will last for years. For more information on replacing windows, read our guide.