The following is an excerpt from Gary Katz's website, GaryMKatz.com, on quick and accurate stool and apron installation. For a more comprehensive look at the details, please visit his website. The images below highlight our Greek Revival Style, but the instructions are applicable to all stool and apron installations.
THE O.D. OF 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.
Below, Traditional Bull-Nose Stool & Apron:
THE 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. 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.
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 the Greek Revival style.
O.D. OF CASING
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
ADD FOR STOOL & COVE PROFILE:
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.
TOTAL LENGTH OF STOOL = 48 inches
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... read more
Special thanks to Gary Katz for providing this photography & guidance. To learn more carpentry techniques check out THISisCarpentry.com