Repair a Rotting Window

Surgical Repair of a Rotten Sill, Trim and Siding.

By Matt Weber

Wooden windows have a classic architectural appeal but require a lot of work to keep them in good condition. Maintenance chores such as repainting and re-sealing the windows can not only improve their appearance but also protect against weather exposure. Whether due to poor maintenance or shoddy installation, unprotected wood can eventually rot and require more remediation than a new coat of paint.

Such is the case with the rotting window sill shown in this article. Over time water had breached the sealant and paint around the sill and seeped into the wood, deteriorating some areas to the point where I could push a hole into the spongy material with my finger. The water had also leeched up the brick molding on each side of the sill, and had run down into the wood siding below the window, rotting those materials as well. This article will detail the repair of a rotten wooden window, a job that demonstrates why routine maintenance is so very important.


Material Roundup & Demolition

This may sound obvious, but make sure to acquire your replacement building materials before diving into your demolition. In this case we had trouble finding a siding supplier that carried a matching profile, which was an unexpected challenge that delayed our start.

To preserve the weather seal, I only removed the rotted part of the window will, cutting it away with an oscillating multi-tool.
Unchecked water intrusion lead to deterioration of the window sill, brick molding, siding and drip edge.

With all your tools and materials on site, carefully proceed with removing the damaged materials. Because only the front of the window sill was rotted, with the area of contact between the sill and the pane still solidly intact, I decided to leave the good wood in place and surgically cut away the bad wood with an oscillating multi-tool. Doing this ensured that I did not compromise the weather seal at the pane-and-sill junction. However, it’s important to make a very straight cut so your replacement piece installs correctly.

Next, I removed the rotted portions of the brick molding, again making straight 90-degree cuts for easy installation of the replacement pieces. Then I removed the siding, beginning at the top and pulling away each panel with a pry bar. The siding was so rotted that it came apart in soft chunks. Avoid the temptation to tear into the wall with a sledge hammer. In our case the siding was installed over rigid foam sheathing, which was still in decent shape so we could re-use it (no sense in destroying it).


Fabricating the Sill

Here’s an example one of those unexpected chores that seem to come up on every remodeling job: The need to fabricate a replica of the partial window sill. To create the replacement piece required some tricky cuts in the pressure-treated stock to mimic the profile. It had a funky shape with two different sized rabbets (grooves) and bevels on each edge. The front bevel of the window sill served as a drip edge, and the rear cut had to interface with the remaining sill that was still attached to the window — which was pitched to shed water.

This type of work generally requires a table saw, utilizing the saw’s rip fence in conjunction with the blade’s ability to adjust depth and cutting angle. Without this crucial tool, I would never have been able to fab up the replacement sill. So, if you’re a DIY’er who has been weighing the benefits of investing in one of these highly versatile saws, a task like this is the exact kind of situation where the right equipment can save the day.

I cut the old sill in half to get to the heart of its profile.
I then removed the damaged trim and pried away the rotten siding.

The old removed sill was in such poor condition that I had to cut it in half in an area that was still fairly intact just to get a good look at its original shape. I then used a cross-section of the old sill as a template to trace its profile onto the end of a 2×4. These traced lines provided my guidelines to make the various rip cuts when recreating the shape. I then used my multi-tool to notch out the corners to fit around the window casing.

Note: Although I used pressure-treated lumber to create the replacement sill, another option might be to laminate 1×4 flat PVC stock into a 2×4 block and cut the profile from the block. Cellular PVC, such as the material available from Fypon (, is a more rot-resistant material but was not in stock in our area at the time.

Before reinstalling the new sill, I wrapped the old sill with self-adhesive window flashing. I covered the cut area of the old sill with flashing and folded it smoothly over the lower framing so that it lapped over the sheathing to divert water.

I then tested the new sill for a good fit, and used shims as necessary to secure it flush and level with the old piece. Once satisfied with its placement, I countersunk several 3-in. decking screws through the sill into the old framing to secure its position.

I used the old sill as a template to fabricate a replacement piece on the table saw.

In addition to screws I anchored and sealed the sill with a liberal application of Siliconized Adhesive Sealant, filling all seams and screw holes. Do not skimp on the quality of adhesive sealant, or you may find yourself repeating this job in the future. The product we used boasted a 52-year warranty. Additionally, make sure your product is labeled as “paintable,” because conventional silicone will not bond with paint. Do your best to tool the fresh sealant smooth, because the siliconized material doesn’t sand easily. The sealant required a couple of applications to accommodate a small degree of shrinkage when dry.

Trim & Siding

I had to rebuild some of the lower window casing from 1×4, installing with adhesive and finish nails. When it came time to replace the brick molding, I used urethane molding from Fypon—a material that will never succumb to the same rot and moisture problems as wood. These pieces had a matching profile, which made them easy to butt to the old molding, fastening with polyurethane construction adhesive and finish nails (and shimming as necessary).

I wrapped the window framing with self-adhesive flashing for proper water protection.
The multi-tool proved handy when notching the sill to fit around the window casing.

Next I installed the wood siding. This was a straightforward procedure in which I followed the original nailing pattern with aluminum finish nails driven through the upper flange of each siding piece. Siding installation always begins at the bottom, with each successive piece overlapping the one beneath it to divert water. I didn’t have room to fit a full siding panel right beneath the window, so I ripped the final piece to size on the table saw and screwed it into place.

I replaced the brick molding with rot-resistant urethane molding from Fypon.
I fastened the sill with a combination for shims, decking screws and siliconized adhesive sealant.

The drip edge at the bottom of the siding had also rotted away. It was made from 1×4 and used to divert water away from the home’s foundation. Again, I had to cut the rotted piece in half to get to the meat of the profile and transferred its shape onto some fresh 1×4 stock, which I ripped on the table saw. The multi-tool also came in handy again when I had to notch the ends of the drip edge to fit over the fascia on the house. I fastened the drip edge with finish nails into solid wall framing and toe-nailed into the siding.


Sealing & Priming

Every window and door installation should be sealed with a high-quality, flexible exterior-grade sealant. I recommend lining all seams with painter’s tape when sealing to localize the mess. Tool the beads of sealant smooth with a wet rag or a flexible caulk shaper. Fill all fastener holes with an exterior grade wood filler. Make sure your sealant and wood filler are paintable products. Follow suit by sealing the seams of the siding and drip edge in the same manner.

Each siding panel overlaps the one beneath it to divert water. I fastened the panels using the original nailing pattern.
Siding installation begins at the bottom.
I had to rip the final siding panel to fit beneath the window.
I had to rip the final siding panel to fit beneath the window.

Once the entire installation is sealed and filled, give all the new materials a fresh coat of exterior-grade primer (or two). The final paint job to follow will not only give the window and wall a fresh coat of color, but will provide a new layer of protection from the water that caused the rot in the first place.

I suggest lining the seams with painter’s tape when applying the window sealant. Use exterior-grade wood filler to fill all fastener holes.
Use a scraper to completely remove all old caulking so your new sealant with have a firm bond with the substrate. Seal the edges of the siding against air and water penetration.
The lower drip edge was also rotted and required me to fabricate a new profile.
A table saw with a rip fence is the perfect tool for cutting the bevels of the drop edge. A multi-tool made quick work of the notches at the rear of the drip edge.
Here’s the repair after priming, ready for a new coat of finish paint.


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