how to extreme

Patching Rotted Window Sills

Construction How-To, Windows & Doors September 24, 2015 Sonia


Surround the damaged area with wood strips to serve as a router guide.

Surround the damaged area with wood strips to serve as a router guide.

Set the depth of the router bit to create a stepped patch to fit into the window sill.

Set the depth of the router bit to create a stepped patch to fit into the window sill.

Carefully rout out the old wood.

Carefully rout out the old wood.

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.

Dust Prevention

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.

Shown here is the completed cut-out.

Shown here is the completed cut-out.

Cut the wood patch on a table saw and seal all edges.

Cut the wood patch on a table saw and seal all edges.

Test-fit the patch and make adjustments as necessary.

Test-fit the patch and make adjustments as necessary.

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.

The patch should fit snugly as into the recess.

The patch should fit snugly as into the recess.