It was in the bathroom that I first noticed the deteriorating 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 strip off and demand to get in the tub on an almost hourly basis.
The first thing I did, well actually my wife, 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 bathroom window. And so, the showers continued, the baths continued, the dog continued to shake excess water off her coat all over the room and my daughter continued to splish-splash.
The rot got worse and 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 which we might have been able to live with had our daughter’s nursery not been next to the bathroom, and if you’re a parent I don’t need to explain how blissful it is it to have your child sleep through the night undisturbed.
So again, we turned to DIY options. Tubes of sealant were applied 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 but once fully extended, with me at the top and my wife below bracing it against sudden slips (I remember all to well when my father fell from a ladder 20 years ago and broke his elbow) 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 is 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 also become completely inoperable, swollen with moisture and stuck in place with various B&Q products and it was then that 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 my alpha masculinity and strength, if anything it’s a window (pardon the pun) into how my brain often proffers up ridiculous routes of action.
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, and a completely new frame made from an engineered wood called Accoya was crafted and installed. 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 and 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.