Patching Rotted Window Sills
Installing a Dutchman Patch
Dutchman patches are known primarily in carpentry, furniture-making and masonry. A Dutchman is a piece of wood or stone that is used to repair a larger piece, shaped so that it fills a void. In our case we cut the void and patched it with new wood—a Dutchman patch.
If windowsill rot is confined to just a small area, a simple patch and fill is all that is needed. For larger sections of damage, most or all the windowsill may need to be removed and replaced.
When performing a Dutchman repair we like to use Western red cedar or Mahogany for the patch. Both wood species have inherent insect- and rot-resistant qualities.
Use a Router to Cut Out Rot
We use a compact router and guide to cut our Dutchman patch because using anything else does not provide a straight enough line for a tight glue joint.
A router kicks up a lot of saw-dust, so sealing off the window is a must.
Prior to cutting out the rot you will need to take precautions to keep sawdust from entering the house. We use a product called Tape & Drape, a pre-taped masking film that unfolds to cover and protect various surfaces while painting.
We then use painter’s tape to completely seal off the sides of the plastic, on the inside of the window, and then raise the lower sash to provide access to cut the sill with a router. This process ensures a dust-free house interior.
Straight Edge to Guide the Router
In order for any tool to cut straight you need to use a guide. We use quarter-inch plywood as our router guide, making the guide with a table saw and miter saw.