Repairing Rotted Window Sills With Dutchman Patches
Look at any wood exterior building and you’ll notice that the windowsills take a beating form mother nature. Windowsills by design are sloped but still basically a horizontal shelf where water and snow can accumulate. Add to that a house without gutters and you’re a candidate for windowsill and related trim rot.
Once water finds its way behind trim or under paint and caulk to rot process begins. Ongoing exposure to this moisture will cause the windowsill to rot, and worsen over time. When caught in time often a small epoxy repair is all that is needed.
But when discovered late or left unchecked, the rot will penetrate deeper into the windowsill, and even wall framing requiring much more and more extensive repair.
Window Repair or Window Replace
The age-old debate of window replacement or window repair is a tough one to answer but I’ll do my best to address my thought process and how I try to guide my clients decision process.
I look at the repair verses the replacement question as a business approach. What makes the most sense; based on time, budget longevity and of course hidden costs.
When dealing with old historic windows, often times repairing is the only option. Many times these windows are protected by a city or town’s historic district commission specifying that the window details remain unchanged. Often times, replacement requires having a custom and costly window fabricated to match the style, look and dimensions of the existing old windows.
Other circumstances where repairing may be an option is on higher end homes with large expensive units, [PHOTO ABOVE] bay windows or multiple mulled windows. Many times these windows will have high-end trim, wainscoting or faux wall finishes that would add a multitude of extra costs if the window is replaced.
I often opt for window replacement when the cost of the window, any interior work relating to the replacement and my replacement labor are equal or less than the time I’d spend performing a quality repair. I often see this as a realistic option on more modern homes when dealing with stand alone window units and simple interior finishes and trim.
Replaceable Sill Nosing
Sometimes a windowsill has rot out the outer nosing only. Many newer windows actually have removable nosing that can easily be duplicated and replaced with Western Red Cedar, Mahogany or PVC. Others are one piece and need to be cut away and patched.
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 such that it fills a void. In our case we cut the void and patch it with new wood – called a the 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 of 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 sawdust so sealing off the window is a must.
Dust Prevention – seal the window opening.
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 and drape,” a pre-taped masking film that unfolds to cover and protect various surfaces while painting.
We then use blue 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.
Fabricate a Straight Edge for Router to Follow
In order for any tool to cut straight you need to use a guide. We use quarter inch plywood as our router guide, making this guide on with a table and miter saw.
Once we determine our rot and where the solid wood is 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 purposely set our “straight cut” router bit to cut approximately 3/8 of an inch deep all the way around. We then added small 1-inch strips to all template sides of the router guide and use the router to cut completely 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.
Dealing with Smaller Rot Sections
When presented with a smaller rot circumstances, not as deep, we only route deep enough to eliminate the and cut into solid wood. [Photo Below]
Prepare Hole for Dutchman 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 after routing. The Dutchman patch is cut on a table and miter saw so we need to have square corners.
Cutting the Dutchman Patch
We then take measurements and make notes to bring to the table saw to make our solid Dutchman patch. The router rode along the sill, sloped 12 to 15 degrees, 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.
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, 2-part, marine-grade epoxy that bonds and coats fiberglass, wood, metal, fabrics and other composite materials to provide superior strength and moisture resistance.
It was designed for boat-building 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 block and gentle hammer tapping is needed to get the Dutchman in place. Once in place a clamp or a few well placed finish nails hold it until the epoxy dries – usually 8 to 24 hours.
Fill Voids with Epoxy
Once the patch is installed we mix up a batch of epoxy with structural filler to fill and overlap the seams of the patch and any nail holes created during the repair.
Sand Dutchman Patch
We wait 24 hours before sanding the patch and applying a quality oil primer to the entire windowsill. The painter will follow us with additional primer and two-coats of paint.
