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Site-built extensions. Either because of job-site complications or forgetfulness, I often mill my own extensions from S4S stock. For paint-grade windows, I use finger-jointed pine. While it’s tempting to use MDF because it’s readily available, mills easily, and paints beautifully, I think MDF is the wrong material for jamb extensions. Fastening through the face of MDF material works well, but nails or screws driven into the edge without predrilling cause flaking and splitting (Figure 2).

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Figure 2. MDF is not a good material to use for extension jambs. While it mills easily and takes paints well, it tends to split when screwed or nailed through the edge.

Since jamb extensions are all about endgrain — extensions are fastened to the jamb through the edge, then the casing is fastened to the extension by nailing into the same edge — it’s best to use real wood. If the job is being stained, pick material that matches the grain pattern of the window jambs.

Milling jamb extensions.

I prefer to mill extensions in my small shop because it’s easier and faster, though I often cut them on site, too. Either way, to increase productivity and save installation time, I measure all window and door extensions before I’m called to install the finish work so that all the material is on the job the first day of work. I purchase stock in widths and lengths that result in the least amount of waste, though I try to avoid ripping more than two extensions from one piece of stock. This reduces milling time and eliminates the need for a surface planer, because I can use the factory edge on the room side. A table saw equipped with a fine-tooth carbide blade makes a smooth enough cut for the inside edge, which is butted against the window jamb. If the extensions are thin, however, I often rip more than two out of a single piece of stock, then pass them through my portable surface planer. I also ease the exposed corner of all extensions with a table-mounted router or hand-held laminate trimmer using a 3/16-inch roundover bit. If you don’t own all those tools, buy the narrowest stock you can find and anticipate a little more waste. Tongues and back-rabbets can be milled quickly with only a table saw. The tongue for an Andersen extension requires several passes, though I often eliminate the tongue for thin extensions, and apply them flat, directly on top of the dado. Back-rabbets can be made in two passes (Figure 3).
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Figure 3. A back-rabbeted extension can be milled on site in two passes with a table saw. The bevel cut begins at the corner of the stock and meets the shoulder cut about 1 inch from the window-side edge. Back-rabbeted extensions can be fastened with a nail gun before the window is installed. The shoulder cut for a back-rabbet should be made about 1 inch from the window-side edge of the stock; the bevel cut begins at the corner and angles to meet the shoulder cut. If the extension can be applied before the window is installed, using a nailgun to fasten the extensions is faster than screws.

Picture Frame Extensions

Most of the jamb extensions I apply are less than 1 inch wide to make up for the thickness of shear paneling that was forgotten when the windows were ordered. For narrow extensions, I measure all the windows on the job, precut the pieces to length using a repetitive stop on my chopsaw, and pin-nail them to the jambs. To find the width of the extensions, I hold a block of wood or a square flat against the wall and measure from the window jamb, then I add 1/16 inch to make it easier to install the casing. I take several measurements on each window, especially at the head and sill, where drywall tends to thicken, then I average the measurements. If the difference is more than 3/16 inch, I make custom rips. When extensions are wider than 1 inch, however, I preassemble them in my shop, where I can work faster on a large waist-high work surface with all the necessary tools in easy reach. I fasten the corners with screws (adding glue won’t hurt, but it’s slower), just like the head and legs of a jamb, so that the joints will never spread (Figure 4). Figure 4. The author preassembles picture-framed extensions in his shop. All four corners are screwed to the keep the joints tight. I approach picture-frame windows differently from windows with stool and apron. Because I preassemble the frames in my shop, I measure for picture-frame jamb extensions while I’m figuring the material takeoff. It’s faster than it sounds, because no matter how many windows a house has, most are the same size. Measure the inside dimension of the jamb and add twice the reveal — 3/8 inch for a 3/16-inch reveal, 1/2 inch for a 1/4-inch reveal. Sometimes, the extension frame has to slip over window stops that are proud of the jamb. In that case, measure outside to outside between stops and add 1/16 inch for clearance. My extension jamb takeoff is usually a short list of window sizes with slash marks for each frame I’ll need. Occasionally, a window size will be listed twice, with different extension widths.