0

Most shelves and workbenches I see all have the same orientation for their horizontal boards (as in, the braces for a shelf or the edges of a workbench, holding up plywood or other horizontal surface). Those horizontal boards are oriented so that the longer face (the 3.5" edge of a 2x4) are oriented vertically, and the shorter edge is horizontal. Why are 2x4 braces so often oriented with their longer-edge vertical, and is it necessary?

I've built a bunch of tables and shelves that way. For a lighter-duty freestanding shelf, I'm wondering if I can have the 2x3s oriented with the 3" edge horizontal and the 2" edge vertical, which would reduce how much vertical space the shelf braces take up. Is there a reason not to do this? So far what I see is that orienting wider-side horizontal means longer screws are needed to make the perpendicular joint of braces. Or pocket screws could be used. I imagine there may also be structural benefit of having more of the brace's thickness oriented vertically to receive heavy vertical loads and spread the vertical load across 2 screws (rather than having screws side-by-side horizontally). Again, this is a light-duty shelf I'm thinking to try this with.

6
  • 2
    2x3' has greater resistance to bending stress in the 3" direction than the 2" direction. Floor and ceiling joists are always oriented with the longer dimension vertical so the load is parallel to the longer dimension. How heavily are you going to load these shelves? Maybe 2x2" edge supports or 1x2" would support the load and give you the clearance you want. Commented Apr 16 at 1:42
  • 1
    Or you could go with 1" plywood cut into 4" strips. The 1" would be less than the 1-1/2" of a 2x4. Maybe even 3/4". Commented Apr 16 at 1:52
  • The loading being parallel to the longer dimension when that's oriented vertically makes sense. I just wonder how important it'd be. Wouldn't a 3x2 with longer dimension horizontal be the same as a 2x2? I'm referring to the braces under each shelf, along each shelf's long and short edges.
    – cr0
    Commented Apr 16 at 1:52
  • I plan on 1/2" plywood shelf, 18"x6', with 3x2s under the 6' edge and with (4) 3x2s as braces under each shelf. Those edges will have vertical supports to transfer the load from shelf-edge to ground. Each half of the shelf, like 3x2' area on the plywood, will have I guess 50-150lbs on it (likely lower end of that load for most/all of the shelves).
    – cr0
    Commented Apr 16 at 1:55
  • 1
    In the end, it comes down to (a) the support provided by the braces/supports, (b) the amount of weight on the shelves and (c) the thickness/type of shelves. Plenty of shelves with books on them are held up by two little plastic or (preferably) metal shelf pins. In the case of shelf pins, it is absolutely critical to make sure the shelves have a really tight fit and don't bend much - shelves on 2x4s that are 3/4" short will slide a little but still probably work. Those same shelves on shelf pins won't stay up for very long, if at all. Commented Apr 16 at 2:00

1 Answer 1

1

"Why are 2x4 braces so often oriented with their longer-edge vertical, and is it necessary?"

The stiffness is proportional to product of the width and square of the height, wh^2. so by turning a 2x4 on flat, the stiffness is only twice that of a single 2x2. But turning it on edge, it is 4 times the stiffness of a single 2x2. These are approximate statements of course, since modern 2x4s are not 2" by 4" ... and note that the modern dimensions are not uniformly downsized, the 2" is now 1.5" for a 25% while the 4" is now 3.5" for a 12% reduction, thus better preserving the rigidity of dimensional framing lumber in the "working" direction, according to wh^2.

If you are trying to minimize the obstruction, you could eliminate it nearly completely if you switch to angle iron rails with the front and back rails oriented so the plywood sits down inside. Discarded bed frame rails are particularly stiff. The 1/2" plywood can be raised up flush by ripping some thin furring strips to use as full-length shims.

Another way to minimize obstruction is to forego the rails completely. Instead use loose vertical dividers to let the shelf below support the shelf above, with the floor supporting the bottom shelf. They are loose/moveable and do not need be kept perfectly vertical, nor one directly above the other. The weight of the stuff on the shelves keeps them in place. Cut the dividers out of say 1/2" plywood. If you want a clearance of H inches and if your shelves are D inches deep, then cut the dividers H by D or H by D-4".

1
  • Thank you for breaking down the math behind stiffness, that is the clarity I was looking for. The alternatives are helpful too.
    – cr0
    Commented Apr 25 at 17:08

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.