No one wants to hear the sound of a tall bookcase filled with heavy tomes crashing to the floor. But in an earthquake zone, or in a home with children or pets, special precautions are needed to prevent bookcases from toppling. Since you generally place books at the front edge of the shelves, bookcases are naturally heaviest at the front, meaning that they are prone to tipping forward. A fully loaded bookcase can weigh several hundred pounds. The chances that you would be able to stop it from falling once it tipped are very small. Since bookcases are so heavy, you must attach them to studs, not to drywall, if they are to be truly safe.
Take all of the books out of the bookcase. On the back of the bookcase, draw a line to indicate the position of the underside of the top shelf of the bookcase. Next, mark the position of a fixed shelf near the middle of the bookcase. Move the bookcase away from the wall, leaving yourself space to work behind it.
Find the studs behind the bookcase using a stud finder. Studs are the vertical pieces of lumber used to frame the wall. They are generally made of wood. Drywall is attached to the studs with drywall screws, which are made of metal. Use a stud finder to find the screws holding the drywall, and that will tell you where the studs are located.
Mark the position of the studs on the wall with a pencil along the horizontal lines you drew to indicate the positions of the underside of the top shelf and a shelf in the middle of the bookcase.
Put the bookcase back in place. Using one 2 1/2-inch L-bracket for each stud along each shelf, mark with a pencil where you need holes to screw the L-brackets through the drywall and into the studs.
Drill 3/32-inch pilot holes at the marks you made to indicate where you will use screws to attach the L-brackets. Use an electric drill. Make the pilot hole about 1 1/4 inches deep to get through the dry wall and start the screw into the stud.
Attach the L-brackets to the studs using 2-inch #8 wood screws. Use an electric driver.
Put the bookcase back up against the wall. The L-brackets will be directly below the bottom of the top shelf and a shelf in the middle of the bookcase. Use a pencil to mark the position of the screws that will attach the L-brackets to the bookcase shelves. Drill 3/32-inch pilot holes in the shelves with an electric drill, then attach the L-brackets to the shelves using 3/4-inch #8 wood screws using an electric driver. Replace the books.
Attaching the bookcase to the floor, or putting the heaviest books on the bottom shelf will not prevent it from falling.