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A few years ago I built a simple 2x4 and plywood shelving unit for my garage. I am planning to build a similar shelf, but I am looking for a way to improve the strength of the joints.

Here's how I made the shelf before: Original shelf design using screws

The circles in my picture represent the screws that are used to fasten everything together.

The problem with this design is that the entire weight of each shelf is a shearing force on the screws. The longer the shelf (and thus the more things I put on the shelf), the more weight on those same few screws.

So, I was trying to come up with a similar design that uses some simple joints to make the shelf a bit stronger. The picture below is what I came up with:

New shelf design using lapped joints

And exploded:

New shelf design using lapped joints (exploded view)

I figured that this would be stronger because the weight of the shelves is now transferred to the vertical supports directly via the join.

My question is, is this truly stronger than the design using screws? Is there a better way to achieve strength in this type of shelf?

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  • Although you haven't identified what the loads will be, I believe that your proposed connection will more than take any regular household loads well. I would recommend that the beam at the bottom span between the front posts and the upper one go from front to back.
    – Ashlar ♦
    Commented Nov 8 at 23:59
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    "The problem with this design is that the entire weight of each shelf is a shearing force on the screws. The longer the shelf (and thus the more things I put on the shelf), the more weight on those same few screws." By all means make something stronger (for any combo of reasons) but just as an FYI you're underestimating the strength of modern screws. Even the lowly, and much-maligned, drywall screw can be much stronger than often stated; but common 'general purpose' screws can individually withstand >80lb in tests. Multiply up from that, use better (and beefier) screws and, "Whoa!"
    – Graphus
    Commented Nov 9 at 7:16
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    "My question is, is this truly stronger than the design using screws?" Yes it could be noticeably stronger. "Is there a better way to achieve strength in this type of shelf?" Any number of ways. Just add glue for one. In conventional furniture-type joinery your second design could be glued only (although you could reinforce if desired). Beyond that, any combination of M&T, dowelled butt joints, housed joints (laps) etc. could be chosen. But unless you're planning on storing barbell weights or engine parts a shelf supported around all sides is a design that's already plenty strong.
    – Graphus
    Commented Nov 9 at 7:27
  • A simple 2x2 added to the front of your original design would create a load path all the way to the floor. (Google ‘site built ladder’ if I’m not describing it well.)
    – Aloysius Defenestrate
    Commented Nov 9 at 23:46

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Your assessment that the first design depended on the shear strength of the screws is accurate.

If you 'boil down' the second design, you can consider that the first cross piece, the one with the notch inboard of the board end, is taking the load primarily on the lower half of the lumber. This means that you can do without half of the plank and change the load capacity in no appreciable manner. I think it's an attractive joint, but not structural above the halfway point.

By the same token, the long plank disappearing off the top right of the image is taking the load on the top half. If your concern is joint strength, the extra thickness below the notch is not going to contribute to that aspect. Still, another attractive assembly is the result.

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  • I wonder if perhaps it would be a good "compromise" to notch the long front 2x4s into the verticals as I have shown, but then just screw the shorter horizontal 2x4s. That way the strength of the long, primary load-bearing studs isn't compromised.
    – Patrick Wright
    Commented Nov 8 at 20:21
  • Then, if I'm really obsessed with strength, I could use joist hangers for those short, horizontal 2x4s instead of just end screws.
    – Patrick Wright
    Commented Nov 8 at 20:27

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