Fixing wood rot in a timber window

Understanding the proper way to repair timber windows so that they will last for generations to come.

Over the years Sealasash has repaired and replaced countless window sills, joints, rails and stiles. We know that wooden windows, some over 100 years old, will require some repairs, even if these are not obvious to the trained eye.

It makes sense that we take care to fix these problems properly as they are discovered and this means that windows will last for many generations to come.

While Sealasash’s core offering is to make wooden windows and doors operate properly and to install draught/acoustic seals, we know that we will have to undertake window repairs as a part of our service.

Beware of patch and paint

The cheap “patch and paint” mentality spells doom to old wooden windows and results in hidden and costly problems for owners. We see some really second rate repairs where rotten patches have been filled with acrylic gap filler, silicone, builders bog, and surprisingly even aluminium foil and plaster of Paris!

These materials “hide” under the paint and no one is less the wiser….until of course the whole process of rot takes off again. At the end of the day these kinds of ‘repairs’ don’t fix the problem at all, they actually make it worse.

Some repairs look great on top, but just mask the problem. Until you cut into the timber, you can’t always see what’s happening.
This is the same piece of sill, shown from above, and with the old repairs stripped off the top.

Repairers and owners don’t always repair rot in the correct manner

There are at least two common reasons for this.

It’s a lot more work to do the job properly and secondly, even old and existing sub standard repairs can be underestimated in the budget. If you are using a contractor and they provide a fixed-price quote, it is possible that they could be tempted to take a shortcut if they discover an unexpected repair half way through the job.

At Sealasash we know that apart from undertaking a thorough assessment flexibility and transparency with pricing is needed so that repairs can be dealt with properly and so that the “unknowns” can be catered for in a contingency sum assigned to the job.

Rot is a fungal infection of timber. In rare cases, the infection can spread into the surrounding wall framing. This shows a wall frame under a sill that was removed.

This customer wanted to reduce the uncertainty in her quote, so she scraped out all the rot before we arrived to quote. This is a good way to see exactly what the problems are.

Here a window stile has been repaired in accordance with the conservation principles in the Burra Charter – doing as much as necessary, and as little as possible.

So what can you do to get started?

When you are looking to fix a rotten component of your window there are a few things to have in mind.

Consider how much rot is there and where is it located?

You can use a sharp object to poke and probe into the wood to locate rot. It’s better that you find the rot than feel “sheepish” about digging in to find the rot. Probing can determine the scale of the problem. A moisture meter is always handy as well and any measurements above 18% swill provide ideal conditions for wood rot to grow. “Dry” wood is 0-12%, “mid-range” is 12-18% moisture and rot is possible but not likely.

The lower parts of a window, where water sits or becomes trapped, is where to start looking. Sills, frames, joints, internal linings at the corners and the bottom of the bottom sashes, particularly on opening sashes.

One of the classic and most common mistakes made by DIY and also handy trades is to fasten and seal/paint bottom sashes shut. This is a slow and inevitable death to the window. While it stops draughts, dust and some noise, what is overlooked is that water/moisture becomes trapped under the rails because there are cavities there. The rot cycle then creeps in working from the inside out!!

Whatever you do, do not paint and permanently seal opening sashes shut or you are likely to be dealing with a costly problem in the future.

Around the windows it’s also wise to inspect the structural elements as well as the cladding and any reveals on the wall and frame. Are the weatherboards rotten? Is the stud work below the window compromised? Has the rot spread up into the pocket cavity and began to destroy the frame lining?

The best way to repair wooden windows

Cutting the rot out to unaffected wood is the best way to deal with removing the fungus and stopping future rot. A good tool for cutting is a multitool. A multitool is very flexible and allows cuts to be made relatively simply and swiftly.

Sometimes this is not entirely possible so a fungicide or epoxy wood treatment system should be used. This system works by soaking into the timber grain and crystallising, making the timber resistant to movement and further rot. It also works to isolate and capture any small portions of rot that may have been missed when you cut out the rest of the rot.

The tools that are handy to repair old wooden windows include a hammer, chisels, drill, table saw, drop saw, multitool and maybe a router.

As you are cutting each component to put back in place it is important to try and match as much of the original joinery as possible. We believe that the old fashioned mortice and tenon joints are excellent and have outlasted many modern fixings and fastenings. Sometimes when fixing these joints we have to use a special flexible two-part epoxy.

After glue and epoxy dries, it’s time to shape corners with a chisel, tidy decorative mouldings, and to get clean and smooth joints. After sanding finally paint all surfaces with a solvent based primer sealer to protect the timber and to prevent any rot penetrating in again. Make sure top coats are applied as soon as possible to protect the new repair.

A word of warning. When you work on old wooden windows make sure you consider the likelihood that lead paint is present. It is very important that work health and safety precautions are followed to stop the ingestion and transfer of old paint dust and chips.

If you would like to talk to a timber window expert who understands all your options, and who will listen to your priorities, then please don’t hesitate to contact us.

Looking for double glazing for your sash windows or wooden windows?

Sealasash can double glaze your old timber, wooden and sash windows with new high-performance glass.  We also upgrade windows to include draught and acoustic seals that further improve the comfort and energy efficiency of your windows. 

Learn more about Sealasash:


Vacuum Insulated Double Glazing

Our ultra-high performance double glazing fits perfectly within your existing timber work, eliminating any bulky or expensive modifications.


Fix Those Annoying Rattles

With our draught and acoustic seals you’ll achieve amazing new comfort levels by reducing infiltration of outside air, noise and dust without the annoying rattles.

Related posts

Using traditional craftsmanship and modern materials, we’ll restore your windows and make them better than new.  This includes high-performance double glazing and draught seals that will dramatically improve the comfort and energy efficiency of old timber, wooden and sash windows.