Gilles' Outlet

January 7, 2008

Window Treatment: Installing Stool and Apron

Filed under: Finish Carpentry — Gilles @ 2:48 am

Stool, Apron and Casing are added to a window.


Skill Level: 3 (Moderate)

Time Taken: About 10 hours

A very blend looking window is enhanced with stool, apron and new casing. Finish carpentry is very rewarding because results are highly visible. It is useful to start by defining a few terms used in this article:


  • Stool: horizontal piece at the bottom of the window opening. Non carpenters call this the "sill". A sill can end being flush with the wall or can extend past the wall for added visual effect,
  • Horn: piece of the stool which extend past the window opening,
  • Apron: piece of molding installed under the stool.

Left: the subject of this article. A fairly new energy efficient vinyl window. While the window is sound, it looks very blend. The existing casing and blind have already been removed. The blind’s mounting brackets are still visible at the top of the window.

I was amazed by the poor craftsmanship: wall texture was sprayed all around the window frame, casing was installed very poorly, paint job was mediocre at best…

Right: a detail of the casing before I removed it. It does not take an expert eye to notice that this window is in desperate need of help: casing does not sit flat on the wall, material end grain shows…

End grain absorbs paint and stain differently than other parts of the board and therefore looks bad when exposed. There are various techniques to hide end grain and a few will be showcased in this article.

In finish carpentry, attention to details is what make results look professional.

The window jamb extension (commonly referred to as "sill" by non carpenters) needs to be removed so the new stool / apron system can be installed.

Left: I cut trough the caulk with a sharp utility knife all around the sill, especially around the window.


Right: The drywall joint compound was cut to expose the side of the sill. This tends to dull the blade very quickly so an old blade was installed prior this operation.

At this point, I am concerned about exposing the edge of the sill so I can pull it out. However , it is critical to be careful not to damage more drywall than necessary. Drywall patching is no fun, especially if the wall was textured.

Left: detail of the left corner after exposing the sill. The sill actually goes under another board (jamb extension) so I was careful to expose the whole sill.

Right: detail of the right corner. The sill also went under the jamb extension so the edge was exposed.

A detail of importance: the left side of the window is almost butted against a wall (see left picture). We will not need to hide end grain on this side of the window but the casing will have to be ripped to fit.

Left: After exposing the edge of the sill, I was able to insert the flat end of a pry bar under the sill. I lifted up gently, just enough to loosen the nails and create a space of about 1/8” in between the framing and the sill. I repeated the operation all along the sill. A little bit of damage to the cut drywall edge is inevitable, but it is important to limit it as much as possible.

Right: I cut all nails holding the sill to the framing with a reciprocating saw equipped with a metal cutting blade.

Left: I gently inserted a mechanic’s bent pry bar between the window frame and the sill and pried very gently. The sill moved toward me. I repeated this procedure all around the window frame.

It is important to pry gently because this operation can actually pry the window off the building. This would have very dire consequences including but not limited to allowing water in the building. Besides, re-installing the window just for an apron/stool addition is something I would like to avoid.

Right: After a few minutes of gently prying, I was able to move the sill forward so I could grab its edge with a pair of channel lock pliers. The sill swung out of the opening. 

Notice the very rough texture on the sill I discussed before. Clearly the texture sprayer team had a lot of liquor for lunch and decided that millwork did not need masking.

After removing the sill, I used a hammer to pound all cut nails into the framing. I also cleaned up the cut of the drywall where the sill used to sit, especially in corners. This concluded the demolition (or more precisely "deconstruction") aspect of this project.

Left: The opening ready to receive stool and apron. The old paint / texture on windows extensions have been scraped down to the bare wood. The wood has been sanded smooth with 200 grit sand paper. 

The wall has received several coats of fresh paint. I like to paint the wall before installing stool and apron because it easier to paint without millwork in the way. However, it is not always possible to use this approach.

Right: I measured the length of the apron, from the wall on the left side to the end of the notch on the right. It happened to be exactly 5′.

I want the new stool to slide under the left and right jamb as it used to.This forces me to use 1/2” thick material. I decided the new stool will be made out of 1/2” x 6 (nominal) hemlock. I explicitly did not use MDF (even though less expensive than hemlock) because MDF is notorious for having very fragile corners and I need the new stool to withstand abuses on all its corners.

Usually, stools stand proud of the window casing by about 3/4”. In my case, it so happened that the casing thickness plus the width of the sill plus 3/4” ended up being exactly 5” 1/2, which is the actual width of a 1x 6 nominal board. If this was not the case, I would have ripped a board to width on a table saw.

The length of the stool is usually computed by the following formula: Window Opening Length + 2 * Casing Width + 2 * Apron Projection. Usually, 3/4” is considered good for apron projection and I am using 2” 1/2 wide casing. However, in my case there will only be one projection (on the right) because the left side is flush with the wall. I decided to cut the stool as if there were two projections instead of one: I can always trim it later.

I made a trip to the miter saw and cut a piece of 1/2” x 6 nominal stock to 67” long. As explained, it is longer than hat I really need.

Left: I dry-fitted the board in the opening. The board slid under the left side jamb extension as planned and came flush with the wall.

Dry fitting may seem a little overkill but it is actually part of the process of marking where the board needs to be notched for horns.

Right: While the board was held in place firmly, I marked the right side notch with a pencil.

In carpentry, it is better to mark locations of notches or other features instead of measuring them. That eliminates potential errors and ensures a tight fit.

Left: I set the steel ruler of a combination square so it would touch the window and I adjusted the moving square to capture the distance between the wall and the window. I then used the square as is to report that distance on the board  (see right picture).

Right: The board with the right horn marked. the combination square allowed me to capture the distance between the side of the board and the vertical line (already cut) on the left.

The area in between the two pencil lines will be cut away. I first use a miter saw to make most of the cut and I finish the corner with a Japanese pull saw. This ensures a very neatly cut corners. It is also possible to cut this on a table saw but the cut is not as nice as a hand cut.

Left: I cut the horn and dry-fitted the stool again. You can see the horn extending on the right of the picture. The horn will be cut to length and a return will be installed.

A return allows to turn 90 degrees and not show end grain. This is one of those details which make the result look very professional.

Right: I marked for cutting the return on the stool. On the picture, from left to right: a 45 degrees miter. The end of this miter marks the end of the horn once installed. It is also marked by the vertical line in the middle of the two miters. There is another miter at 90 degrees from the first one. This is actually the return piece which will be used to hide end grain.

Left: The three cuts were made using a power miter saw. The left piece is the horn, the equilateral triangle in the middle of the picture is the waste and the small triangle on the right is the return.

Right: to complete the return, the waste is moved aside and the return is then turned 90 degrees counterclockwise. The end of the return shows the same grain as the face of the stool. this technique allows us to easily and effectively conceal end-grain.

Nowadays, only outstanding finish carpenters bother going through the lengths of cutting and fitting returns. Clearly, results are nowhere as clean without returns, especially with stain grade projects (projects where the wood is visible, in contrast with "paint grade" projects for which the paint covers many imperfections).

Left: The return gets glued to the horn with good quality interior carpenter glue. I did not nail the return with a brad nail to further prevent the miter to open up but it would be a good idea.



Right: I left the piece aside for the glue to set. The next day, I sanded the whole board lightly with 200 grit sand paper.

Left: I applied a coat of latex based primer on all sides of the board. It is a good idea to prime the back of the board even if you do not plan to paint the visible parts because the primer acts as a moisture barrier. It prevents moisture from being absorbed and makes the wood more dimensionally stable.

I also treated all visible sides of the board with two coats of good quality satin latex paint. Satin latex paint is a good choice for places which need to be cleaned often (think fingerprints removal) like casing or windows stools.

Right: I set the paint apron in place and checked for level. It just happened to sit firmly on the framing and be level.If it was not, I would have had to shim under it: a hassle.

Left: I pre-drilled for 15 gauge 8d (2” 1/2) finish nails. I decided to install 3 nails, equally spread over the 5′ distance. 


Right: I used hammer to drive nails home but I did not fully drive them. I left them protruding 1/8” above the stool. This is a trick to avoid marring the surface with the hammer.

When manually nailing a board, pre-drilling is almost always a good idea to prevent the board from splitting, especially when nailing close to the edge of the board.

Left: I used a punch chisel to drive nails about 1/16” below the surface the wood. The business end of the nail punch is flat and tends to slip on top of the nail. It worked but I had to be careful not to pinch my fingers.

A $1 nail set would be much better tool for this. I know I own one but I could not find it…

Right: I filled nail holes with wood putty and a small 3/4” putty knife. I used epoxy wood putty which cures very hard and greatly reduce risks of nail popping through later. This material is similar to auto body filler (Bondo Brand).

Left: I let the putty dry (the manufacturer recommended 1h) and lightly sanded the nail holes with 120 grit paper and then 200 grit. This created a few spots where I will need to touch up the paint.


Right: Now the caulk around the window also needs to be re-applied. I applied masking tape around the window to prepare for the final caulking.

Alert readers have noticed that window extensions were also given two fresh coats of good quality stain latex paint.

Left: I applied a generous bead of siliconized acrylic caulk. Pure acrylic caulk cures hard, becomes brittle and will crack as the wood and / or the vinyl expand and contract.

Right: I smoothed all beads of caulk. A wet finger is one of the best way to smooth a bead of siliconized acrylic caulk. Keep your finger wet and make sure you smooth it out in long, continuous strokes. Consider making one last pass with a very wet finger to give the bead a glazed look.

After all beads were smoothed, I immediately removed the masking tape.

The wet finger technique will not work with pure silicone caulk because silicone is not soluble in water. It works with siliconized acrylic caulk because water is a solvent for this kind of caulk.

At this point, we are left with cutting and installing the casing and the apron. In this article, I will focus on the apron. You can refer to this previous post on how to install casing (it is for a door but it is similar for windows).

Left: The apron will also need to be fitted with a return so I cut a miter on the apron out of a stock piece of 092 molding.

All moldings have a number so it is easy to refer to them in a unique way.

I hold the stock so the saw enters the visible part of the molding and exits at the back of the molding. This setup gives the cleanest cut on the visible edge because the saw rarely tears the material where it enters but has a tendency to tear the material where it exits.

Right: The resulting miter.

Left: I took another piece of scrap 092 molding and cut the opposite miter. the picture shows how the return will fit. I only need to trim the return to length.

Right: The return after trimming. It is a very small piece and cutting this on a miter saw can be tricky. Make sure your hands stay at least 1′ away from the blade at any time. Also, the miter saw has a tendency to send small pieces like this to the darkest corner of the room so be careful. Some carpenters make this cut with a hand saw, a much safer way of making this cut.

Left: The finished return held in place for the purpose of the demonstration. It will be glued before the molding gets installed.

Right: Detail of the return glued onto the piece of molding. With a little sanding and a coat of paint, no one will notice the little edge at the top.

The whole point of doing all this work was to cut a return so the end grain of this 092 MDF molding would not show and paint would be absorbed evenly even in corners.

Left:  After letting the glue cure overnight, I picked up the apron and positioned it tightly under the stool. I secured it in place with about five 23 gauge 1” 5/8 pin nails.

Pin nails are so thin, they leave a hole which usually does not need filling. A little bit of paint on top of it and the surface will appear perfect. It is true that they do not hold as strongly as regular nails, but they hold strong enough for apron / window casing.

Right: I finished installing the casing all around the window. I also used  23 gauge 1” 5/8 pin nails to secure the casing.

The finished product:

Left: Detail of the left side of the apron. I decided that the casing will butt against the wall. Essentially, this is visually equivalent to having the wall "cut" through the casing: a common practice in finish carpentry.

Right: Detail of the return on the apron. The end grain is concealed and the return contributes to the general look of the apron.

Notice how all millwork is tightly fitting against the wall: no big or uneven gaps. This is fairly different from the previously installed casing.

Left: Detail of the stool and casing on the left side of the window. The casing was also ripped to width on a table saw to accommodate for the wall and yet expose a reveal around the window jamb extensions.


Right: The apron and casing on the right side of the window. The horn is clearly visible. It does protrude more than 3/4” after the casing but this visual effect was appropriate for this very large room.

Tools Used:  

  • Utility Knife
  • Level
  • Pry Bar
  • Combination Square
  • Basic Carpentry Tools (Tape Measure, Pencil…)
  • reciprocating Saw w/ Metal Cutting Blade
  • Miter Saw
  • Power Sander
  • Pneumatic Micro Pinner

Materials Used:

  • 1/2” x 6 x 6 Clear Hemlock
  • Carpenter Glue
  • 100 and 200 Grit Sand Paper
  • 8d (2” 1/2) 15 gauge finish nails
  • Latex paint and Primer
  • 092 casing (Apron)
  • 019 Casing (Window Casing)
  • 23 gauge, 1” 5/8 pin nails


March 26, 2007

Installing door casing

Filed under: Finish Carpentry — Gilles @ 6:05 am

Door jambs are prepared. Molding is cut to length and installed.


Skill Level: 2 (Basic)

Time Taken: About Two Hours

Finish carpentry is very rewarding because results are highly visible.

Left: the door around which casing will be installed. The yellow substance around it is expanding insulating foam. It was allowed to cure over several days. It prevents cold air infiltrations around the door frame.


Right: the molding we will use. It is pre-primed MDF and was purchased at a local lumberyard for about $3 a piece.

Before installation, it received two coats of good quality satin latex paint. 

Left: close-up detail of the bottom left door jamb. I decided to install the molding all the way down to the plywood step at the bottom. The molding will need to be cut to clear the fondation.


Right: close-up detail of the bottom right door jamb. The molding will have to be cut on the right side too.

Left: I marked a 1/8” reveal (amount of the jamb edge that will be visible) using a combination square. This $15 tool is worth every penny.

It is only necessary to mark the top corners.

Right: I measured the width of the door opening. That is the inner distance between the left jamb to the right jamb. It ended up being 31 1/2”

I measured the width of the molding. It was 2 1/4”. I calculated the length of the top piece of casing along the longest side by:

L = Door Opening + (2 * Reveal) + (2 * Casing Width) = 31 1/2 + 1/4 +  4 1/2 = 36 1/4”

I always use the longest edge when miters are involved. It is much easier to hook up the tape measure to the longest edge of the piece. 

Left: I used a power miter saw to cut a 45 degrees miter at one end of a piece of molding.

I hooked up the tape measure to the longest end of the miter and marked 36 1/4”. I used the miter saw to cut the second miter to length.

It is easy to cut the miter on the wrong side so do not hesitate to make a sketch on a notepad before actually cutting.

Right: The top piece cut to length. It is exactly 36 1/4” on the longest edge, that is the top edge on the picture. 

I checked the drywall around the door to ensure it was flush with the door jamb. It was flush. 

If the drywall is not flush with the door jamb, there will be large gaps between the wall and the molding. When faced with this situation, I peel the drywall paper at high points and use a rasp to remove enough gypsum for the wall to be flush with the door jamb so the casing can rest flat on the wall and the door jamb. This kind of attention to detail is what makes the installation look "professional".

Some carpenters just caulk these gaps but the result does not look as good.

I positionned the top piece in place and made sure the reveal was the same everywhere. I used a pneumatic nailer to drive 1” brad nails (18 gauge) in the door jamb because they need to penetrate the door jamb to hold.

Nails are usually driven in a groove of the molding. This helps conceal the small hole left by the brad nailer.

I first secure a corner and then the other corner. I then drove a few more brads in between. That is approximately 4~5 brads total.

The top of the molding should be secured to the framing with 2” 16 gauge finish nails. This requires me to switch nailers so I’ll do it later.

Left: for the right piece of the casing, I cut a piece of molding 1” longer than needed.

I laid the necessary notches with a pencil and used a Japanese pull saw to notch the molding. A pull saw cuts on the pull stroke. It is easy to use and mine is fine toothed so it makes very clean cuts.

Right: The bottom of the molding after the cut was finished.

I prepared and cut the left side the same way.

I then positionned the left side and aligned its miter with the miter on the top piece. I checked the reveal was about the same over the whole length. I secured it using brad nails, starting with the top corner then the bottom. I added about 6 brad nails in between, spaced at regular intervals. I repeated the procedure for the right side of the door.

Left: I used a pneumatic finish nailer with 2” 16 gauge nails to secure the molding to the house framing. I drove about 4~5 finish nails equally spaced.

I repeated the procedure on the right side of the door. I also secured the top piece with about 3 finish nails.


Right: the finished door. Nail holes were filled with lightweight spackling paste. The paste was left to dry and paint touch ups were made after a light sanding.

It is also possible to fill nail holes with caulk, for instance DAP Alex Plus Siliconized Latex caulk.

Left: detail of the bottom left of the door with the molding installed. It fits snuggly around the fondation.


Right: detail of the bottom right of the door with the molding installed. It fits snuggly at the hinges and around the fondation.

Tools Used:  

  • Power Miter Saw
  • Pneumatic Brad Nailer
  • Pneumatic Finish Nailer 
  • Tape Measure 
  • Basic Carpentry Tools

Materials Used:

  • MDF Molding (3 x 7ft)
  • Finish Nails (2”, 16 gauge) 
  • Brad Nails (1”, 18 gauge)
  • Sating Latex Paint
  • Splackling Paste
  • 400 grit sand paper

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