How can I prevent a drawer bottom from sagging?

Question

I am building drawers to finish a bed frame I built. The sides are made of 1x6 cedar (actual 0.75 by 5.5 inches, 19 x 140 mm). There are four drawers, each with exterior dimensions of 39 x 36 in (100 x 91 cm).

I hand cut dovetails to join the boxes and plan to rout the insides to hold the bottom, which is a fine laminated finish plywood about 3/16 inches (4.8 mm) thick. The 24-inch rails are rated at 50 pounds (20 kg) total, but I have to expect the bottom needs additional support for such a width and depth.

I was originally thinking some straight steel rods perhaps 1/4 inch (6.4 mm) in diameter placed under the bottom oriented front to back to prevent sag and relieve much of the load from the edges. There appears to be little supply of such materials at hardware stores except coarse threaded rods, which seem tacky. I imagined something stainless and smooth.

Woodworking sites such as woodweb, solowoodworker, woodsmithshop, and sawmillcreek mention several alternatives:

• Put the rails under the drawer with guides at the edges
• Install wooden cross bracing in the drawer
• Make the bottom thicker, like 1/2 inch
• Don't do anything special but do glue the bottom firmly to the box
• use narrower drawers instead

I can rule out smaller drawers as we want to store comforters and heavy blankets in there. Using more of the height for a thicker bottom I had initially ruled out, but the more I think about it, the more I am warming up to the idea.

If anyone has done a comparison of various bottom structures, I would love to know about it. Potentially, since I have four drawers, I could try four techniques to see what works out best.

Answer 1

You don't explicitly state your question in the post, but I'm going to answer assuming that it's along the lines of

What can I do for the drawer bottoms to reduce sag?

To address your points:

I was originally thinking some straight steel rods perhaps 1/4 inch (6.4 mm) in diameter placed under the bottom oriented front to back

I'm trying to think of a good way to mount these that doesn't end up taking up more depth than increasing the bottom's thickness, but nothing is coming to mind. Also, stainless rods are pretty expensive compared to thicker plywood (at least, in my experience).

Put the rails under the drawer with guides at the edges

This will work, but you might need a thicker bottom to mount your rails (here I'm assuming you're using metal ball-bearing type rails like one would buy at a big-box store, similar to the image below).

(source: Rockler)

Install wooden cross bracing in the drawer

This will help reduce sag, but can be a bit tedious to make it fit right. Personally, I think you're better off with the next solution.

Make the bottom thicker, like 1/2 inch

Usually, the quickest means of reducing drawer sag is to use a thicker bottom, as you've indicated was suggested in another forum. For example, if you double the bottom thickness, you increase its stiffness by a factor of eight (stiffness is based on the cube of the thickness). This is usually my first and only solution for large drawers.

You can also keep a smaller groove in your drawer sides (1/4" is probably ok) and bevel or rabbet the edges of the drawer bottom to fit the groove. The drawer needs to be thickest at midspan to resist the loading, so narrowing the edges really doesn't affect its strength.

Groove Drawer Sides & Rabbet Bottoms
(source: WoodworkingTalk.com)

Groove Drawer Sides & Bevel Bottoms
(source: Fine Woodworking)

Don't do anything special but do glue the bottom firmly to the box

Usually, it's not good practice to glue in a drawer bottom due to expansion and contraction of the bottom. However, if you are using plywood for the bottom, expansion/contraction is a non-issue since plywood is dimensionally-stable.

use narrower drawers instead

You've indicated that this is not an option, so I won't address it.

Answer 2

It's worth noting right from the start that 3/16" plywood is very thin for the bottom of a 36"x39" drawer. You're correct in thinking that you will need to do something to prevent the bottom from sagging too much. But on the bright side, your drawer bottom doesn't need to support 50 lbs--or, more correctly, it cannot support a full 50 lbs without exceeding the capacity of the rails you've chosen.

The easiest way to strengthen your drawer bottom is to make it thicker. However, the obvious tradeoff is that, with a limit of 50 lbs on your rails, the heavier you make your drawer, the less capacity you have leftover for the stuff you want to put in the drawer.

Cedar has a density of about 23 lbs/cu.ft., and your drawer frame (sides, back, and front) is about 303 cu.in., or 0.175 cu.ft.

23 lbs/cu.ft. * 0.175 cu.ft. * = 4 lbs

So assuming your design uses an integral, inset drawer face, you have 46 lbs. of capacity available for your drawer bottom and whatever you're going to put in the drawer. If you add a false front, you'll have to account for the weight of that, as well. Once you decide how thick you want your drawer bottom to be, you can find the weight of a full sheet of that type of plywood, divide by the number of square feet, and multiply by the size of the drawer bottom. Subtract that from 46 lbs., and you'll have the maximum remaining load capacity.

Moving onto some of the remedies you're considering...

Put the rails under the drawer with guides at the edges

If I'm understanding correctly what you mean, this is not a good solution, for multiple reasons:

1. All you're doing is effectively extending the support of the left and right sides by an inch or two on each side. The middle, which needs the most support, is still unsupported.
2. If the drawer bottom sags significantly, it will produce a radial force along the sliding axis, so the slides won't roll as smoothly and may wear out prematurely.

A center-mount drawer slide will do more to support the bottom, but the bottom of your drawer will need to be thicker than 3/16" anyway in order to screw the slide to the bottom of the drawer.

Install wooden cross bracing in the drawer

You can add support ribs either on the underside or inside the drawer. If you're going to add them on the inside, you might as well just make the bottom thicker instead.

Make the bottom thicker, like 1/2 inch

A 1/2" bottom is more than adequate. 13/32 is borderline for a 50 lbs uniformly-distributed load, and anything thicker than that should support a 50 lbs. load just fine.

Don't do anything special but do glue the bottom firmly to the box

This will help to the extent that loading the drawer will produce tension across the drawer bottom rather than pulling the edges of the bottom out of the grooves. Unless it's under significant tension even when it is empty, the bottom will still sag--but it may not sag quite as much.

use narrower drawers instead

This would certainly work but you mentioned you aren't interested in this option. Another variation on this idea is to put dividers in the drawer and attach the bottom to the dividers.

Answer 3

I recently built two 72" wide tool cabinets with sq. tubing, 16ga sheet metal and wrapped with 18ga ss including drawer facings. The drawers are 65" wide by 30" deep and 4" to 12" tall. I knew I would have an issue with sagging. My solution for the 7 - 4" drawers was to fabricate hat channel 4" tall by 30" long with an extra 1/2" leg 90 degrees off one of the channel legs. Put to pieces together standing up and sitting on the two extra legs in the drawer. I then welded the channel to the drawer. I know this is way more extreme than what you need and you're dealing with wood instead of metal. Just showing you how well stiffeners can work when done correctly. I also added a straight piece of 16ga between the two hat channels to maintain they're strength, then cut thin cuts with cut off abrasive blade every inch in the hat channel. Then added more hat channels and use them as drawer dividers with 18ga pieces of metal that slides in the slot. No more sag. If this will work on 65" wide drawers holding up to 600 lbs, some variation of this should work for you. Hope something here was helpful for you.