How to Repair the Interior Window Sill and Trim

It’s always a great time for small home improvements, and repairing window sills is actually an easy home repair project!

A window sill is the bottom portion of the window, and usually- but not always- has a lip that overhangs the trim molding and over time these can get damaged or even rot. Repairing window sills isn’t a hard DIY and I try to share simple ways to make DIY improvements to your home. For this tutorial, the same principles apply whether it is replacing an interior or exterior sill. We have been working on small projects such as this so this is republished from an older article in the hopes that we can all be inspired that the DIY doesn’t stop when the temperatures drop.

So, first, we have the crime. There’s actually 3 of these that look this bad. This damage is all from a wonderful pup we rescued from some dire circumstances. Unfortunately, he has severe separation anxiety, but it seems to have calmed down after a year of working with him. So this week, we are replacing the windowsills and repairing the window trim underneath.

Tools and Materials

• Hammer or prybar
• Utility Knife
• Nails or Brad Nailer (This is the top rated kit we use)
• A board and router or a millwork board to match the other sills in your home
• A product to stop rot (you may or may not need this but we have used this one in the past)
• Level
• Trim Caulk

How to Remove Window Framing

The first step is always the most worrisome, right? Every window should be installed into the wall framing so that the window itself does not sit on the windowsill or trim. The removal of the sill is actually quite easy.

The seams are usually covered by caulk and painted to create a smooth look so you are breaking that line of caulk. First, we ran a cutting blade (This is my favorite caulk removing tool.) around all the edges including between the underneath trim and the window sill. Then we used a hammer and banged/popped the sill upwards, being careful to avoid any further damage. We pulled out the rest of the nails that were left behind.

At this stage, the biggest thing to watch for is any rot that may be around the frame. Most rot will need to be removed, or it will continue to spread. If you do find rotted wood, replace or use a product like this to stop rotting. And, of course, make sure that you are fixing the source of the water or weather damage that caused the rot.

Creating A Custom Windowsill

We ended up replacing all four window sills along the front because we made them an inch shorter than the old ones. If you have a basic router and intend to paint the window trim, leftover wood is fine. We used leftover poplar because it is slightly harder than pine and easier to find a piece without knots.

We cut the sides a bit inwards to fit the window and dry fitted this several times. A simple run across the router to create a bullnose edge was all that was needed. If you do not have a router, most window sill material can be found in the millwork section at a home improvement store like Home Depot. Here is an example similar to ours. 

Repairing Window Trim

The window trim had some gouges and dog chewing damage as well. Although, we could have replaced the window trim pretty easily, I elected to fill it because the edge cuts on trim like this are not easy to get perfect. I have used a lot of wood filler and this is my favorite product. I thinned it out with a bit of water to get in the smaller areas. I used several thin layers to fill the holes, lightly sanding and cleaning between each layer, until I was able to fill the damage while also following the lines of the trim. For filling this type of wood, I love these sponges because they have edges that I can easily control and get into the ridges.

After the trim was filled and smoothed, I caulked around the edges of all the window trim and frame. Then I primed and painted everything to match the rest of the trim in the house.

LOOKING FOR MORE SIMPLE WAYS TO IMPROVE YOUR HOME? 

• The Easiest Way to Make Baseboards look Beautiful Again
• Make Basic Cabinets Look Expensive with Trim
• Building A Custom Closet
• How to Paint Old Kitchen Cabinets
• Kitchen Backsplash with Peel and Stick Glass Tile

Pin It for Later!

Well, now that’s over, the dining room makeover will look much more finished. It’s hard to make a room inviting, cozy, and beautiful with eye sores such as these. If you want to see our finished dining room makeover, check out our home tour tab with all of our before and after makeovers.

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