Sagging drawers are incredibly frustrating to me. When I put something away in a drawer, I don't want to worry about how heavy it is, but a lot of furniture comes with flimsy hardboard bottoms in the drawers. You can ignore it until the problem eventually destroys the furniture, or you can repair it before it ruins the drawer frame and ensure that the furniture will last.
Step 1: Broken Dresser Drawer
My daughter's dresser drawers are always getting overstuffed with clothes. Between her own clothes and the endless stream of hand-me-downs she inherits, it was more than the cheap drawer bottoms could take. We bought this dresser from a store that rhymes with tortilla and although it's not the cheapest they make, the drawer bottoms are very flimsy. Like most flat-pack furniture, it came with 3mm hardboard for the drawer bottoms that fit into a dado groove in each of the drawer sides, face and back. The hardboard eventually pulled out of the groove and needed to be replaced, but I never like to do any repair a second time, so I upgraded to 1/4" plywood.
Step 2: Disassembling the Drawers
This step is pretty self explanatory, just disassemble the drawer the same way you put it together originally. You could find the instructions or just work through the parts and carefully disassemble everything without forcing it.
I will also mention that you have to take everything off of the drawer. I even take the dowels out of their holes so they don't get lost or damaged while I make my cuts later. Especially since I ran these pieces through my table saw, you really don't want any loose pieces that could cause the saw to jam in any way.
Step 3: Cutting a New Bottom
I bought a couple 2' x 4' sheets of 1/4" plywood to use for the replacement drawer bottoms. They are a little thicker than the original 3mm hardboard, but the store I went to doesn't sell plywood thinner than 1/4" in these small sheets and I don't think I would want it if they did. I laid out the original on top of a good corner of the plywood and traced it onto the plywood. You can use spring clamps to hold it in place while you trace the outline.
Step 4: Widen the Dadoes
This process takes a little time and a bit of trail and error to get the fence set just right. For this dresser, I can only widen the dado on the upper edge of the groove, I will show you why in the next step.
The sides of the drawer and the back have the dado the same distance from the bottom edge. I laid the dado of the first piece on the blade of the saw and adjusted the blade height so it started to pop the board up, then backed it off slightly. I then adjusted the fence so it left a little gap at the edge of the board. Start small and expect that you will make a few passes with the first board until you fine tune the cut to something that will fit the new drawer bottom. Because the dado only needs to be widened less than the width of a standard saw blade (no dado blades used), once the first board is ready, you can run the other drawer side and back of the drawer through the saw and they should be just like the first one.
Depending on the drawer you are working with, the drawer front may be more or less like the other 3 sides. For me, there was a larger distance between the dado and the edge of the board, so I had to start the dado process over again. I fit the dado over the saw blade and set the fence so there was a small gap at the edge of the board. Make sure to go slow and only take a little material with each pass. Make fine adjustments and don't rush or try to take too much material.
Step 5: Reassemble
Now you can see why I had to make the dado wider only on the upper edge of the groove. There is a flange on the drawer slide that wouldn't have matched up with the dado if I made it wider on the lower edge. The flange is a nice feature, because it will usually make the hardboard stay in the dado, but the hardboard is still just too thin.
When reassembling the drawer, the drawer bottom might be a little tighter than it was originally. You want to allow for expansion, but if the plywood is just a little tight in the dado it will firm up the drawer a bit and prevent it from racking when it is pulled from one side or the other. This is another reason I think this is a huge improvement over the hardboard. Once the bottom is replaced, the drawers feel much more solid.
Step 6: Finish
You might want to hit the plywood with a little sandpaper before putting the drawer back together because you might have some splinters otherwise. This drawer is nice and solid now and it can be the heavy duty drawer of the dresser. It can easily take all the clothes that are crammed into it and the bottom stays firm and in place in the dado.
Step 7: TV Cabinet
This was actually the prototype project for this process, but I didn't take enough pictures to document it. You can still see that the process was the same as for the dresser. These drawers at the bottom of the basement entertainment center get packed with all kinds of heavy stuff.
The drawer that gave out is where I store old batteries. Once I have enough, I bring them to the store that recycles them. Sometimes these little zipper bags of old batteries get pretty heavy.
I also store new batteries, and game controllers, and DVDs in this drawer, so the flimsy hardboard bottom never stood a chance. I replaced this about 7 months ago and have not had a problem with it since.
Step 8: Update: Additional Repair Methods
As a result of some of the comments I received, I decided to include some other ways to repair these drawer problems. Below are a few other fixes that would also work and might take fewer tools to complete.
This one looks simple enough and you could probably do it with a hand saw and a drill. It would firm up the drawer itself and lock the bottom in the dado groove.
This one will work on the front and back to back up drawer dadoes, but because this dresser's drawer slides have a bit of this built in for the sides, this won't work on the side pieces. I think you could probably use some pieces of quarter round molding or shoe molding as long as it isn't wider than the distance between the bottom of the drawer and the outer edge of the frame.
This one would make a very solid and permanent repair.