Stacking Jamb Extensions Doubling up jamb extensions is another way to trim thick walls. Double extension frames, or stacked extensions, can be faster than wide, one-piece extensions because double frames are easy to preassemble from square-cut stock, and they can be installed in stacks, one after the other, using a nail gun. For double extensions, use smaller reveals, which allow more wood for fastening. Measure for the first frame just as for any other window, then add 3/8 inch for the second layer, which leaves a 3/16-inch reveal. To speed up installation, I use a reveal block — a 3/4x3-inch square piece of wood with a 3/16-inch-deep rabbet all the way around — which can be positioned quickly in each corner, in any direction (Figure 8).

Figure 8. To achieve a consistent reveal when installing stacked extension frames, the author uses a wood block milled on all four edges.

I cut and assemble the frames in my shop, then fasten them to the window jamb, one on top of the other. If the second extension is 1 inch thick or less, I attach the pieces one at a time.

Scribing for Irregular Walls

Framing isn’t always plumb, and occasionally a window or door jamb can’t be set parallel or in plane with a wall. Some trim carpenters install extensions proud of the wall, then use a portable power plane to cut them almost flush to the wall. If the drywall is installed, a 1/16-inch-thick spacer taped to bottom of the plane prevents planing the drywall and ruining the blades. But extensions can also be scribed. I use two methods: One is fast and crude; the other, slow but perfect. For apartments and tract housing, I position each piece of the frame against the jamb, then lay my pencil against the drywall and scribe a line on the back of the extension (Figure 9).

	Figure 9. When the window is out of plane with the wall, paint-grade extensions can be individually scribed (top), then ripped with a panel saw and sanded (bottom). For stain-grade extensions, however, the author prefers to measure both ends of each piece to make sure the ends mate perfectly.

I use my panel saw to cut along the scribed line, then sand the edge near the reveal line and ease it with my laminate trimmer and a roundover bit. I take a little more time in custom homes, particularly with stain-grade material. Before assembling the frame, I measure and find the widest point of the jamb extension. Starting from that corner of the jamb, I mark and then measure both ends of each extension, so that the mating ends are exactly the same dimension. I rip the pieces with my panel saw, or freehand on a table saw, then sand and ease the edges before assembling the frame. With either method, always apply the apron under the stool last. That final detail is the easiest and yet the most gratifying part of the whole job. Although using screws eliminates the need for shims, on especially wide extension jambs, shims are still a good way to prevent the jambs from splaying and the joints from spreading. It’s good practice to place a shim on both sides of each corner, and at the center of the opening on all sides. Also, while everything is open, it only takes a second to stick a couple of shims between the sill and the stool to support the weight of exploring children. I don’t like the spongy quality of packaged shims, so I make my own. Wide MDF shims are easy to cut on a chop saw, and they chisel off easily. But for most applications, I use a sled on my table saw to crosscut scraps of 2x6 into endgrain shims. The sled is an 8x12-inch scrap of 3/8- or 1/2-inch plywood with an angled notch cut into one end. The shoulder of the notch is 5/16 inch deep, and sets the thickness of the butt end of the shim. The angled cut is 53/8 inches long, just shy of the width of a 2x6. A large block of wood screwed to the back of the sled serves as a handle to keep my hand far away from the blade. To cut shims, slide the saw fence over until the sled is just beside the blade. Then, hold a short length of 2x6 in the sled’s notch and push both through the table saw blade. When the shim falls off onto the sled or behind the blade, slide the sled back for the next cut. I cut shims until the angle on the 2x6 gets too steep, then I turn the piece of 2x6 around and cut in the other direction. The author prefers to make his own endgrain shims, which are strong but easy to snap off. He crosscuts 2x6 scraps using a sled made out of plywood with a tapered notch cut along one side. When the angle on the 2x6 gets too steep, he turns it around and cuts from the other end.Gary Katz is a finish carpenter and writer in Reseda, Calif.