I made a modified version of this for sheetgoods

http://www.scrgeek.com/woodwork/storageSheets.html

I wanted to optimize space, so I used 1/2" conduit for the bars, stood
all my end 2x's on edge and put one hole out on the 8' 2x,and lined up
a pair of 2x's to support the wheels which I put in about 20" from
each end. Time will tell if that all works out. The dimensions are
2' x 8, which has the same capacity as the one in the link above. I
did not want to take up any more space than necessary.

Next I'm onto a rolling version of a lumber cart. I was thinking to
make 4 - 2'x8' platforms from 2x4s, and hold them all vertically in
place with 4 2x4's uprights with dados cut out for the platforms (3.5"
wide dado). The dados should give it ridgidity and a single bolt or
pair of bolts would hold the platforms to the upgrights. So basically
4 platforms held together by 4 legs. To the bottom platform I'd bolt
4 heavy duty casters. If I seperate the platforms 16" apart that
means each could hold 24" x 96" x 16" of lumber. Assuming I just
keep the top open for sorting, I'd have 3 "bays". That's about 64
cubit feet of lumber storage. Oak is about 50 lbs / cu ft, so
completely maxed-out that would be 3200 lbs. But that's ridiculous,
so figure 50% or about 1600 lbs.

Is that even reasonable for what I'm describing?