If the bottom falls out of a drawer, it's usually because the drawer itself has become unglued. The bottom typically fits into grooves cut into the sides of the drawer, and when the sides start separating, there's nothing to hold the bottom piece in place. The most secure way to repair the drawer is to take it apart and re-glue it.
Before you take a drawer apart, it's good to know what to expect when reassembling it. The drawers in higher-quality desks and cabinets often have dovetail joints; these rarely come apart, and when they do, you simply need to fit them back together with fresh glue.
Less expensive drawers made from softwood, plywood or particle board are often screwed together, and these are the ones that most often separate. Disassembling them involves removing screws, but it's an unchallenging procedure. Problems arise, though, if the wood or particle board has split or chipped out -- then you may have to use wood filler.
Disassembling and Reassembling a Drawer
Before removing the drawer for repair, take everything out of it. Removal may involve freeing a catch on the glides on either side of the drawer; do this by lifting a lever on each glide. If the drawer slides along a bottom glide, you usually have to tilt it up to free it from the glide.
Set the drawer on a flat surface and examine the joints holing the sides to the front and back. If you see screws, remove them using an appropriate screwdriver. If you don't see screw, the joints are glued together. Knock each joint apart, using a rubber mallet.
Scrape dried glue off each joint, using a sharp chisel. Do not use water to soften the glue; it will swell the wood and make reassembly difficult.
Repair split wood or chipped-out particle board, using epoxy wood filler. Mix the filler with hardener, according to the instructions on the container, and apply it with a putty knife. Use the knife to mold the filler around corners and edges that have broken away. Let filler set completely before proceeding.
Assemble the sides and bottom of the drawer without glue to ensure that everything fits. Slide the bottom into the grooves and ensure that it fits securely, and then take it out. Remove each of the sides in turn and spread carpenter's glue on the back end, and then reassemble the drawer.
If the drawer has dovetail joints, you may have to tap the joints together with a rubber mallet.
Put a small amount of glue in each of the grooves on the drawer's sides and back, and then slide the bottom into the grooves. Apply glue to the front end of each side piece, and assemble the front of the drawer. If the drawer has held together by screws, replace each of the screws. Otherwise, secure the drawer with bar clamps to hold it tightly together while the glue sets.