Window jambs make up the top, bottom and sides of the window like a frame. They hold the pieces of the window together that allow the window to be opened and closed. The jambs also mate with with the decorative molding that surrounds the inside of the window and the brick molding that protects the window from the elements on the outside. On the inside, extension jambs are often built to extend the window jamb to the interior wall surface so that the finished molding has something to be nailed to.
Temporarily place the window in the opening and make it flush with the outside sheathing. Measure the depth of the extension jambs between the finished wall and the existing jambs.
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Set up the table saw to cut the lumber to the width measured in Step 1. Cut all the jamb material in this step, using care to make smooth cuts.
Lightly sand the cut edges of the lumber to remove any burrs or bumps. The cut edge should fit tightly against the existing jambs without gaps.
Hold the lumber against a side jamb and mark it for cutting. Cross-cut the piece on the table saw with the good (visible) side up.
Hold the lumber against the side jamb again, aligning the ends. Mark the lumber and jamb for a biscuit about every six inches. With the biscuit cutter, cut biscuits slots in the jamb and in the lumber. Repeat steps 4 and 5 for the other side.
Dry fit the two side extension jambs to the window with biscuits in the slots. Then measure and cut the top and bottom jambs.
Hold the top extension jamb against the top jamb of the window. Mark the extension jamb, the window jamb and the two side jambs for biscuits. Cut the biscuit slots with the biscuit cutter. Repeat this step for the bottom extension jamb.
Remove the side extension jambs from the window and dry fit all four extension jambs together with biscuits. Test fit the extension jamb assembly to the window also using biscuits. Make any final adjustments before gluing together.
Remove the extension jamb assembly from the window and disassemble it. Pull all of the biscuits from the slots in both the window and the extension jamb pieces.
Apply glue to the biscuit slots that join the extension jamb pieces together. Apply glue to the matching biscuits and place them in the slots. Add additional glue to the edges that will join together and bring the pieces together snugly.
Apply glue to all the remaining slots and biscuits, and the edges that will join together. Join the pieces together and add clamps to hold the pieces tightly to the window and to each other, but do not over-tighten, as window damage may occur. Ise a damp rag to wipe up any glue that squeezes out and allow it to dry one to two hours. The window will be ready to install after cleaning up any remaining glue residue with 220 grit sandpaper.
Things You'll Need
3/4-inch-thick lumber for the extension jambs
Table saw with rip blade
Biscuit cutter and biscuits
Wood glue with brush
220 grit sandpaper and block
Mark the unseen edges of the window jamb and extension jamb pieces, instead of the visible parts. The biscuit cutter fence won't get in the way and you won't have marks to remove later.
Be ready to glue. You'll have to work quickly and get all the pieces glued and together before the glue begins to set, typically within 15 to 30 minutes. Have the clamps ready, but don't use them until all the pieces are glued and in place.
Most home centers and rental companies can rent you a biscuit cutter for a half day or full day.
For this project, #0 or #10 wood biscuit sizes are good; #20 size biscuits may be too long and deep.
Simply nailing the extension jambs in place is an option, but getting them to line up for a seamless installation is the hard part. Biscuits solve this problem.
Always wear eye protection and a dust mask when cutting wood. Always wear eye protection when using any kind of power tool.