While newer houses often take advantage of rot-proof window materials like vinyl or aluminum, windows tend to be areas where moisture congregates, so unless your windows get consistent preventative maintenance – they’re likely to suffer from water damage.
One of the tricky things about rotted windowsills is that they often appear to be in good shape. Water can work its way into the frame or sill and siding create rot without much visual damage.
The best way to check if the wood is rotten is by pressing into it with a screwdriver. If it has a spongy texture – you’ve got yourself wood rot. The most common spots for this to happen are in the lower corners of window frames – particularly on windows that aren’t protected by some type of overhang.
Another good indicator of rotted wood is chipped and missing paint on the sill. Siding under windows is subject to rain, snow, and condensation.
So now that you’ve found some dry rot under your window, how do you replace the rotted wood?
If the rot is more severe (more than about 10% of the overall length), then you’ll need to remove the rotted piece and replace it with a wooden insert. This is a more labor-intensive repair but will result in a more durable and longer-lasting repair.