I stored mine on its side too, for the first few months I had it. I kept bumping it when I would get out other rulers, and it would come apart in two pieces, which I had to put back together. So, I slid it under an easy chair here in my room, which means it is not stored flat. I was afraid I might have warped it, but I just tried it, and it is okay. I've only used it twice. Glad I will be able to use it again.
Mary, thanks for the warning on this. I probably should have stored mine flat from day one.
Dina