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Book shelves I made from MDF 15 years ago are sagging pretty bad. Over the years my wife added more and more books. Now in retirement, I have decided to replace them. First I found this link for shelf design. http://www.woodbin.com/calcs/sagulator.htm. Then I spent an hour at HD sorting through their 3/4" Birch plywood looking for the flattest sheet. Since my longest shelf is 63", I had them cut the sheet at 65" making it easier to get home. I then ripped three shelves to 11" wide and cut 3-1/2" rails for the front and back. After everything was cut I noticed the shelves had twisted almost 1" end to end, (e.g. left rear clamped down - right front 1" up). I have had these clamped to a flat surface for a week now and they are still twisted. I know humidity plays a role but here in the Las Vegas area it is pretty dry. Any ideas on how to get the twist out?

You are propbly going to have to stiffen the shelves up with a thin board running the length of ths shelf. I"m finding myself having to do this this summer with bookcases I built for my classroom. It doesn't have to be all that wide; I'm thinking a 1" wide board should do the trick.
BPR

You are probably going to have to stiffen the shelves up with a thin board running the length of ths shelf. I"m finding myself having to do this this summer with bookcases I built for my classroom. It doesn't have to be all that wide; I'm thinking a 1" wide board should do the trick.
BPR

jmadden wrote:Book shelves I made from MDF 15 years ago are sagging pretty bad. Over the years my wife added more and more books. Now in retirement, I have decided to replace them. First I found this link for shelf design. http://www.woodbin.com/calcs/sagulator.htm. Then I spent an hour at HD sorting through their 3/4" Birch plywood looking for the flattest sheet. Since my longest shelf is 63", I had them cut the sheet at 65" making it easier to get home. I then ripped three shelves to 11" wide and cut 3-1/2" rails for the front and back. After everything was cut I noticed the shelves had twisted almost 1" end to end, (e.g. left rear clamped down - right front 1" up). I have had these clamped to a flat surface for a week now and they are still twisted. I know humidity plays a role but here in the Las Vegas area it is pretty dry. Any ideas on how to get the twist out?

Check out this Charles Niel Video I think it will give some in-site on cupping.http://www.cn-woodworking.com/uncup-the-cup/

I wish my pieces were just cupped. I could straighten them with the rails. But being twisted makes it a real problem. I tried placing 1/4" x 1/2" strip diagonally corner to corner and then clamping the two high corners past the twist. No luck. Maybe I need to add a little moisture since I am in the desert.

Darn! twisted plywood is a hard thing to work around for sure. Here's what I'd do -
make some pieces of hardwood 1/2"X1/2". Glue them horizontally to the rails exactly the same width apart as the plywood. (a little less than 3/4") They will form slots into which you can slide the shelf... twist gone!

Now to keep the shelf from sagging - rip another 1 inch strip of plywood and glue and maybe screw it to the bottom of the shelf with the 3/4" side on the shelf. If the shelf is not supported in the back you will want to make another 1" strip for the back. Of course this will reduce the space of the shelf below, but that can be fixed by relocating.