Patching Rotted Window Sills
Once we determine where the rot ends and solid wood begins, we use that measurement plus the width of our router base to determine what size router guide we need. Small
4-penny finish nails hold the guide in place, and these holes are later filled with epoxy.
Cutting the Dutchman Hole
We set our “straight cut” router bit to cut approximately 3/8 inch deep all the way around. We then added small 1-inch strips to all template sides of the router guide and used the router to make a “stepped back cut,” cutting completely through the window sill. This takes several passes, each time deepening the router cut.
We use this “stepped method” to increase our gluing surface and to provide a shelf for the Dutchman patch to index into and rest on.
Smaller Rot Sections
When presented with smaller rot circumstances, not as deep, we only rout deep enough to eliminate the rot and cut into solid wood.
Prepare for the Patch
Once the routing is completed we vacuum the entire area and inspect the framing sub-sill for damage. If rotting, we may end up removing the window after all. If not, the repair continues.
We use a chisel to square off the corners of the hole after routing. The Dutchman patch is cut on a table and miter saw to ensure square corners.
Cutting the Dutchman Patch
We then take measurements and notes to make our solid Dutchman patch on the table saw. The router rode along the sill, sloped 12 to 15 degrees, so we try to match the angle on our patch.
We duplicate this slope with the intent of getting all the sides of the new patch to touch all the sides of the hole in the sill. This takes time to get right and ensure that we have a tight “dry fit” patch. Cut the patch into shape with intersecting rip cuts to create the stepped key effect.
Gluing the Patch
Once we have a solid dry fit with tight joints we mix up our epoxy.
We use West System epoxy, a versatile two-part, marine-grade epoxy that bonds and coats fiberglass, wood, metal, fabrics and other composite materials to provide superior strength and moisture resistance.
The epoxy was designed for boatbuilding and repair and works great on windowsills, columns and other areas where gluing, filling and shaping are needed.
We use the epoxy in two ways, as glue and as filler. First we brush on the epoxy to all wood surfaces and then tap in the piece. It swells a bit, and a gentle hammer-tap against a wood block is needed to get the Dutchman in place.