Considering that windows are exposed to all sorts of inclement weather all year-round for up to decades at a time, windowsills can start to degrade if not taken care of properly. This is especially true for wooden window sills, as they are not as tough as metal and are more susceptible to the elements and even termites plus other insects that burrow into wood. Although simply keeping them clean and covering them with a protective layer of paint can ensure that they will last for decades, even a small amount of neglect can lead to disaster. Indeed, if water or other liquids get caught inside or around them, this can lead to it deteriorating. Fortunately, there are ways to replace rotted windowsills.
For minor damage, where the rot has only spread to about a tenth of the sill, a basic epoxy repair will usually solve the problem. However, for sills in need of more serious repair, using PVC plastic to replace rotted windowsills is a better idea. Not only does is this material affordable, but it will not begin to rot, no matter how many times water gets onto it. To begin, just remove the side casings by prying them free. Then, create a plumb cut flush along the rotten windowsill. Circular saws are often ideal for the job, but for places that it can not reach, try using a multitool with a small saw blade. Afterwards, smooth the cut over with a hand plane. The third step is to repair the rotten areas with epoxy. Once that is down, cut the edges of your new sill accordingly, so that it will fit into the siding.
Then, drill pilot holes into the newer windowsill on the back and front edges, about every fifteen inches or so. After that is done, apply a high quality adhesive to the length of your old sill. After, put your new sill onto the adhesive, immediately connecting them together by screwing the plastic onto the wood, stopping when they are about seventy five percent of the way through. Generally, it is best to use 3/4 of an inch screws. Remember to wipe away any adhesive that escapes through the screw holes. The second to the last step is to cover the screw heads with white adhesive. After they have dried, sand them over. Finally, nail the new casing into place, over the new windowsill.
Please note: This post is for informational purposes only and results may vary. Please contact a professional/specialist to discuss your project in more detail.