Alcoves can be ideal places to mount some shelves as their structure offers ready support. However, the limited space can feel crowded once you install the shelves and their support brackets. One solution to this is to install floating shelves -- shelves that hide the support structure inside the hollow casing of the shelving itself. Floating shelves are a more complex and demanding do-it-yourself project than regular shelves. If you're doing it for the first time, consider getting an experienced helper to assist you with your first shelf.
Measure and cut
Measure the width and depth of your alcove.
Cut one pine beam to a length equal to the width of your alcove. Cut the other two beams to a length equal to its depth, minus 3.7 cm (1 1/2 inches). An alcove 90 cm (36 inches) wide and 40 cm (24 inches) deep would require one beam 90 cm (36 inches) long and two beams 56.25 cm (22 1/2 inches) long.
Cut two furring strips to match the depth of the alcove, minus 3.7 cm (1 1/2 inches). Cut the other two strips to match the width, minus 12.5 cm (5 inches). The alcove above would need two furring strips 56.25 cm (22 1/2 inches) long and two furring strips 77.5 cm (31 inches) long.
Cut the two plywood sheets so that both exactly match the width and depth of your alcove.
Sand all faces of the timber until they're smooth to the touch.
Build the shelf
Set the four furring strips in a rectangular frame. The ends of the width-cut strips should abut the inside faces of the depth-cut strips. All strips should rest on their narrow, 2.5 cm (1 inch) wide edges.
Glue one corner in place and reinforce it with two nails from a nail gun. Wipe off the excess glue and allow it to dry.
Repeat Step 2 for the other three corners of the frame. You may go quicker if you work on other corners while the glue dries on the first one, but the dried glue can add some stability that will make the job easier.
Set one plywood sheet in place on top of the frame. One of its width-cut edges should align evenly with the outside edge of one width-cut furring strip. The other three edges should overlap the other strips by 3/7 cm (1 1/2 inches).
Glue the sheet in place along the tops of the furring strips. Reinforce it with one nail in each corner and one nail for every 7.5 cm (3 inches) along each furring strip. Wipe off the excess glue and allow it to dry.
Turn the shelf over and repeat Steps 4 and 5 to attach the remaining plywood sheet to the open face of the frame.
Paint the plywood and visible side of the front furring strip. For best results, use long and even strokes in the same direction as the grain of the timber. Allow it to dry. You may need multiple coats to get the colour you want.
Mount the shelves
Use a stud finder to locate the studs in all three walls of your alcove. Mark their locations with a pencil.
Draw a line on the back wall of the alcove at the height you want the shelf. Use a level as a straightedge and to make sure the line is level. Do the same for both side walls.
Set the width-cut beam against the back wall with its top edge lying along the pencil line. Screw it in place with one 10 cm (4 inch) wood screw for each stud it crosses.
Install the depth-cut beams on the side walls using the same method you did in Step 3.
Slide the shelf frame over the beams you've attached to the walls. The three open sides should fit over them snugly.
Screw the shelf in place with two 2.5 cm (1 inch) wood screws evenly spaced along each of the three beams. Screw through the top of high-mounted shelves and through the bottom of low-mounted shelves.
These plans call for plywood and pine timber, the cheapest and most readily available wood. If you want higher-end shelves, you can use the same plan but replace the visible timber with more expensive wood.