A reader sent me a photo of a wood window in his girlfriend’s bathroom. The window is suffering from rot, and the sash frame is starting to separate. He asked if I was able to determine what was causing the issues.

When I looked closely at the photo, I could see a depressed part of the sash profile in the lower right corner. Water vapor from hot showers no doubt condensed on the glass pane, then rolled down the window and found a tiny crack at the sash corner where it could enter the wood.

I would guess that the problem has been going on for some time, and the weight of the glass pushing down on the lower, horizontal sash frame has caused the two pieces of wood to separate at the corner. As the crack has gotten bigger, more water has entered, which has accelerated the rotting of the wood.

Fortunately, this sash can be salvaged with a small amount of effort. You may prefer to wait for warmer weather to do the job, but you can start a repair in the winter if you have the skills to cut a piece of plywood the same size as the sash. You can then pull out the damaged sash to work on it, replacing it with the temporary piece. When the weather is warmer, a thick piece of cardboard is enough to close up the opening while repairing the sash.

The first thing that needs to be done is to remove the mildew from the sash. Start by using a fan to dry the wood at the corner. Once the wood is fairly dry, use a long squeeze clamp to try to close the gap between the two pieces of wood that make up the corner.

Then apply yellow carpenter’s glue in the crack and use a Popsicle stick or other thin piece of wood to spread it around in the crack. Tighten the clamp and check to see if the corner draws up tight. If it does, leave the clamp in place for a few hours to let the glue dry.


While the clamp is in place, drill a pilot hole in the bottom of the sash about 1/2 inch from the outside corner in order to install a 3-inch-long, stainless-steel wood screw up through the horizontal piece into the vertical piece. Drill the pilot hole carefully so the screw is centered in the wood frame.

If the corner can’t be drawn tight with the clamp, the crack can be filled with wood epoxy. I’ve posted two videos at AsktheBuilder.com that show how to mix, apply and sand this material.

As for the rot on this window, that problem could have been avoided. If you have a water vapor problem on your bathroom windows, all you have to do is place a towel in contact with the bottom of the window glass each time you shower. The towel will capture the dripping condensate before it can get to the wood.

After you’ve showered, use the towel to wipe off the glass pane, as well as any other visible water on the window frame. Leave the bathroom door open to lower the overall humidity in the room as rapidly as possible.

Tim Carter has worked as a home improvement professional for more than 30 years. To submit a question or to learn more, visit AsktheBuilder.com.