How to Replace Rotted Wood Around a Window?

If you live in a rainy, wet climate with plenty of precipitation, then the wood around your windows is at risk of rotting. It’s really as simple as that.

While newer houses often take advantage of rot-proof window materials like vinyl or aluminum, many older houses have window frames made from regular old hardwood. Windows tend to be areas where moisture congregates, so unless your wooden windows get consistent preventative maintenance – they’re likely to suffer from water damage.

One of the tricky things about rotted window sills is that they often appear to be in good shape. Water can work its way into the frame or sill and create rot without much visual damage.

The best way to check if the wood is rotten is by pressing into it with a screwdriver. If it has a spongy texture – you’ve got yourself wood rot. The most common spots for this to happen are in the lower corners of window frames – particularly on windows that aren’t protected by some type of overhang.

Another good indicator of rotted wood is chipped and missing paint on the sill. Wood window frames are subject to rain, snow, and condensation, and the only thing protecting them is their paint/primer.

Repairing and Replacing Rotting Wood around Windows – Guide

So now that you’ve found some dry rot around your window, how do you replace the rotted wood?

There are several options for repairing rotted wood around windows. If the damage is relatively small – less than a few inches of rot – then you can patch it effectively with a wood filler epoxy.

If the rot is more severe (more than about 10% of the overall length), then you’ll need to remove the rotted piece and replace it with a wooden insert. This is a more labor-intensive repair but will result in a more durable and longer-lasting repair.

If the wood rot is really bad, then you may want to shell out the dough for a full window replacement. Of course, this will cost a lot more than a DIY repair, but in severe cases, it can be the only viable option.

Let’s take a look at the first two repair methods in greater detail.

Method 1: Epoxy Wood Filler

If your wood rot isn’t that extensive, then patching it with wood filler is an excellent option. This method involves removing all of the rotted material, cleaning out the gap, and then filling it with a wood filler epoxy.

For smaller rot problems, wood filler works surprisingly well. It will blend in with the existing wood, and after it’s sanded and painted, you won’t be able to spot the repair from the original wood.

Tools & Materials

Step 1: Assess the Level of Wood Rot

First up, you’ll want to get down and dirty by checking the extent of the water damage. Use a screwdriver or putty knife to press into the wood and remove as much as possible with your hands.

Once you’ve removed the soft, crumbly rotted wood, you should have a good idea of the level of damage. You’ll be able to see how deep the rot has permeated, and whether a patch job will get the job done.

Step 2: Clean out Gap

Next, use your hammer and construction chisel to clean up the surrounding wood. If you have an oscillating multitool with a saw attachment, you’ll be able to slice through wood trim rapidly and accurately.

removing wood rot with chisel
Removing rotted wood with a hammer and chisel.

Once the gap is cleaned up, and only solid dry wood remains, you’ll want to clean out any wood chips, caulking or debris left inside the gap.

Step 3: Prepare & Apply Epoxy Wood Filler

Now it’s time to mix up a batch of wood filler to fill up the gap. There are a number of specialized wood fillers that work well for this task, but you can also use regular Bondo for wood repair with great effectiveness.

Protective gloves and a facemask are a good idea when mixing these products, as they often give off harsh fumes, plus you want to avoid getting the stuff on your skin.

Follow the instructions on the back of the package when mixing the 2-part epoxy. Once you’ve blended the two parts thoroughly, use a putty knife to scoop and stuff the filler into the gap.

wood filler inside gap in window frame

Try to shape the filler as flush with the original wood as possible. Once the wood filler has dried, it’ll be much more of a pain to sand to your desired shape.

Depending on the depth of the repair, you may need to apply the filler in several thinner coats. For thicker repairs, driving several all-purpose construction screws into the gap will give the epoxy something to ‘grip’ onto.

Step 4: Sand and Paint

Follow the instructions that come with the wood filler and let the epoxy cure completely. Then use a bit of sandpaper to knock it down until it’s flush with the original piece of the window frame.

Then wipe it clean, caulk any seams, and prepare to apply a few coats of quality exterior paint with primer. Quality wood filler won’t absorb moisture and will keep water out of your repair – so the painting is merely for cosmetic purposes.

If you haven’t painted your wooden window frames in a while, now’s a good time to paint the entire frame. This will give you a nice even look, so the repair won’t be visible to the casual observer.

Method 2: Create a Wood Insert

The second method is to replace the rotted wood around your window. This method is more labor-intensive and will require you to do a little basic carpentry.

When wood sills or trim is significantly rotted, patching it with wood filler won’t provide a long-lasting solution. Other than replacing the entire window frame, the only alternative is to remove the rotted wood around the window and replace it with a custom-made insert.

As it’s a more involved repair, you’ll need a few additional supplies.

Tools & Materials

Step 1: Remove Rotted Trim

Once you’ve decided to replace the rotted wood around your window, it’s time to get to work.

prying rotted wood from window frame
A prybar will make quick work of removing rotted wood.

Use a prybar and hammer/chisel to extract the rotted piece free from the rest of the window frame. If you can get the prybar behind the rotted wood, then removing it all in one piece should be doable. If the rot is severe, then you may need to remove it in several pieces with the hammer and chisel.

Step 2: Clean up Gap

Once you’ve removed the rotted wood, clean up any wood chips or debris left inside the gap. Pull out any remaining nails, and other loose material.

cleaning up rotted wood

Also, take some time to inspect the condition of the wood and house wrap underneath. If it’s in poor shape, you may want to consider installing some window flashing tape to prevent future water damage.

Step 3: Measure the Piece(s) you removed

If the rotted piece you removed is still in one section, then use your measuring tape to record its exact dimensions. If the piece deteriorates then you’ll have to measure the gap and remaining intact pieces to get the right dimensions.

Also, pay attention to any special features – like mitered corners – that will need to be recreated later on.

If you’re lucky, the measurements of the trim you’ll need will be a standard dimensional lumber piece. This will make cutting it to size easier, as you’ll only need to make crosscuts to get the right size. In my case, the piece required was 1 ¾” thick, so I had to rip the right thickness from a 2 x 4.

Step 4: Cut your Lumber to Size

Use the measurements you recorded earlier to cut your piece of lumber to size. If you have to make rip cuts, then a table saw will make the task much easier, but it can also be done with a circular saw using a rip guide.

cutting lumber for window trim
Ripping cutting the replacement piece.
A word about wood:
Wood for window exteriors needs to be highly durable, naturally rot-resistant and must hold paint well. Some of the best options are Douglass fir and cedar. Both are tough and rot-resistant, with cedar being softer, lighter, and easier to work with than Douglass Fir.

If you’re open to using materials other than wood, you can also get a replacement piece made from PVC. PVC is essentially indestructible and will never rot, but doesn’t have the same look as wood.

Step 5: Check Fit

Next up, cut your lumber to size, including any 45-degree mitered cuts. Try to cut as cleanly and accurately as possible, as you need the fit to be precise. A saw guide or speed square can be helpful for this.

checking fit for replacement wood piece
Checking the fit.

After you’ve made the necessary cuts, you can take the piece and see how it fits in the gap around the window. It should fit snuggly, but not so tight that it’s impossible to fit in place.

Step 6: Paint & Primer

Next up, you’ll want to coat the entire surface of the new piece in a few coats of primer and paint.

replacement pieces after paint and primer

Make sure to paint the entire surface, including any surfaces not directly exposed to the elements. This will protect the wood from absorbing moisture and rotting – no matter where the water is coming from.

Step 7: Fasten the New Piece(s)

Place the new piece into the gap and tap it into place with a soft rubber mallet. Once it’s in position, drive a few exterior-grade construction screws through the new piece into the framing members on the opposite side.

fastening replacement wood trim
Fastening the replacement trim.

You can also use larger galvanized nails for fastening. If the replacement piece is long, then you’ll want to fasten additional nails/screws every foot or so.

Once you’ve completed this, use a little wood filler/putty to fill in the holes left by the nails or screws.

Step 8: Caulk and Paint

Last but certainly not least is caulking and painting your repair. Use a high-quality exterior-grade caulk and caulking gun to seal off all the seams around your wood insert. Make sure to caulk both the interior and exterior seams.

After the caulk has had a chance to cure completely, paint the new wood so it matches the rest of the window frame. If you can, paint the entire window frame at this point – it will make your repair blend in with the existing wood frame.

painting the window frame

And that’s about it! Hopefully, you’ve got a better idea about how to replace rotted wood around a window.

Denis Gardner

I've loved tinkering and fixing things for as long as I can remember. So, naturally, I gravitated towards DIY and home improvement when I bought my first home. Nowadays you can find me writing about my passions or messing around with my newest tool!

5 thoughts on “How to Replace Rotted Wood Around a Window?”

  1. Excellent step by step description. You provided what could be done in different degrees of rotted window frames. Your suggestions for types of wood replacement and the actual progress of completing the word was excellent too. When I hire someone to do this work, I know exactly what to expect. Thank you so very much.

  2. Awesome! It’s a genuinely awesome article, I have got a much clear idea regarding from this piece of writing. Very nice and helpful information has been given in this article. I like the way you explain things. Keep posting Thanks.

  3. Can I replace exterior bottom of rotten window sill if interior bottom window sill is not damaged. I was going to remove and replace with cedar, caulk, prime and paint. Is this possible?


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