How to repair a rotten wooden window

The first thing that i noticed on the deteriorating bathroom window was the rotten sill. The hot moisture from our daily showers. Not to mention the splashes from bathing the dog after a fox poo heavy walk through the woods. Or the many baths with my daughter who, at age 2, would demand to get in the tub on an almost hourly basis. But just how do you repair a repair a rotten wooden window sill?

The first thing we did was try a bit of DIY to save some pennies. We slathered the sill in B&Q’s finest wood filler, gave it a sanding then sloshed on some white paint. Jobs a good’n! At least is was for a few weeks until we started noticing telltale water stain marks tinging the newly applied paint.

We ignored it, we had better things to spend our time and money on than a rotten casement window. And so, the showers continued, the baths continued, the dog continued to shake excess water off her coat all over the bathroom.

The rot got worse

It became almost impossible to open and even harder to close the window. Then the rattling started. The pane of glass had become loose and any breeze would set it off making a right racket. We might have been able to live with it had our daughter’s nursery not been next to the bathroom. Any parent knows how blissful it is it to have your child sleep through the night undisturbed.

So again, we turned to DIY options. We applied tubes of sealant liberally around the pane of glass, both inside and out. The latter via a ladder I borrowed from my father which while concertinaed seemed sturdy. However once fully extended, with me at the top and my wife below bracing it against sudden slips felt disturbingly flimsy.

We had friends stay over one weekend and during the quick tour around the room they would be staying in and the bathroom we had to explicitly say “do not try to open or even touch that window, in fact try not to look at it at all”.

At this point the window sill was more filler and sealant than wood but we’d managed to stop the rattling. We got a few more months out of the window which had now become completely inoperable. Swollen with moisture and stuck in place with various B&Q products.

The final straw

It was on a particularly hot summers day, with steam building up in the bathroom that I decided to open the window using my shoulder. That didn’t go well. And this is no boast of strength.

The frame splinted and snapped, the sill split down its length and the glass slid down only stopping when it became stuck on a piece of sealant. Luckily, we were able to keep the glass from smashing into the courtyard below which would have been an almighty clean up job to make safe for doggy paws and baby feet.

In the end we bit the bullet and got the experts in. After a survey it was deemed uneconomical to try and save the frame so out it went. The experts crafted and installed a completely new frame from an engineered wood called Accoya. It even matched the other windows in the house. We painted it ourselves and now we have a fully functional casement window in the bathroom that can be opened and closed with ease, doesn’t look a complete mess and is no longer an embarrassment.

To answer the question posed in the title, the best way to repair a rotten wooden window sill is to get a professional in. True it will cost more than a shelf full of wood filler, but the job will be done once and done properly.