While most bookcases have a full back installed, sometimes you want to leave the back open. Since the back provides much of a bookcase’s stability, it needs bracing of another sort to make up for the missing back. Corner braces are the most common substitute.
Corner braces can be made inexpensively from 1/4-inch plywood; however, you can also buy ready-made L-shaped metal braces that are virtually invisible from the front of the bookcase. The bracing process is the same, except that the metal braces use screws; you will need to drill pilot holes before attaching them. This how-to article will explain how to make corner braces from plywood.
Place the bookcase face down on a flat surface.
Use the measuring tape to check the distance from one corner of the bookcase to the corner diagonally across from it. Write this number down if necessary.
Measure the diagonal distance between the two other, opposite corners of the bookcase. Make a note of this number as well.
Compare the two numbers. If the two numbers match, the bookcase is square; skip to step 8 and begin the bracing process. If they are not the same, the bookcase is out-of-square and must be adjusted.
Determine which of the two diagonals is longest. You will want to square the bookcase using those two corners.
Butt one of the corners against an immobile object, such as a wall, and gently push on the other corner with your hands.
Recheck the measurements across both diagonals of the bookcase. If they do not match, repeat steps 5 and 6 until they do. When both measurements are the same, your bookcase is square and you can begin bracing it.
Cut four right triangles from 1/4-inch plywood; these will be our braces. A right triangle has a 90-degree angle; the two sides that form that right angle should each be 3 inches long.
Place a brace on one corner of the bookcase and carefully nail through it into the edges of the case. Use four nails, two on each of the sides that form the right angle.
Repeat the step 9 on each of the remaining corners, using the other three braces.
While 3 inches is a good size for a typical brace, you can make them larger if your bookcase is very large or will be carrying a very heavy load. Use more nails if you make larger braces. You can use glue in addition to nails, if you desire.