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I built a cabinet (from a big box store plywood). I used 1/2" for tops and bottoms and 3/4" for the sides. This 16"dx22"hx30"w deep cabinet will be inset into the wall and fastened to the studs (so the back 12.5" will be hanging with no support). It will then be trimmed and painted so if a joint ever fails, it will be a big problem.

I used 1 1/4" screws, with pre-drilling and countersinking. But, tbh, it didn't feel like there was enough grab. Did I undersize the screws?

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Or did I predrill it with too big a bit?

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    I’m this question because it would be better suited for woodworking.stackexchange.com.
    – DoxyLover
    Commented Sep 30, 2023 at 0:10
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    The application may be cabinetry, but I think that the issue of plywood and fasteners is of interest to the community.
    – Wynne
    Commented Sep 30, 2023 at 0:26
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    Your problem is trying to screw into the butt end of plywood. Plywood just doesn't have that kind of strength. The screws basically pry the layers apart, and even if you pilot hole you just don't have enough grip for the threads. IMO you'd have better luck using metal angle brackets with short screws (bolts would be better).
    – Huesmann
    Commented Sep 30, 2023 at 13:13
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    The bit you're showing is the one with which you drilled in the 1/2", with the countersink. Which bit did you use for the pilot hole in the sides?
    – njzk2
    Commented Sep 30, 2023 at 15:59
  • @DoxyLover I think that most people in the woodworking forum would be unfairly dismissive of this sort of job. Apart from that I'd add that if screwing into ply in this way it's important to drill square to avoid the screw busting out.
    – Mark Morgan Lloyd
    Commented Sep 30, 2023 at 23:11

3 Answers 3

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Most woodworkers would have glued the joints and used the screws just to hold things in place as the glue set. It's not too late to take it apart, add the glue, and reassemble. You could probably even reuse your questionable screws and screw holes.

The detailing of the bottom is a little goofed. The screws oriented up through the bottom means that they'll be in tension and their withdrawal strength provides the bottom's strength. Redetailing with the bottom butting into the sides, screws through the sides into the bottom would be acting in shear to provide more strength against the bottom falling out.

You can put a piece of cross framing across the wall cavity below the cabinet to carry its weight in bearing, relieving much of the stress on the bottom's screws.

And be sure to put a back on that thing so that it stays square.

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Yes, you should use 2" screws; 1.5" are the bare minimum. The countersink being a little too big isn't a huge problem, but the lack of depth of the screw into the plys are weak.

I would recommend removing the screws, pre-drilling to 1.5" on the 3/4" panels, and installing 2" screws.

Alternatively, you can add a back (1/4" plywood) and not worry about it falling apart or anything falling out the backside.

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  • I'm planning on adding a back and fastening it with 1 1/4 screws. Are you saying this supersedes the need for longer screws?
    – Wynne
    Commented Sep 30, 2023 at 3:43
  • What size screws for this application - IDC what it is; nothing ever calls for 1-1/4", +1. Only that that's what you could get away with.
    – Mazura
    Commented Sep 30, 2023 at 10:16
  • @Wynne a back prevents racking and makes it very difficult to come apart. Additionally, the inset and trim provides additional support and helps hold it together. It would be better to replace the screws and do the back, but just the back would likely be fine unless you are placing VERY heavy things in that box.
    – UnhandledExcepSean
    Commented Sep 30, 2023 at 12:41
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"it didn't feel like there was enough grab" - well, gotta go with what you said. Sounds about right for using 1-1/4"; I don't use that length for anything.

You're screwing into the side of 3/4 ply and you got it to work? Count your blessings.

The strength comes from the backing, it sitting on the sill of the RO, and the shims on the sides against the studs. Those screws just need to not come lose, which they won't, as long as you're nice to it while putting it in.

IDK about taking them out at this point; you're going to run out of 1/2 ply that already has a CS in it. Do three more in the middles with pre-drill but no counter sink; just drive 'em hard.

If the head of a screw sticking out 1/32" is going to be a problem, then your RO isn't big enough by a long shot.

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