Fixing rotted wood window sills.

Window Repair:

Fixing A Rotted
Wood Window Sill

In This Article:

The old window sill is cut off and the surface is cleaned up. A new sill is cut from a piece of cedar and nailed in place.

Related Articles:
Skill Level: 3 (Intermediate) Time Taken: About 1 Hour

By , Editor

Start:

While repainting the exterior of my house, I discovered that this window sill was badly rotted at the ends (red arrows). Wood window with rot damage on sill.

On older wood windows, the sill usually extends deep into the house, below the lower window sash. Completely replacing the sill is possible, but it's a lot of work and may require removing the entire window from the rough opening. I have repaired damaged window sills by merely replacing the section that sticks out beyond the siding. This repair is simple and usually effective.

Using a metal detector to locate nails.

Optional:

Before cutting into the wood, I used a small metal detector to check for nails that might be driven through the face of the sill. I didn't find any.

Using a metal detector isn't necessary. Another way to find nails is to scrape all the paint off the sill.

 

I placed a long wood-cutting blade in my reciprocating saw and proceeded to cut off the protruding sill.

I tried to keep the cut flush with the siding below the window. Note that I mounted the reciprocating saw blade with the teeth up, which meant that I could hold the saw in its normal orientation while cutting.

Cutting off window sill with a Sawzall and long blade.

 

Using Fein Multimaster to cut protruding sill flush with wall. I also tried my Fein Multimaster with a wood-cutting blade. This tool cut the sill flush with the siding, but it was slow, so I went back to the reciprocating saw.

(This tool is the reason I used the metal detector earlier... these blades cost $25 and they get ruined if they hit any metal.)

 

After a few minutes of cutting, the protruding part of the sill was removed. Removing part of window sill.

 

Old rusted nails next to rot-damaged wood sill. There were some rusty nails (red arrow) in the rotted wood. I pulled these out with a pair of pincer-pliers.

 

Fixing The Deeper Rot At The Ends:

Normally I would take the extra time to remove all the rotted wood at the ends of the sill. This can be done by cutting away the bad wood with a chisel or drilling a series of holes with a large drill bit. Then the cavity can be filled with a block of wood which can be glued in place and/or fastened to the surrounding wood.

But I did this repair the quick-and-dirty way... I just covered over the rotted wood that was behind the face of the siding. I DO NOT RECOMMEND THIS PRACTICE. I did this cheap fix because I plan on remodeling the house in the next few years, at which time this old window will be replaced. If the new sill is sealed properly to all surrounding materials, then water should stay out and the concealed rot should not pose any problems. It's important that any rotted wood be dry before covering it.

If removing all rotted wood is too difficult, it can be treated with an epoxy consolidant and filled with an epoxy or polyester wood filler as seen in this article about repairing rotted door jambs.

 

When I cut off the protruding part of the sill, I didn't get a perfect cut and the center section still protruded a bit.

So I used a block plane to remove any excess wood.

Planing back the wood surface with a hand-held block plane.

 

Checking the angle where the window sill meets the casing. The original sill was sloped so water would drain off, and the new sill needs to duplicate that slope.

To determine that angle, I made a miter cut on a 2x4 and placed it on the wall beneath the original side casing.

After trying a couple of different angles, I determined that the bottom of the casing made a 15 degree angle with the siding.

 

On my table saw, I ripped a piece of cedar 2x4 with a 15-degree bevel.

I cut this piece about 1 inch wide. Note that this piece of wood is a simple parallelogram, meaning that opposite faces are parallel.

Ripping a new sill on a table saw.

 

Cutting drip kerf on a table saw.

Cutting A Drip-Groove
An Easy Extra Step:

I lowered the table saw blade until it was sticking up about 1/4 inch. I removed the splitter/guard and ran the board through the saw to cut a shallow groove (or kerf) on the bottom of the sill, just inboard from the outer face of the sill.

 

After I cut the new sill to the exact length (on a miter saw), I fastened the new sill to the old sill area.

I used 2 inch galvanized ring-shank siding nails, which are very skinny and less prone to splitting the wood. Galvanized finish nails or box nails are also good choices, but it may be necessary to pre-drill the holes near the ends of the board.

Nailing the new window sill in place.

 

Drinving nails upward through the sill into the casing. At the ends of the new sill, I drove a nail upward from the bottom of the sill into the side casings.

I really needed this step because I didn't address the rooted wood problem at each end of the sill. I just covered over the problem, which is not how I prefer to fix things.

 

The completed sill after installation. I nailed the sill in 6 or 7 places.

Next I applied a bead of siliconized acrylic latex caulk around every edge of the new sill.

Window sill after repair.

 

New window sill after painting. After covering the new sill with primer and exterior paint, it looked as good as new.

 

More Info:

Tools Used:
  • Basic Carpentry Tools
  • Table Saw
  • Miter Saw
  • Reciprocating Saw
  • Block Plane
Materials Used:
  • 2x4, Western Red Cedar
  • Siding Nails
  • Caulk
  • Exterior Primer and Paint
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Written October 12, 2009