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How To Replace A Rotted Window Sill


  • Rotted window sill can cause lots of problems, including mold, termite and other pest issues
  • Window should be replaced at the same time as replacing a window sill


I think it was my third or fourth window out of 8 replacement windows when I encountered this rotted window sill. 

The warning was as plain as day:  this particular window was the only one with it’s exterior window trims and window sill covered with aluminum flashing and even clearer warning of loose finishing nails on top of the flashing that I could pull out with my finger nails. 

With my pregnant wife expecting to deliver our bay any day, these warnings should have triggered a loud scream in my head but alas, being a true all thumbs DIYer, I proceeded with my project.

It did not take too much time or effort to remove all nails and metal flashing from the sill and trim.  As soon as the sill flashing was removed, a large chunk of the sill fell out as seen from fig.3.2.  I am going to go out on a limb and guess that:

  • rain water + rain water bouncing off the roof + top finishing nails +flashing trapping moisture = wood rot

I have to be honest and tell you that I ABSOLUTELY PANICKED.  In all my 10 years of DIY projects, including many window replacements, I have never replaced a window sill before.  And I really did not know how the window framing was put together so I was also afraid that taking apart the window sill would be like opening up a can of worms.

After staring at this gaping window opening for about an hour thinking how it was going to be a buggy night, I finally worked up enough courage and researched the internet for more information and stumbled through the process.  If you are in a similar situation, I hope you can gain some useful information from my wild how-to-replace-a-rotted-window-sill-at-the-last-minute journey.


I don’t know why the architect and/or builder decided to place a window so close to the adjoining roof line.  I guess form trumps function, heh?

After cleaning off the debris, I estimated that approximately 8 inches of the exterior window trim was rotted and approximately 15 inches x 4 inches of the window sill nose and body  were rotted (see Sharpie outlines in fig.3.3) .

My initial hope was to use some wood epoxy products to repair it (check out the products mentioned in RotDoctor, NYTimes article and Journal of Light Construction Beating Wood Rot – click here if the PDF link is broken). However, based on the extensive rot damage, there was a legit concern that additional damages might be hiding underneath the sill as the water runoff from the adjoining roof was getting trapped between the outside sill nose and siding.

Although it was tempting to do a quick fix and move on, I decided to replace the entire sill because I did not want to come back years later to tear everything apart to fix the sill rot again.

If your window is fairly new and are in good condition, you may be able to replace your window sill WITHOUT removing the window. There is an excellent article posted at DoItYourSelf’s forum for your reference (click here for the How To Install A New Window Sill Without Taking The Window Out article – click here for the PDF version if the link is broken).

Temporary Fix

Before doing anything else, I had to do something about the gaping hole where the window used to be.  Luckily, I was in this predicament during May, which meant I only had to contend with bugs and heat, not snow and cold.

I thought about installing a large piece of plywood to cover up the window opening but quickly abandoned that idea because of the erratic wind condition.

My next choice was to take some heavy duty 6-MIL (for a quick tutorial on MIL terminology, click here) plastic sheeting (10′ x 10′ which goes for around $20 bucks at local hardware store), double up and roll the ends on to some 1×2 scrap lumber and screw them in.  I used the lumber to increase the holding power as screws alone would most likely rip off the plastic sheathing.


Because I was TOTALLY unprepared for this exercise, I guesstimated that it would take at least a day or two to purchase necessary materials.  So my strategy was:

  1. Remove and save the rotted window sill;
  2. Locate / purchase a new window sill with matching profile (does not have to be exact but should be close);
  3. Purchase a new window stool with matching profile;
  4. Purchase shims, loose insulation, foam insulation, deck screws, caulking, wood preservative and exterior grade paint;
  5. Re-install window sill, replacement window, window stool and interior/exterior trims

Required Materials

My major problem was finding a replacement window sill. Because I had already purchased a new replacement window based on measurements taken with the rotted window sill, I had to find a close, if not exact match to maintain the correct installation height of this particular window opening. There were literally hundreds of window sill varieties with different angles, cut patterns on the bottom, etc. To add to the problem, window sills were hard to find, especially at Home Depots and Lowes.

The best option would be to custom make your own window sill.  It does NOT require lot of skills but you need to have the right equipment and patience. Head on over to How To Make Your Own Window Sill article for more information

After searching for 2 solid days (yes, God took sympathy on me and did not allow to rain), I found a very close match at a local place called Somerville Lumber yard. It was fairly expensive, selling for $14 per linear foot (1 3/8 x 6 3/4 Clear Sill) for about $12 per foot. Very expensive, but beggars can’t be choosers, right?

Step 4 – Replacing / Repairing

Now that I had the necessary materials, I was ready to cut away the old and replace it with a new window sill. Fig. 4.1 is a simple schematic of what it looks like from the side (please keep in mind the sill pictured is one design out of many variations. Your sill design may be different):

Fig. 4.1 – Side View, Window Anatomy
  1. Remove the window as described in this section
  2. Carefully remove the interior window stool. Window stool is usually nailed to the bottom molding and/or sill from the top surface. The easiest way is to gently tap the stool from the bottom side to separate the nails.
  3. Remove the bottom molding;
Exposed side jambs and sill after removing molding pieces
  1. Set a shallow blade depth on the circular saw and cut down the middle of the window sill (you can also use a reciprocating saw but I prefer the circular saw for stability). Don’t worry if you cut into the bottom frame slightly.
cutting off the rotted window sill in half
  1. Using a wood chisel and hammer, gently slide out the remaining sill away from the side molding
  2. Clean up any nails and debris
  3. Check to see if there is any more rot (thankfully my jambs and framing members were in sound shape)
  4. Cut the new window sill to proper length and using the old window sill as a template, cut each corner (Can’t find the matching window sill from your local lumber yard? I wrote a separate article on how you can make your own window sill)
New Replacement Window Sill
  1. You will need a Top Handle Jigsaw to cut away the end pieces. Although I do use cordless drills, I do not recommend a cordless jig saw because I found them to be under powered and unbalanced with heavy feel to it. Or if you have the muscles and plenty of patience, you can even use a small hand saw. I recommend Bosch JS470E 120-Volt 7.0-Amp Top-Handle Jigsaw
    because it is fairly light and has ample power to cut through the wood. BTW I consider Bosch tools to be higher in quality when compared to DeWalt.
  2. After cutting, I painted the freshly cut ends with a Rust-Oleum Wolman Woodlife CopperCoat Green Wood Preservative to minimize future rot.
  3. Next, I grabbed some loose left over insulation and stuffed the bottom framing member to reduce the air flow.
Adding loose insulation underneath the window sill
  1. Now, tap in the new sill (from the outside to inside); use shims as required to level
  1. Temporarily install 2 screws to hold the new window sill in place (you may want to screw in where were the shims are)
  2. Trial fit your new replacement window to make sure that it fits with room to spare (You do not want the window to be tight against the opening frames. Window frame will swell and shrink depending on the weather outside so you want to have at least 1/8 to 1/4″ gap all around the window). If the fit is good, install additional nails/screws.
  3. Add caulking around the perimeter of the window, both inside and outside.
  4. Reverse steps to install interior trim moldings
  5. Now you are done! 😉

If you want any more details or clarifications, please let me know and I will do my best! Good luck!! 🙂

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Thursday 24th of September 2015

Can the entire job be done from the inside ie without using a 30 ft ladder on the out side for access?


Monday 28th of September 2015


That's some height you got there. Personally, I was able to replace my rotted sill from the inside without disturbing the outside siding but your situation may differ.

When I use my ladders, I make sure to install a cleat at the feet (or rebar in case of dirt) and some sort of stabilizer on the top (like this like this on

Good luck and tell us how you made out




Thursday 5th of September 2013

Thank you for the very informative article . It, and your others, are very well done and as such highly useful. I have only one concern: the article fails to explain how you actually remove the old sill. Instead, jumps from cutting the sill with a circular saw to "using a wood chisel and hammer gently slide out the remaining sill." ????? The "remaining sill?" Where did the rest of it go? Is the sill not fastened anywhere, or does it just float loosely? The reason I read the article was for any tips on actually removing the sill. I got everything but that advise. I'll figure it out. Thanks anyway!


Thursday 5th of September 2013

Thanks for your kind words.

I see that my web page is not loading all my pictures so I will take a look at it right away.

As far as removing the sill is concerned, usually a wood sill will slide in between left and right side jambs (The picture I had showed you a groove) at an angle.

Whether you use a circular or reciprocating saw, I would make sure to not cut through the bottom window framing by cutting in small increments.

Check back in few hours to see if the pictures load for ya.

Good luck