Well, I had resisted contributing to the debate on bottom boards but my
strenght of will would be better used regarding that diet I keep
breaking...
 
First the issues:
1) Rot, caused by moisture
2) Weight
3)  Ease of use and manufacture.
 
Most of the comments made so far indicate that often 2 or 3 is not an
issue (for small beekeepers) so they go with some concrete or metal stand.
 
For myself, all three issues are critical:  Here's what I do.
 
1.  Find yourself some old cross trees from telephone or telegraph poles
(these are cedar ussually, and are naturally resistant to rot).
2.  Rip these in half with table saw to get a flat side.
3.  Cut result of 2 to beehive width.
4.  Nail result to bottom board as appropriate.
5.  boil the whole works in parafine wax for 3 mins.
6.  Grow old and die without having to make any more (local conditions
may vary) (actually its fairly self evident that I have yet to complete
this last step).
I put old boards under the bottom boards, but this is really just to keep
them from freezing down in the fall so I don't have to pry them loose
when it is time to haul them in for wintering.  I have bottoms made this
way in the '50's by my grandfather, who learned it on a trip to NZ during
WWII.
 
BTW, make sure you use fir plywood, otherwise you will be constantly
replacing that part of the B. board.  Also, for god's sake don't make the
dado'd style board that seems so popular.  Here's a crude ascii version
of my board (viewed from front).
 
 
        =               =       <-  Lip made from plywood strips.
        =================       <-  Plywood sheet
        OOOOOOOOOOOOOOOOO       <-  Cedar leg.
 
Note that the legs run from side to side, not front to back.  One
advantage of this is that if you put the leg as far forward as possible,
so that there is little overhang at the entrance, then bees shaken out
at the entrance walk up it rather than clustering underneath.  That is:
 
           |    Super     |
         * =================
        *===================
*********000             000  <-  Bees (*'s) not cluster'd under ledge
 
Rather than:
 
         |              |
         ================
       ==================
*********000          000  <-  Bees clustered under ledge
 
Well, that's my $.02's worth.
 --
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Phil Veldhuis           | If I must be a fool, as all those who reason
Winnipeg. MB, Canada    | or believe any thing certainly are, my follies
[log in to unmask] | shall at least be natural and agreeable.
                                                David Hume (1739)