Can you repair rotten window sills?

Prolonged exposure to the rain and sun makes wooden window sills prone to easily getting damaged. Because of this, you need to regularly maintain your home’s window sills to ensure they remain functional for an extended duration. If not, water might eventually find its way inside, and after a while, this leads to rotting, which can be a challenge to repair.

Fortunately, all hope isn’t lost. There are steps you can follow to repair the rotten window sills. All you need to do is get some new sills from a reputable vendor like the Skirting Board Shop, some tools, and some safety gear. If you don’t know where to start, here’s an exhaustive guide on how to repair your window sills: 

  • Take out the side casings 

Before you can get to the window sills, you’ll need first to remove the casings. You need to do this by cutting through the caulk bead positioned between the casing. Once you’re done, go ahead and remove the casing gently, ideally using a utility knife to ensure wood chips don’t get stuck on the sliding. You should also ensure the trim remains in excellent condition by partially taking out every nail and pulling it out. Doing this is important as it ensures the trim remains intact so it can be reused.

  • Pry off the old window sill 

Once the window is open, you can now easily inspect the sill. If the sill’s edges are against the window frame, this allows you to remove all of it as one unit. You can do this by taking out the pry bar below the nails, fastening it in place, and then making the nail pop by pushing up the board. Then, extract the board and remove the remaining nails the exact way you did the window sill. Next, pry off the window sill section that expands beyond the window siding.

  • Apply adhesive

The next thing to do is getting epoxy to repair the window sill sections that are rotten. You should trim the edge of this new sill to ensure its back edge perfectly fits against the old wood, and its lower edge firmly fits in place against the siding. You should then create holes after every 16 inches or so across the back edges and the new window sill’s front side.

After doing this, go ahead and apply a marine-grade and water-resistant adhesive on the sill. But before you even spread this epoxy agent using a paintbrush on all the destroyed areas, start by mixing it with a hardener as stated in the manufacturer’s manual. In most cases, the ideal mixing ratio is one-to-one. After mixing these two components, spread them with a knife across the sheet of clear, firm plastic.

  • Fix the new window sill 

Using 3½–inch deck screws, attach the new window sill onto the adhesive while passing through the plastic and through the wood. Once the head is around ¼–inch under the window sill’s surface, stop and wipe away and excessive adhesive oozing out. Get adhesive to fill the gaps between the new and old sill.

  • Cover up the screw heads     

To ensure the window’s aesthetic appeal isn’t ruined by the presence of screw heads, you need to cover them up. A great way you can achieve this is by using a drop of white, two-part acrylic adhesive. Once the adhesive has hardened completely, which takes around 30 minutes, go ahead sand it properly.

  • Install side casing 

This is done by pressing the caulk next to the siding’s end and then nailing the new casing. You need to do this using an 8d stainless-steel ring-shank nails and cellular PVC trim. As you do this, confirm that the nail heads are marginally underneath the surface. After that, cover all the nail heads by placing a two-part adhesive over them.

You should also confirm that the crew holes are filed with two-part epoxy wood filler. After this filler hardens, get 120-grit sandpaper and sand it thoroughly. Using acrylic latex caulk, fill the new sill’s edges and go ahead and change the trim, seal the nail holes. Applying an exterior wood primer is also necessary and once it dries, paint the window trim and sill with external trim paint.


Extreme weather is the enemy of wood sills and, in most cases, leads to its interior starting to rot. This is a challenge many homeowners struggle dealing with, and dealing with such issues can be quite frustrating. If you didn’t know what needs to be done to repair the rotten wooden sills, this guide has provided you with the steps you need to follow. With this in mind, you can now effortlessly go ahead and replace your window sills and perhaps consider PVC sills for the future to prevent rotten sills altogether!

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