Rotten wooden window frames

Rotten wooden window frames
Rotten wooden window frames

Rotten wooden window frames are extremely common in older homes. Rot can develop in wooden frames for a lot of reasons. We’ll explore the most common reason for rot in just a moment. Rot is caused by a type of fungus. When any wood is left exposed to the elements either without paint or the proper treatment, rot can form, and it basically eats the wood.

However, there are ways of treating the rot in your wooden windows. In fact, there are even simple DIY methods you could use to solve issues with rotten windows, as long as you only have very minor rot in your frames. Let’s take a look at rot and how to spot it in your frames and then explore what you can do to solve it, shall we?

Is my wooden window frame rotten?

If you aren’t sure if your wooden window frame is rotten, there are a few clues. Firstly, you may be able to see some discoloured paint or paint that is peeling away. This doesn’t mean that you have rot, but these are good areas for the rot to get into the wood of the frame, so they are an ideal place to check for rot.

example of rotten windows timber frame
rotted wood – window frame

Take a close look and have a feel of the timber. If you notice any crumbling, swelling paint or softwood, there is a good chance there is rot.

You see, most wooden windows in the UK are made of dense hardwood, so there shouldn’t be any softness or brittleness in healthy wood. You should check common problem areas too. The bottom corners of the window frame, the sill and the lower strip along the bottom, even if the paint looks to be intact.

rot under crumbling and swelling paint
rot under paint
wet rot on the window frame
rot on windows frame joints
Rot on window frame joints after the window was previously spliced repaired

Take your time and give the timber a really good going over. You can even use a chisel to test suspect areas. If you notice any wood that isn’t solid, this is a sign of rot.

Why does rot happen?

Rot occurs due to exposure to water. Rot absolutely loves damp conditions, and so the longer the wood is exposed to the severe weather, the bigger risk of a big problem.

If your wooden windows have been in your home for hundreds of years, they can also be very dry. So, once they are exposed to water (via cracked or peeling paint), they will try and do what wood does, soak up the water, which will create rot.

wet rotten window
wet rot on the window frame
rotted wood: window frame
rotted wood: window frame

Moisture content in timber window frames 

moisture content in timber window frame

Every piece of timber has a moisture content, which fluctuates depending on the humidity and the seasons. Even the wood in your home is affected by humidity.

In England, exterior wood, including timber frame windows, sits at between 9% and 14% moisture content most of the year as long as it’s protected from moisture getting in.

If the wood isn’t protected, the moisture content can rise. If it rises to over 20%, this creates the perfect conditions for dry rot to grow.

Too much moisture content in wood can also create warping and cracking as the timber dries out. So, for wooden window frames and other exterior wood, it must stay within 9% and 14% moisture content.

40-60% moisture content in timber window frames creates ideal conditions for wet rot to develop and grow into a serious problem.

Wood Moisture Meter

If you aren’t sure whether your window frames are dry enough, you can buy a wood moisture meter to test them.

A moisture meter is perfect if you are planning on working on your windows when it is particularly humid. If it stormed the day before, for example. There may still be a lot of moisture in the air, and if you strip the paint off of your windows, you may introduce moisture into the wood, which can lead to rot and other issues.

If the moisture level in the wood is too high when you paint your windows, it can also lead to bubbling in the paint.

moisture meter for window frames
bubbling paint on windows

If the moisture content of the frames is over 17%, this can ruin the paintwork as the water tries to escape the frame. So, a wood moisture meter is really handy when doing anything with wood, but particularly when you are trying to protect it from moisture. Knowing that the wood is stable before you start is a must.

Windows are at a big risk of getting rot, as the fungus preys on the natural character of the wood. Never fear, though; there are some solutions to rot in wood.

Fixing rotten window frames: What solutions are available?

There are plenty of solutions to rotting wood in window frames.

The key is to identify which method of treating wooden window rot is right for you. This comes down to budget, time and expectation. As well as the maintenance cycle you would like to achieve (some methods require more care than others).

Some of these rot treatment methods work fantastically well on frames with a small amount of rot present. While others are better suited to windows that are extremely rotten. Some methods are much cheaper than others but will not protect your timber windows from rot for a long time.

Some methods also require an extensive range of tooling and joinery skills while others can be performed by savvy DIYers.

repair rotten window frames

Solutions for rot repair on wooden windows

Wood treatments

There are certain wood treatments available that work very well on rotting wood. These treatments work by killing the fungus and penetrating into the wood and making it solid again. It cannot replace any wood that is very far gone, but you can use a good quality wood filler to fill any gaps.

Reverse the wet rot 

There’s a good chance of reversing the wet rot in the timber. This requires a special wood treatment that is available at most DIY stores and online. These treatments work fantastically on wet rot, as long as the source of water has been found and fixed.

A pro tip to use with this wet rot treatment is to treat all of the timber, not just the areas that you can see are affected by the rot. That way, you can be sure that the rot has been reversed, and you won’t run into trouble moving forward.

wood treatment solution

Repair rotten frames with Epoxy wood fillers

There are also epoxy treatments available for wood rot. It is best to buy a rot repair kit as it has a couple of epoxies in it that are perfect for the job.

The first epoxy is thin, penetrating one that is designed to go deep inside the wood and hold everything together. It kills the fungus and ensures that the wood is solid again. The second epoxy is then used as an epoxy filler.

Epoxy fillers are excellent for keeping water out of wood and provide a very solid surface ready for paint.

Epoxy wood fillers for Rotten wooden window frames
Two components epoxy filler

Repair rotten windows by replacing parts of the window

If there are large areas of the window frame that are affected by rot, you could consider replacing the whole frame or doing spliced repairs. When our team repairs sash windows, we replace rotten parts of the window with Accoya wood or Tricoya MDF.

replacing sash windows parts
replaced sash window parts

Replacing the whole parts of the frame

Here is the list of timber window parts that can be easily matched and replaced cost-effectively. You will probably need to order these parts from a local joinery workshop. 

replacing the whole parts of the window frame
  • Window sill 
  • Architraves/ outer lining 
  • Bottom rail  
  • Sash stile
  • Sashes 
new window parts in workshop edited

This is a tricky job and one that requires extensive tooling. However, it is the most rewarding way of fixing wooden window frames. This type of repair will last a lot longer than repairs with wood fillers or spliced repair. Plus, it will look great for years, and you’ll have the pleasure of learning some new woodworking skills or enhancing the skills you already have.

Cost of repairing rotted window sashes and frame

It depends on how you will approach this task. If you just decide to replace one rotten window sill, the sash window companies charge around £250 for this job. Spliced repairs and draught-proofing will be priced at around £700 per sash window (does not includes painting).

Our wooden window restoration company has different approaches to sash window restoration, you can read about our sash window refurbishment prices here.

Wooden window frames spliced repairs

This method of repairing rotten window frames involves cutting out the rotten wood and then splicing (sometimes called scarfing-in) fresh timber. These are shaped to match existing profiles and to maximise the strength in the frame as well.

Rotten wooden window frames

While excessive trimming of the timber should be avoided, you must remove all of the rotten wood as you can. You should also work with any deformation in the existing frame as well. That way, your new splice won’t affect the operation of the window.

Spliced repairs must also keep water directed to the outside of the window. This means that your repair must not enter a joint already present in the window, as this may lead to moisture entering the joint and causing more issues.

Timber for spliced repairs

If available, it is always best to use the same timber as the window frame already has. If you can’t find the timber locally, you can try online or try and match the timber as close as possible with a good-quality wood that has a similar moisture content (your local timber yard will be able to help you with this).

You should be selective with your timber, though. Anything with knots shakes, or sapwood should be avoided if at all possible. These are weaknesses in the wood that can cause further issues down the road.

low quality timber used for spliced repairs
Example low-quality timber used for spliced repairs

If you plan on doing any perseveration to the existing timber, the ideal time to do this is after the rotten wood has been removed. It is well worth using a dry rot treatment on your rotten wooden window frames while doing splice repairs, though, as it can make sure that the existing timber is nice and stable.

To remove the rotten wood, you can use hand tools like a handsaw, chisels, a hammer and a plane. You can also use power tools like routers and make jigs and things like that. The method you use to splice repair your windows greatly depends on your current skill level with tools.

Once your splice repairs are glued in and dry, you can then finalise the shape according to the existing window frame with sanding. Give them a finish sanding and then paint and prime the rest of the window. If there are any gaps in your splice repairs, though, do make sure to fill them with an exterior-grade wood filler prior to priming and painting.

Replacing the entire window

As the last option, instead of repairing your rotten wooden window frames, you can replace the whole window. Replacing the window isn’t an easy job. We certainly wouldn’t recommend a DIYer tries to change a wooden frame, as they are tricky to do and windows fitters have to be FENCA or Certass registered.

If the person who fits your new window can not give you certification upon the window replacement, this can cause trouble. It might even lead to delays when selling your home, as your new windows must comply with building regulations.

So, it is best to get windows installed by a certified professional who is registered with regulations bodies and has had good-quality training.


What is dry rot in timber?

Dry rot is a type of fungus. It decays the wood when the wood becomes too wet. It is the most serious form of wood rot and occurs when there is over 20% moisture content in wood. Dry rot is extremely destructive, and it can spread across the brickwork and masonry of your home and attack other wood too, including structural timbers and flooring. If left untreated, dry rot can damage the structural integrity of your home. The reason that dry rot is so dangerous is that it eats the wood but also draws the moisture away from the wood. This causes any remaining wood to be very brittle.

What causes dry rot in timber?

Dry rot starts as an airborne fungus. When this fungus comes into contact with damp wood, these spores will germinate. As the spores grow, you will begin to see a thick cotton-like substance that covers the timber.


Dry rot needs damp conditions in order to thrive. So, if your window frames are not protected properly with a good layer of paint, they can get wet, and if dry rot spores land on this damp timber, it will cause dry rot.

Can you repair a rotted window frame?

Yes, you can repair a rotting frame, but it isn’t easy, and it isn’t always recommended. You see, if there is too much rot in the window frame, it will make the frame very unstable no matter how much filler you add.

You can, of course, replace sections of heavily rotted wood with new wood, but this isn’t easy. Unless you are a trained joiner who knows how to use power tools and cut joinery, you may struggle to replace the rotting areas of the frame.

You certainly can repair a window frame with minor rot, though. You can use penetrating wood treatments and epoxy to rebuild the wood and fill up any missing areas. You can then sand and repaint the window frame. If you do have a lot of windows that are showing signs of rot, though, it may be worth giving us a call. 

Can you fix wooden window frames?

Yes, absolutely. One of the best things about wooden window frames is that they are almost endlessly repairable. Even if one part of the frame is beyond fixing, you can simply cut a fresh piece of wood and replace it.

Fixing a window isn’t the easiest thing to do. While a professional bench joiner could make light work of all of the joints and finishing process with hand tools, even an avid DIYer could struggle. If your windows need fixing, it’s worth considering getting professional help. It’ll save you a lot of time!

How do you stop windows from rotting?

The best way to stop windows from rotting is regular maintenance and painting them every 7 years or so. This will bring the window frames back to their original condition, which is awesome. However, it also stops the older paint from failing and allowing moisture to get into the wood and start the rotting process. Of course, if you notice any areas of concern in your windows before this, do take action. If you catch problem areas quickly. You can sometimes repaint without needing to treat for rot. Still, do check for any signs of rot before you paint!

If you’ve just moved into a property with wooden windows and aren’t sure when they were painted, it is worth giving them a coat of paint now. You can also remove the older paint and check for early signs of rot, then prime and paint with good quality products to eliminate rot.


If you want to keep or old timber windows and professionally renovate them, give Chameleon a call today.

We are specialists in sash windows repair. Chameleon can completely eliminate rot from all of your outside wood while keeping the appearance of your home exactly the same.

We can also improve your draught-proofing and even add double glazing to our existing sash window frame, along with a lot more.