Skip to Content

Complete Guide to Making Your Own Window Sill


  • If a window sill is rotted, it needs to be replaced right away and this guide will shows you how to make your own exterior window sill
  • Also contains step-by-step instructions on installing a new window sill along with a replacement window

Table of Contents


Since posting the original article back in 2007, there more places selling a variety of window sills.

However, if you are unable to locate one to replace your rotted piece, you may be able to use the information in this post to make your own.

Please note that there seems to be some confusion between the words “window sill” and “window stool”:

  • Window Sill – primarily visible from the outside, an exterior window sill is angled to channel rain away from a window; this is commonly found in traditional, double-hung windows
  • Window Stool – if you are describing a window ledge or window bottom from the inside, a correct terminology is window stool; it is an interior trim work that is located inside your house, installed as a flat horizontal piece, along with an apron to provide structural stability as well as to hide any unfinished framing work

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.

Why Do We Need a Window Sill

The concept of a window sill has been around since Egyptian days.

In a modern residential setting, it is commonly found with double-hung windows to protect the house from water penetration as well as to provide a solid base frame to stabilize the window frame from flexing.


Window sill is required to attach the interior trim pieces (i.e. window stool and apron).

^ Return to top

Replacement Windows vs. New Construction Window

A replacement window installation requires you to remove all window parts EXCEPT the frame.

Fig 5.1 show below is an actual double-hung window frame that was removed for another project.

Fig 5.1 – Old Double-hung
Window Frame

When installing a new replacement window, the entire window is installed inside this existing window frame. In essence, you are installing a new window frame inside your old window frame, reducing the glass area.

If you want to see how I installed my replacement windows, read my post “Installing Replacement Windows

Because it is a drop-in installation, the outside siding and trim work is not disturbed.

However, if the window sill on an old frame is rotted, you will need to replace it before installing your new replacement window.

On the other hand, a new construction window will require you to remove the remove all parts of your existing window, including the window frame.

Unfortunately, that means you may have to re-work the exterior siding near the window. Re-working the exterior siding isn’t too difficult but definitely adds more work.

DIY or Hire a Contractor?

As a DIYer, I think the process of making your own window sill and installing it isn’t too difficult of a task as long as you have some basic carpentry skills and and are comfortable working with power tools.

The only specialized equipment would be the inclinometer to measure the sill angle and you can even use my inclinometer cardboard cutout template for free to take a simple measurement..

Few inconvenience things to keep in mind:

  • If replacing a window, you will need to remove the interior trim/molding pieces to take an accurate measurement
  • You will have to remove the entire window to gain access to the window sill (so have your replacement window on hand before starting this step)
  • You may encounter unforeseen challenges so you need to remain calm and flexible

I decided to do the work myself because many contractors wanted to charge over to $500 just to replace a single window sill (not including the price of a window).

^ Return to top

Anatomy of a Window Sill

Window sill is exposed to outside weather which means rain.

Believe it or not, capillary action (i.e. wicking) and surface tension allow water to flow up or cling to the underside of a surface.

Also known as a drip groove, channels cut on top and bottom of a window sill act as a stop to break this capillary action. Without the drip grooves , rain can potentially wick up to the siding (leaving unsightly stains) or worse, penetrate into the unprotected interior cavity (causing rot).

Channel grooves will pool water so it can be dripped off.

BTW, when you make a fresh cut, the new exposed area should be sealed off with a wood preservative.

Buy or Make a New Window Sill

Since I wrote my post back in 2007, many more stores are selling the exterior window sills.

I just did a quick search and saw that (external site does not allow direct linking; copy and paste the following; )

If you are going to buy one, keep in mind that the sill thickness impacts the overall window height so you need to make sure to match this thickness.

Of course, building your own window sill gives you maximum flexibility to match your existing sills.

Required Materials and Tools



^ Return to top


Length, Width and Thickness

  • STEP # 1 – Measure the window sill from the outside
    • Verify the thickness of the window sill by looking at the bottom part of the window from the outside
  • STEP # 2 – Remove the interior trim pieces to verify the thickness and length from inside
    • Using a sharp utility knife, lightly score the space between the quarter round and molding to “break” the paint seal
  • STEP # 3 – Remove the quarter-round molding
    • Using a hammer, gently squeeze in the 4″ metal putty knife between the quarter round and side moldings. 
    • Make sure to lightly rock it side-to-side, then up-and-down motions to create a small gap
    • Slide the putty knife up and down to locate the finishing nails
    • Use a craw bar to gently pry off nails (if you don’t plan on re-using the quarter round molding, you can just break it off)
  • STEP # 4 – Remove side moldings
    • Use the same process in STEP #3 to remove the side molding
  • STEP # 5 – Remove the window stool and apron and measure
    • Using a 3 pound hammer, gently tap the underside of the window stool to remove it
    • Again, if you plan on replacing it, you can just whack it away!
    • 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)
    • Although two pictures below have their windows removed, do not remove the old window until you have your replacement window and sill (or else you will have a large hole in the wall)

^ Return to top



Having an accurate window sill angle measurement ensures tight fit with windows and more importantly, minimizes water infiltration via wicking. Some replacement window manufacturers like Andersen has 3 sill angle choices so providing Andersen with accurate angle measurement will ensure good fit.

So what is the correct angle for a sill? In general, a window sill should have 3-12 or 3? by 12? slope or pitch for good rain drainage. 3-12 slope means that for ever 12 inches of horizontal distance (run), the vertical measurement (rise) is 3?.

Looking at Table 1.1, this corresponds to 14 degrees of slope. There are three easy ways of measuring this sill angle:

Method A – Inclinometer

An inclinometer or clinometer is a simple instrument for measuring angles, slopes or tilt. It is also known as a tilt meter, tilt indicator, slope alert, slope gauge, gradient meter, gradiometer, level gauge, level meter, declinometer, and pitch & roll indicator. Clinometers measure both inclines (positive slopes, as seen by an observer looking upwards) and declines (negative slopes, as seen by an observer looking downward).

In our case, using an inclinometer is the simplest and fastest way of measuring the angle of a window sill. You can buy one (see fig.6.1 and click here to read AllThumbsDIY inclinometer reviews and recommendations), use a paper template (fig.6.2 – click to download the PDF template) or a cardboard angle finder (fig.6.3). The last two are provided free by Andersen Windows (To obtain the one in fig.6.3, you have to visit one of their retail locations – call ahead to confirm that they have one in stock for free).

Please note that both mechanical or electronic inclinometers are far more accurate and can also be used in other settings (like when setting a blade angle on a table saw, etc.) whereas the paper angle finder is less accurate.

Method B – Torpedo Level and Two Rulers

Another method is to use a torpedo level and 2 small ruler sticks to measure the angle.

However, unlike an inclinometer, you *MAY* have to remove any exterior storm windows and storm window frames to get the necessary clearance to get the accurate measurement. Please note that if your window sill has 2 surface levels, the vertical ruler must rest on the uppermost level.

To calculate the appropriate angle, simply place 1 ruler horizontally (use a level) and read off the height from the second ruler.  You can then calculate the angle by looking at the table below.

RiseRunSill Angle
1/8 “2″4 degrees
1/4 “2″7 degrees
3/8 “2″11 degrees
1/2 “2″14 degrees

Method C – Framing Square and a Ruler

Another alternative method is to instead use a small framing square against the outside blind stop.

This way you don’t have to fuddle around with a level and 2 ruler sticks. However, you are assuming that your blind stop is truly plumb (vertical).

Once you obtain the measurement, please use the table above to calculate the sill angle.

^ Return to top

Review Dimensions

Before making the initial cut, let’s go over the dimensions one more time:

Length and Width

Making the Cut

Although the final length of my new window sill will be 40″, that is too short when working with power tools so I will be rough cutting it to approximately 6 feet.

This way, I can keep my fingers and hands safe from circular saw and router bits.

  • STEP # 1 – Double Check Your lumber piece
    • It absolutely has to be straight and flat
    • Cannot have any cups, warps or bends
  • STEP # 2 – Cut the Rear side
    • Mark the top side at 6 7/8″ and the bottom side at 6 7/16″. This will roughly correlate to 15 degree angle.
  • STEP # 3 – Cut the front nose
    • The front nose is a bit steeper at 10 degrees.
    • Mark the front top side at 1/4″ and make the cut using your circular saw
  • STEP # 4 – Cut the top groove
    • Mark at 1 7/8″ from the front side and use your router to cut the groove
  • STEP # 5 – Cut the bottom groove(s)
    • The “A” groove near the front is required
    • “B” groove is designed to rest the sill on top of the exterior wall; not absolutely required
    • “C” groove is designed to increase insulation cavity; not required
    • “D” groove is designed to rest the rear side of the sill on top of window frame
    • For B,C and D grooves, you will need to use “dovetail” router bit
  • STEP # 6 – Thickness test
    • The stock lumber is a bit thicker than my old window sill but since it was only 1/8″, I decided to leave it as is.
    • If you prefer, you can use a planer to thin it down
  • STEP # 7 – Apply wood preservative
    • Any freshly cut exposed area must be sealed with a wood preservative
    • Make sure apply it in a well-ventilated area

^ Return to top


If you need a refresher on how to remove a window, read my post “Installing Replacement Windows

  • STEP # 1 –Inspect the window frame
    • With your old window removed, thoroughly check the window frame perimeter for any rot
    • As show in Fig 7.4, I had a pretty bad rot on the left side of my window sill (where it met my garage roof line)
    • Pay special attention to side jambs
Fig 7.3
  • STEP # 2- Remove the sill
    • The window sill sits inside a dado groove on the left and right window jambs
    • Because the sill is attached to the side jamb via staples, I could not slide it out so I had to cut it in half
    • Quickest way to cut it out is to set the circular saw depth to 1/2″ and cut it in half
    • You can use a hand saw to do the same thing
  • STEP # 3- Remove any nail heads
    • The new sill will rest on the bottom frame so it needs to be free of any exposed nail heads and other debris
  • STEP # 4- Add window flashing tape
    • Apply windows flashing tape around the base frame
  • STEP # 5 – Add insulation and shims
    • Add some loose insulation before sliding in your new sill into the groove
    • install shims to three spots (about 2 inches away from left and right and center) and adjust the sill angle
    • You may have to cut away some sheetrock to install shims which will be covered by apron later
    • Once level and angle is correct, install a 2″ Deck Mate screw per shim location
    • Re-check to make sure that it is level

With a new window sill installed, you can now proceed to install your new window

^ Return to top


If you want to print these scaled drawings, for use, you need to make sure that they are printed in actual size. If you don’t know how to setup your printer to do that, read this article.

I hope you found this article to be useful for your DIY project and sign up for my newsletter. The signup form is found on the upper right hand corner of your screen.

Thanks and good luck!



Wednesday 9th of August 2023

I wanted to thank you for such a wonderful post!!! This was truly thorough and amazing. I ended up here because I am trying to make a window from scratch and I am trying to figure out what angle my window sill should have. Is a 32x 32 window for a barn/utility window. If you have the time to give me an idea or opinion I would appreciate it.