Mats Bengtsson More Advanced Wood Working and Do It Yourself

How to repair window frame or sash by replacing rotten wood

Mats Bengtsson mib over the years

Replace rotten wood to repair window frame

I referred this to the wood working section due to the use of a router and a special inlay guide bush set. Otherwise it is a very typical DIY exercise.

a window frame from a window being reconditioned. In the frame is an inlay from a reapir done through exchanging a piece of the bad frame with a piece of new wood. A Veritas No 5 plane is on the window frame.We have an old house, and with that comes the need to repair window frames of wood that has some bad or rotten wood and need maintenance. The wood is old and good, so given the right care, the repaired frame will last longer than new windows. For the window in the picture, there was an old and badly done repair of the window frame applied to the wood. I removed the unwanted part of the window frame wood, and replaced it with a piece of fresh wood as seen in the picture. There is a number of approaches to replacing the wood that can be followed.

Using the router and inlay guide bush method

a Festool router with a Trend Micro router template for inlay routing, and an MFS fence dialed in for routing a rectangleFor this job, I have used a router inlay guide bush set and template. The work to repair the frame is straight forward. The template I needed was a rectangle. For that, I used the the Festool MFS which is an excellent routing aid (but not as speedy as templates you have already created and do not need to tune.

I decided for the size I wanted. A size which removed enough material from the window frame. I dialed in the lengths of the sides on the Festool MFT (at ② in the picure).

To be sure I did not make mistakes with the size, I select to create a wooden template (at ③ in the picture) from the MFT. That wooden template was later the template used to route the actual hole in the window frame to remove the bad wood. Doing this extra (not needed) template does not make the job easier. But it gave me a clear visible hole telling me if I had thought right when tuning in the MFS. First time I had done a mistake, and I needed to change the size before I was sure it would give me the right size hole when routing in the window frame. This way felt safer than risking that the window frame hole became bigger than intended due to a miscalculation.

Removing the wood from the Window frame

an old window with a bad wooden frame covered by an inlay template ready to remove enough wood to recondition the window through a new inlayWith the right size template created, route away all bad material from the window frame within the template. The depth was increased in steps. I stopped increasing the depth when I thought the frame beneath the hole only contained good enough solid wood.

Determinining the needed depth in advance can be hard

close up of the routing template used to remove bad wood from a window frame with the bad wood to be removed clearly visible inside the templateIt was not possible to immediately set the final height to remove. In this case, the bad wood (seen at the small picture) was a terrible done previous repair. It could also have been some rotten wood. On the top, there was cavities between the differen round wooden repair bits sen in the picture. I needed to go so deep that these cavities where eliminated, or so small they could be ignored.

No matter what the bad wood is, you have to tune down the router in steps until what you see after the routing is good enough wood. If instead of an old reapir, what you would see is wood damaged from moisture, you would still not know how deep to go until you tried.

measuring the thickness needed for the new inlay that will replace the removed bad wood from he window frameWhen I had routed away enough wood (all cavities from the previous fix removed), I measured the height of the hole created. This height is the minimum thickness needed for the plug to be inserted into the window frame.

Creating the plug to be insterted into the window frame

an inlay bit routed out from a pieve of wood using a router and the same template as was previously used to remove wood from the window frameNext step was to create the new wood plug to use for an inlay. I used a piece of wood that was about 1 mm higher than the hole. The very same template as used for routing the hole in the frame was now used to route a suitable piece of wood to fit in the hole (an inlay).

Fitting the inlay into the frame

the routed inlay fitted into the hole previously routed in the frame after the whole has been squared in the corners usign a chiselWith the piece in hand comes the most scary part of the exercise. The piece need to fit in the whole created in the frame. I did not use rounded corners in the template for the inlay. I selected instead to chisel the corners square in the window frame. This is in my mind easier to do than rounding the corners of the piece.

With the corners squared, the piece if tested for fit. And it fit perfectly. In fact, it fitted too perfectly. It needed to be adjusted a bit. And that is scary. Adjusting too little, and the piece will get stuck when pressed halfway down in the hole it is to heel. Adjusting too much, and it will not take the glue well. An old rule is that it should be so tight you cannot press it down with your thumb, but so loose you do not need a hammer. The rule does not help too much, because if the piece is put too far down the hole while testing, it will not be possible to take up again. So I straightened the edges of the hole, and the edges of the inlay. Until the point when it was time to try.

inlay piece is now placed in the hole in the frame but not yet planed into the same height as the rest of the frameOn with glue (I use Titebond III when the fit is tight and I need some additional opening time. I would have used white glue otherwise, unless a more filling glue would be required). The glue was spread evenly using a brush. Do not spend to much time. The glue will cause a slight swelling of the wood, so it should not get too much time for doing too much swelling. Also, it is important the glue is still open when the piece is fitted in place (open time is defined by the glue manufactorer and differs from glue to glue) .

the inlay piece of wood finally fitted into the window frame and planed to the correct depth.In my case, I needed some blows from my hammer, and the piece was down in correct depth. Best way I have found to remove the excess wood is by using a plane. A special purpose japan saw could be a good start, but a plane is perfect for the final adjustement.

inlay piece glued n place in frame and planed with veritas hand plane No 5All done, a perfect fit and no risk it will be visible in the future.



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