Wood vs Plywood Strength

[Steve Benson]

Ok, so i screwed up i used poplar for slats for my bed and they are super flexible. It would be fine if it was a normal bed frame but it is a platform bed. I was wondering if i use slats of 3/4 ply would it be a lot stronger and less flexible, i mean they wont be seen anyway. Is ply really that much stronger? and will it flex a lot less?
thanks

[Brian Peters]

it all depends on how it is used... sometimes ply can be just as strong as solidwood, sometimes it can be just as floppy and have just as much give as mdf.. baltic birch vs regular birch ply, lots of variables. In a bed I would want to be going with a solid wood design. If you're on a budget poplar is the way to go, just go thicker, but for me I'de atleast go for soft maple.

[Jamie Buxton]

One way to make your poplar slats stiffer is to make them thicker. If you double the thickness, they'll be eight times stiffer. You could glue more poplar on to the existing slats.

[Steve Benson]

They are already 3/4 in thick, will it be a lot better if the boards are shorter. Currently they are length wise would it be alot better if they were width wise, they are 8" wide and there is a center beam so they are running 72" long or width wise it would only go 39". I cant really make them thicker as it is kind of set. Would alternating hard maple or some other stronger harder wood be better.

[John Schreiber]

. . .Currently they are length wise would it be alot better if they were width wise, they are 8" wide and there is a center beam so they are running 72" long or width wise it would only go 39".
If I understand what you are saying, then yes that's exactly the thing to do. The same board over a shorter distance will bend less. If you change from a 72" span to 39", much less. I've never seen bed slats go the long way.

[Jamie Buxton]

You can make only a little change in the stiffness by changing to a different species. Doug fir is among the stiffest of commonly-available species in the US, and it is only 25% stiffer. As I said before, a much better way is to make the slats thicker. You can get 25% more stiffness by increasing the thickness only 8%, and you can get 800% more stiffness by doubling the thickness.

[mreza Salav]

Definitely going shorter lengths will help.
If it still flexes, you can stiffen them by gluing a board perpendicular to it beneath it to form a T with the flat top of the T being on the top. Even a 2" board this way will make them much stiffer.

[Wayne Cannon]

According to a reference on shelving, solid poplar, oak, etc., are all substantially stiffer than the equivalent thickness of plywood.

If you can't go thicker, as many suggested, you can also add a lip below the edges as you often find with bookshelves, even if the lip stops an inch or so short of the ends.

[Wayne Cannon]

This "Sagulator" may help. It's designed to calculate sag for a shelf, but should also work for bed slats.

http://www.woodbin.com/calcs/sagulator.htm

[Jason Roehl]

Long before I was "into woodworking", I built a loft bed out of pine for an apartment. I used 1x (3/4") construction pine for the slats--running width-wise. Held my wife and I easily without noticeable sag. Poplar should be even a tad stronger. I definitely think that your problem is the length-wise run. Even oak or hickory would probably sag at those lengths with the weight of a person on them.

[Greg Heppeard]

I had the same problem when making a bed....I just made a 3/4" dado in the bottom of the slats and glued and screwed in a 1 1/2" piece (making a T) it made the slats almost un saggable.

[Jim Becker]

The slats that Ikea supplies are beach or something similar...a much denser wood than poplar. That said, no matter what the material, cross-wise is the way to go and with the center support. Or you could go "true platform" and just deck it with solid plywood. But be sure you're using a mattress that is designed for that kind of foundation...there are differences.

[Prashun Patel]

You can also add more 'beams' to distribute the load. Do you have enough clearance on the underside to be able to run 2x4 stock in between the midspan beam and the rails?

[eric auer]

Definitely going shorter lengths will help.
If it still flexes, you can stiffen them by gluing a board perpendicular to it beneath it to form a T with the flat top of the T being on the top. Even a 2" board this way will make them much stiffer.


+1

And making it an "I" would help even more, like small beams!:D

[Cliff Rohrabacher]

forget the slats and use ply as a one piece sheet.

[Jason Beam]

Run 'em shortways - tie 'em together if you need even more stiffness (run a board underneath 'em that ties 'em all together. This way they'll distribute the load as evenly as possible and spread the flex out over a wider surface.

[Tim Marks]

slats made out of plywood would be a big mistake.

The advantage of plywood is it is strong in both directions. If you cut the plywood in narrow slats, then the inner layers with the grain running across the short width of the slat would add no strength, and you would have a very weak slat which would break easily.

Use solid wood for the slats. I would bet that the existing poplar slats will work just fine if you run them cross-wise (the short way).

[Peter Quinn]

Plywood has considerable shear strength but limited strength to resist bending across its thickness. Think about its use in construction as sheathing. It ties the studs together like the shell of an insect. A house sheathed in plywood is considerably more rigid than one sheathed in T&G solid wood.

Slats ripped from 3/4" ply would be very weak, more so than poplar given their orientation. The plywood laid flat simply will not resist the load of a mattress without a box spring on a queen or king sized bed. I have made slats from 15/16" QSWO that were more than sufficient for a queen sized bed.

Try creating a T as suggested above, and consider using plywood for the vertical leg of the T. Simply rip strips 1 1/4"-1 3/4" of 3/4" plywood, cut to length to fit between the rails, screw and glue these to the bottom side of each slat, use a dado if you wish for orientation, it need not be perfectly tight. You will gain considerable stiffness this way. Think about an I joist used for construction. A few pieces of pine and a strip of OSB can hold the load of a house!

[Howard Rosenberg]

Glue and screw vertical full-length cleats into every single slat.

At that length and 3/4" thick, most species will sag to some degree.

Those cleats will add the beam strength you'll need.

HTH

Howard