How to Frame Existing Basement Windows

Measure the window opening's top edge and subtract one-quarter inch from this measurement. This leaves a one-eighth-inch reveal (a visible portion of the window's wooden edge) around the window.

Set a miter saw to a left-hand 45-degree miter cut. Cut about 1 inch off the end of a frame piece. Transfer the measurement from the previous step to this frame piece. Measure along the piece's bottom edge from the mitered end and make a mark. Swivel the miter saw blade to a right-hand 45-degree miter cut, slide the frame piece down and cut through the mark.

Lift the frame piece over the window. Leave a one-eighth-inch reveal along the window's top edge. Set a level on the frame piece and adjust it as necessary. Trace along the piece's bottom edge with a pencil.

Lay the frame piece upside down on a flat surface. Place a tube of construction adhesive into a caulk gun and remove the tip with a utility knife. Apply a wiggly bead of the adhesive along the piece's back.

Lift the piece back over the window and line up the bottom edge with the pencil line you drew. Press the piece firmly in place. Drive three or four finishing nails through the frame piece and into the window's wooden edge to help hold the piece in place.

Measure the window's height on both the left and right sides. Miter two new frame pieces' ends, one with a right-hand and one with a left-hand 45-degree miter. Transfer the measurements from the previous step onto the two pieces, measuring along the inside edges. Miter the two boards' ends to 45-degree angles.

Hold the frame piece with the right-hand 45-degree angle cut on the top edge against the window's left side. Press the mitered end together with the top piece's mitered end to form a 90-degree angle. Hold a level against the frame piece and adjust it as necessary. Trace a line down the piece's inner edge. Repeat the earlier steps to mount the frame piece in place. Repeat these steps with the second frame piece against the window's right-hand side.

Measure the window's bottom edge. Miter a left-hand 45-degree angle on a new frame piece's end. Measure along the piece's upper edge to transfer the measurement to the frame piece. Test that the piece fits properly between the two side pieces, then secure it in place the same way you secured the three previous frame pieces.