How To Make Your Own Window Sill – Part 1


If your exterior window sill is rotted and can’t find a matching sill from your large box retailers or lumber yards, I may be able to help you.

Please note that How to Make Your Own Window Sill post has Part 1 (you are here), Part 2 and Part 3).


I know I have mistakenly used the term “window sill” to describe various parts of windows so let me clear that up first.

The very first thing is to take a look at your existing window sill, both from the inside and outside.

Profiling the sill nose and other finishing touches can be done AFTER we remove the old window sill but the two most important parts to know BEFORE removing the window sill are the angle and thickness of your window sill.

Other than judging the condition of the window sill, it is very difficult to accurately gauge the sill angle just by looking at it so we will need some way to measure the sill angle.

We will obtain the sill thickness by measuring the exterior sill nose as well as from the inside by removing the window stool and apron.



  • 2″ X 6″ kiln dried lumber (or other sizes depending on your needs)
  • 1 lb box of 2 1/2″ Deckmate screws
  • Wood or composite shims
  • Loose insulation
  • Window caulk
  • 1″ finishing nails


  • Tape measure
  • Table saw or circular saw (additional setup required)
  • Inclinometer (to measure angles, you can buy or make one)
  • 4″ metal drywall spatula
  • Wonderbar / crawbar
  • Claw hammer
  • Mash hammer
  • Router
  • Jig saw


Measuring an Old Window Sill – Thickness

Step 1: Take your tape measure, go outside and take a look the bottom part of the window.  You should be able to see a protruding wood trim.  Take a measurement of the window sill nose.





Remove Interior Window Trim

Step 2: Break the paint seal (fig.3.1).  Use a sharp utility knife to score between the quarter round- and side trim moldings.

Step 3: Separate two moldings (fig.3.2).  Using a hammer, gently tap the 4″ metal drywall spatula between the quarter round and side trim moldings.  Using a light rocking side-to-side, then up-and-down motions, gently create a small gap between the quarter round and side trim moldings.  Then find and mark finishing nails by sliding up and down with the spatula in between the gap.

Step 4:  Using a flat tipped craw bar (i.e. Wonderbar), gently pry off the nails, making sure to work all nail points little bit at a time (fig.3.3).  Of course if you don’t plan on re-using the quarter round molding, you can be less careful and just pry them off. Repeat these steps for the other quarter round molding.




Step 6:  Now we need to remove the side trim moldings.  We normally do not remove these moldings when installing replacement windows but we need clear access to the window stool so we have to get it out. Use the same method as before to gently pry off the side moldings.  In order to remove the window without damaging the drywall, use the 4″ metal spatula as a backerboard against the Wonderbar.

Step 7: To remove the window stool, use a 3 lb mash hammer to gently tap the underside of the window stool (fig.4.2). A claw hammer is not recommended because you must strike it harder and it has a less surface area so it will damage the wood surface. Again, if you don’t plan on saving it, whack it away!

Step 8: Use the same method to remove the window apron (fig.4.4)

Please note that some window pictures may show drywall damages. That’s because I could not find ideal pictures from the same project so I had to “borrow” pictures from my kitchen remodeling project.

Step 9: Take the measurement. It should be approximately 1.5″ thick (fig.4.6)








Measuring an Old Window Sill – Width

Step 10: Now that the window sill is exposed inside, its a simple 2 step process to measure the width. From the inside, take the measurement from end-to-end (fig.5.1). Pay special attention to the side jamb and window sill. My side jamb was notched at an angle to accept the window sill (fig.5.2).
Step 11: From the outside, take the measurement from end-to-end.




That’s it for measuring your window sill. It’s a lot of work but these steps will ensure that your replacement will is of correct height, angle and depth.

  • NEXT >> How To Make Your Own Window Sill – Part 2




  • Trackbacks

    Leave a Reply

    Your email address will not be published. Required fields are marked *