Plywood strength cross outer grain vs. along it

[Tom Frank]

I am planning to use 3/4" plywood (5 ply) for shelves of a shoe rack in my garage. I plan on having 6 shelves each 40" wide without center support. I can get all the shelves out one sheet of plywood if I cross cut against the grain on the outer plies. My question is would the shelves be more likely to bow or sag over time if cross cut instead of ripped, or do the interior plies with adjacent grain balance out the strength evenly for both dimensions? If there are any other reasons not to cut the selves across the outer grain, please do share. I am always looking to learn more. Thanks!

[Leo Graywacz]

40" is too long either way. I limit mine to 32" with a 1" glued front support.

[Roger Jensen]

I don't think it makes a difference, but I can't find a web reference to support that. However, the outside veneer layers are probably the thinnest layers out of all of them, so I don't think you would gain much going "with the grain".

However, 40 inche spans are too long. Perhaps you can build them up with hardwood strips on the front and back. Angle iron bolted to the back edge would be even better.

Roger

[Leo Graywacz]

You would be shocked at waht the veneer does for strength. Take a pc of plywood and plane or saw off one side of the veneer. You will have a very flexible pc.

[daniel lane]

+1 for Leo's comment in #4. The outer veneer does a lot for tensile strength. I used to skin hot-wired Styrofoam wing cores for RC aircraft using 1/32" balsa. As long as the grain was parallel to the wing spar, that wing was SOLID. (0.5oz fiberglass helped reduce "dingability", too.)

Also, re: others on the 40" being too long - since it's for a garage shoe rack, I wouldn't worry too much - especially if you put a 1" lip on it like Leo suggested in #2. If you put anything besides shoes on it, expect bowing. (No steel toes!) :)


daniel

[Gary Breckenridge]

40 inches is too long and it would sag. Put a back on the shoe rack like on a bookcase and put some knees in the middle for added support.:)

[Tom Frank]

Thanks for the advice, guys.

I should have mentioned that I am planning to have a back panel of 1/4" plywood, and I'll be doing a quick job with the brad gun on the whole project. I was hoping with the back support and the lack of weight from the shoes, I could get away with 40" and no middle support. Technically, I am planning about 38" for the width of the shelves, about 40" total width after the legs/side panels. The shelves will be 12" deep.

I understand 40" is pushing it, but I figure I can always wedge support in the middle if I need it. I'll try the suggestion with the 1" lip. Thanks for that. I have lots of quarter sawn oak, which should be strong and will add something nice to look at on this otherwise utility shelf. Now I'm wondering if I should use it on the sides too. No, I can't get carried away. This isn't furniture...I told my wife it would be a quick one!

Thanks again!
Tom

[Leo Graywacz]

With the back of the shelf secured and the 1 to 1 1/4" lip you shouldn't have much of a sag. That should work. You never said about securing the shelf, that makes a big difference.

[Tom Frank]

Nice, thanks Leo!

Just to double check, does anyone advise against the grain of the outer plies running along the shorter dimension of the shelves (they are 38" x 12" x 3/4")? This is my biggest concern about sag. Leo and Daniel mentioned the significance of the outer ply in regards to strength, but I never heard anyone actually advise against my idea.

Again, thanks for the great advice. It's amazing how helpful members of this forum are.

[Rick Moyer]

Tom, I've always wondered about that too and I suspect it might be better to cut the pieces along the outer skin grain, as opposed to cross grain, but I really don't know. That being said, I believe if you put a 1" lip across the front that will overcome any difference(if there is any). The front stiffener will make a big difference. Plus we are talking about a shoe rack, not books. I wouldn't worry about the direction of the skin in this case.

[Leo Graywacz]

It would be best to run with the grain. But supported and under a light load it shouldn't be that much of a concern.

[Gary McKown]

If all you want is 6 shelves 40" X 12" from a single sheet of plywood, it shouldn't make any difference which way you cut it except for the number and sizes of left overs. Rips provide three full-width pieces (plus one extra that would be a tad bit less in width). Each of these would provide two shelves, with four 16" X 12" cutoffs (or one 16" X 48" piece if you crosscut that off first before ripping).

Crosscutting, you would get 6 full-width shelves with a piece roughly 24" X 48" left over, and six 8" X 12" cutoffs (all sizes minus saw kerfs).

Getting 7 12" shelves could only be done the crosscut way. On the other hand, if you want 8 all the same width (little bit less than 12"), the width would be greater for the rip way as there would be fewer saw kerfs to account for.

Want 10 shelves? Crosscut two, then rip the remaining 8. Total left over is two pieces 8" X 12".

[Tom Frank]

Thanks for pointing that out, Gary! You're not missing anything, I was. How the heck I missed that, I don't know. Now it seems like I wasted about 5 people's time asking my original question. I learned some new tips in the process though, for which I am grateful. Thanks again.

[Steve Clardy]

I've ran ply grain in both directions for shelves.
Doesn't make any difference in strength that I've seen.

[Josiah Bartlett]

If you think of it like a beam (it is a 2 dimensional beam), the deflection strength comes almost entirely from the skin. The deflection strength when the outer plies are removed is quite a bit lower. However, if you support the shelves and don't break the outer ply, it should be fine.

[Joe Spear]

The deflection strength when the outer plies are removed is quite a bit lower.

The fact that the deflection strength is quite a bit lower in that case--wouldn't that be, at least in part, because your plywood is now thinner than it was before removing the outer plies?

[Tom Veatch]

Plywood laid up so that the grain in a given ply is perpendicular to the grain in adjacent plies will be marginally stiffer with the face grain running the long way on the shelf. How wide that "margin" is depends on the thickness of the face ply and the ratio of the elastic modulus of the face ply parallel to and perpendicular to the grain. As thin as the face plies are on some of the plywood I've had the misfortune to encounter, I strongly doubt you'd see any difference in stiffness anywhere outside a testing laboratory.

For an extreme example of the difference in stiffness parallel to and perpendicular to the grain, pick up a piece of "bender board" AKA "wiggle-wood" - plywood laid up with the grain in all plies running the same direction.

[Tom Frank]

The plywood I will be using is not hardwood veneer with the thin outer ply. It's some sort of pine, with one nice side for workbenches, etc, and one knotty side. All 5 plies are equal in thickness, roughly 1/8". I don't know if this matters for the debate brewing here, but thought I'd throw it out there.

Several people mentioned a lip for support on the front edge of the 38" long shelves I'll be building. I have a lot of quarter sawn oak, so I was thinking of cutting the lip from that. My question is do the thickness of the lip have to be much thinner than the height of it to keep it strong? For instance, a 2x4 is much stronger in one dimension than the other for this reason. Specifically, if my lip was 1" wide (or high when installed) ripped from 3/4" thick quarter sawn oak, are those two dimensions too close in size to render the lip as a proper support?

Sorry for all of the theoretical questions. I do much more thinking about the physics of things than I do actually trying them out.