Hiding the Window Frame Within the Wall

Run a straightedge across the window frame against the surface of the wall while watching as the straightedge touches the frame. Look for any lifting of the straightedge over the frame, which would indicate that the frame extends over the surface of the sidewalls. Remove any excess frame material with a hand planer. Push the blade of the planer along the frame surface to remove thin slivers of the frame until its flush with the wall surface. If the frame is lower than the wall surface, then place pieces of wood shims against the frame to make a level surface from the frame to the wall. Attach the shims to the frame using 4d finishing nails and a hammer.

Measure the height and width of the base of the frame with a tape measure for the placement of the window stool, which is the wooden sill that runs along the length of the frame over the window opening. Pencil the measurements onto the bottom of the stool you wish to use. Mark two cutting lines onto the stool using the penciled measures. The first cutting line will run along the stool length, while the second marks the height of the frame on the rear of the stool. Use a router with a rabbet bit to cut out the marked section.

Return the stool to the window and place it in front of the window opening, centered across the opening. Mark the edges of the inside of the window opening on the stool width with a pencil. Cut the stool along the marked line with a circular saw from the rear of the stool towards the front until you reach the point of the rabbet cut running the length of the stool on the base. Make two cuts from the outside edges of the stool to the two new cuts. The result should be a window stool piece that fits through the opening at the center right up to the base of the frame with the top of the stool level to the top of the base. The sides of the stool should sit flush with the window opening sides in the center, and with the front of the wall at the edges. Apply a layer of wood glue to the base of the stool with a paintbrush. Set the stool in place along the bottom of the window opening, and then nail it to the sill using 4d finishing nails.

Measure the height of the window frame from the top of the frame base to the top of the window opening. Cut two side trim casings to match the height. Measure the width of the window and cut the header casing to match the window width.

Apply a layer of wood glue to the rear of the side trim pieces and then set them over the frame’s side, covering the joint between the frame and the wall flush with the stool. Nail the trim into the frame with the 4d finishing nails, and then into the wall beside the frame with 8d finish nails. Nail upward through the stool into the trim bottom with 6d nails.

Glue the head casing to the wall above the frame, concealing the frame edge. Nail the casing to the frame with 4d finish nails and into the wall with 8d finishing nails.

Place the corner blocks at the corners between the head casing and the side casings. Plane the edges to adjust them if needed to create a gap-free fit on both sides to the casings. Glue the edges of the block that will touch the casings and the rear, then press the block to the wall. Drive 8d finishing nails through the blocks at each corner into the wall.

Cut a piece of apron trim matching the length of the window plus the side casings. Glue the trim to the wall and the base of the stool. Nail it into place with 8d finishing nails.

Place a nail set over each nail used and tap it with a hammer to drive the nails beneath the surfaces of the trim. Fill the nail holes with wood putty using a putty knife, allow it to dry overnight, then sand the putty smooth with 120-grit sandpaper. Use the sandpaper to sand down the sides of the corner blocks, apron and stool as well. Wipe the sanded wood clean with a tack cloth.