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  Installing Traditional Stool and Apron  
Trimming a window correctly is one job that begins with good design and ends with proper technique, especially if the design is anything more elaborate than picture-framed casing. 
Stool forms the foundation for every piece of trim around a traditional window, so always begin with a dimensional drawing, even if it's a rough sketch, so you can calculate the length of the stool precisely.     To create a pleasing design, make the outside of the window casing plumb with the ends of the window apron beneath the stool.





O.D. Of The Casing

To find the length of the stool, start by determining the O.D. of the casing (O.D. = outside dimension; I.D. = inside dimension).  Measure the I.D. of the window jamb and add twice the width of the casing (include the width of the backband), and twice the jamb reveal--that's the amount of jamb which is exposed just before the inside edge of the casing.  The OD. of the casing is also the O.D. of the apron.

O.D. Of The Stool

To get the length of the stool, double the Stool & Cove Profile, then add that measurement to the O.D. of the casing. First add twice the thickness of the cove molding that projects past the edge of the apron--once for each end of the apron. If the stool has a bullnose edge, add the extension of the bullnose twice (that's the amount the bullnose extends beyond the bottom of the stool).  Finally, add the stool reveal.

The Stool Reveal

The stool reveal is the space between the cove molding and the bullnose on the front of the stool (see Fig. B).  On window stool that doesn't have bullnose, the reveal is the distance between the cove molding and the bottom edge of the stool (even stool without a bullnose profile isn't usually milled square but beveled about 5 degrees out toward the top). The reveal can be any number--1/4 in. to 3/4 in., though the larger the reveal, the deeper the stool.  Deeper stools are more attractive in spacious homes with tall ceilings and windows.  But the reveal should be consistent all the way around the stool--on the sides and across the front.  It's easy to maintain a consistent reveal because most stool is manufactured much wider than necessary.  The extra width is meant to be ripped off the back so that reveals can be consistent.  To determine the width of the stool pictured above, I added the following:

1/4 in. {reveal}  +  3/4 in.  {cove molding} +  3/4 in. {apron}  +  1 1/2 in. {the distance from the face of the wall to the sash--minus 1/8 in. so the stool will just clear the sash!}  =  3 1/4 in. = Width of Stool

Simple Arithmatic

Determining the length of the stool may seem confusing, but it's really simple arithmatic. Here's an example taken from the window pictured above, using Windsor One Greek Revival moldings.


[3 1/2 in. {casing} + 3/4 in. {backband} = 4 1/4 in.  + 1/4 in. jamb reveal  = 4 1/2 in. 

4 1/2 in. X  2 {both sides} = 9 in.

Width of window jamb = 35 1/2 in. + 9 in. = 44 1/2 in. =  O.D. of Casing and Length of Apron



3/4 cove molding    x   2  =  1 1/2 in.

3/4 in. Bullnose   x   2  =  1 1/2 in.

1/4 in. Stool reveal x   2  =  1/2 in.                3 1/2 in.  =  Addition for Stool & Cove Profile


Total Add for Casing & Stool/Cove = 12 1/2 in.



After ripping the stool (see Stool Reveal above), cut the stool to length.  To maintain the bullnose profile on the ends, cut outside-corner miters on both ends.  The length of the stool is measured from the longpoint or acute angle of the miters (see "Miter Terminology").
Next, cut self-return caps from a scrap of molding, one cap for the right end and one for the left.
Glue the caps to the sill...
and secure them temporarly with spring clamps.
That way, when you nail through the mitre, the self-return caps won't slide around and ruin the miter.
Nail through the long-point of the miter, too. 18 ga. 1 1/4 in. brads work well.
Temporarily secure the stool against the window opening.  Use a pair of scribes or a tape measure to make sure the stool is perfectly parallel to the sill.
Shim the stool away from the wall if it's closer to the sill at one end than the other.
Only then can you scribe the horns or ears.  If the stool is parallel to the sill before scribing, then once the ears are cut, the stool be will meet the still perfectly just as the ears touch the wall.
Use a small panel saw to cut the ears...
and a jig saw or handsaw.
Notice the second little scribe I made to follow the profile of the jamb.
That way the stool will lap over the jamb slightly and reach closer to the sash for a tight-weather-seal fit.
Fasten the stool to the sill with nails (more on fastening in a minute).
Install the jamb extension next (if it's needed) with 18ga. brads.
The casing can be mitered, glued, held with spring clamps, then nailed in place, but...
You'll get a tighter and more durable miter with a spline joint.  Be sure to clamp the material to a work bench.  Hold the router firmly against the casing, but don't push hard against the material or the router will rock and the joint won't be flush.
Use a 1/4 in. slot cutter in a router.  Mark an X on the bottom of the casing and make sure the X is always down.
A spline cut from 1/4 in. mdf will fit snugley in that slot, ensuring a perfectly flush joint.
Glue the slot before inserting the spline, and glue the shoulders of the joint, too.
Be sure to rip the spline 1/8 in. narrower than the combined depth of both slots, or the miter won't close up tightly.
Nail the casing to the jamb with 18 ga. brads; nail the casing to the wall with 15 ga. or 16 ga. 8d finish nails.
Install the backband next, and start with the head.  Measure the o.d. of the casing and cut the backband head to that dimension, from short-point to short-point (obtuse angles--see "Short-points and Long-Points" under Trim Techniques)
Glue and cross-nail the backband miters.
Use the same o.d.-of-casing dimension for the ogee molding, but measure to the long-points of the miters. Start with the head.
18ga. 1 in. brads are perfect for installing the ogee molding and will leave small nail holes.
Now the stool can be fastened up into the bottom of the casing, with nails or screws.
Install the apron next. Notice that the apron is cut with self-returns on both ends and the o.d. of the apron, from long-point to long-point, equals the o.d. of the casing.  The stool should also be nailed down into the apron.
The cove molding--from short-point to short-point--is cut to the same dimension as the apron.
The cove m olding self-return caps must be 3/4 in. long, from the short point to the butt cut. Be sure to cut these pieces off lengths that are long enough to hold safely against the miter saw fence (at least 8 in. long).
The cove self-return caps can be glued with hot glue or...
fastened with a 23ga. micro pinner.



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