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Stool and Apron Extensions Window stool is applied directly to the jamb and serves as the jamb extension as well as the sill, so it must be cut to fit before the extension legs and head can be installed. This work must be done on site, but production techniques can speed installation. As with every piece of repetitive door and window trim, first measure and cut all the stool to width and length, then scatter the pieces to every window, along with precut extension legs and heads. Next scribe and cut each piece of stool, and test the fit. After all of the stool is cut, fasten the extension jambs to the stool, then attach the complete frame to the window. This method minimizes the number of times I change tools, and dramatically speeds an otherwise slow and expensive process. The stool has to be long enough to catch the ends of the casings, but finding the dimension is easy arithmetic. First, double the width of the casing; then add twice the casing reveal on the jamb plus twice the casing reveal at the end of the stool. Now add that total to the inside dimension of the window jamb. For example, a 3-1/2-inch-wide casing with 1/4-inch reveals at both the jamb and stool requires a piece of stool that is 8 inches longer than the inside jamb measurement. Scribing the stool to fit isn’t too difficult, either. After cutting the stool to length, measure in at each end and make a mark at the width of the casing plus the jamb and stool reveals (4 inches for 3-1/2-inch casing with 1/4-inch reveals). Then hold the stool against the finished wall and align the marks with the inside edges of the window jamb (Figure 5).

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Figure 5. On windows trimmed with a stool and apron, the stool horns must be long enough to catch the casings (top). If the horns need to be scribed for a tight fit against the wall (middle), the author creates a slight back-cut using a panel saw (bottom).

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While still holding the stool in place, get ready to scribe the horns so they’ll fit tightly against the walls and the windowjamb. First, take a quick measurement to verify that the stool and jamb are parallel. If they’re not, slip a small behind one end of the stool to correct the problem. Then spread your scribes the distance between the stool and the jamb, and scribe the horns from the finished wall. This scribing technique works well for bullnosed returns, too, and always results in a tight fit. I use a small panel saw to cut scribes because the combination of high rpm and a small blade help in making fine, slightly-curved cuts. I keep the blade square to the stock at the start of the cut, so the end of the stool will meet the wall square, but I back-bevel the inner portion of the notch so that the piece will fit tightly against the wall without any struggle. I finish the inside corner of the cut with a back saw, also back-beveling slightly.