How to Repair Rotten Window Frames and Sills

Chilly spring rain and layers of snow can conspire to introduce wet rot on residential wood. This also includes fascia boards, door thresholds and jambs, and wooden garage doors. While checking for this problem, take time to inspect the weatherstripping.

How to Spot Signs of Wet Rot

Obviously, a visual inspection is a preliminary way to detect damage due to moisture retention. What are some signs? Bubbling paintwork or spot flaking indicates that something is going on beneath the surface to make the paint lose its bond.

But a visual isn’t conclusive. Use an awl to test the wood surface for tell-tale softness. In one sense, this is a better test because it might mean that the damage is still very limited.

Be on the lookout for rotten wood that is accompanied by a powdery white fungus. This is indicative of a different condition called dry rot. This can be treated with strong fungicides; this is essential to keep the condition from spreading. If you have a mold problem in the house read this article.

Treating Minimal Water Damage

If caught early, like cancer, treatment is easier and less invasive. A minimum approach is to dig/scrape out the softwood. Then, drill holes into the wood and insert fungicidal rods (similar in appearance to wooden dowels) along with borate preservatives.

Then use a putty knife to repair the void with an epoxy wood filler. This may take more than one pass. Once it has dried completely, sand it down and prime it with an epoxy sealer. Finally, paint. It’s advisable to take this opportunity to paint the entire window frame or sill for two reasons – for a good paint color match and because it’s just prudent home preventative maintenance.

Treating More Severe Cases

When the problem is too extensive to treat minimally, it’s time to resort to good old-fashioned surgery. The bad wood will need to be cut out, just past the point where dry, solid wood is encountered. The tools needed vary with the circumstances, but will likely include an electric jigsaw, hammer, and chisel, coping saw, and perhaps a Roto-Zip rotary tool.

On door frames or jambs, the damage is most likely to occur down low. The chunk of wood cut out should be step-shaped to ensure maximum open grain and support.

When repairing window sills (the horizontal piece sticking out at the bottom of the window) note that there should be a drip groove on the underside. This is to enable rainwater to fall to the ground rather than traveling to the home itself.

This groove will need to be replicated in a patch. Use a chisel and hammer or a router.

Patching and Finishing the Job

It is generally a simple matter to cut and install the wood patch. If needed, cut it a bit larger to begin with and make it perfect, with a process of dry-fitting and fine-tuning.

Once it’s just right, secure the patch using brass or stainless steel wood screws (pre-drill the holes to prevent splitting), and finish it all off as explained in the minimal damage section above.