I know there are plastic bottom boards offered in several bee supply catalogs.
But having never used one, I have no reference to recommend them.  I have
always used wood if for no other reason than that I know it has worked well for
me so far. In ten years of bee keeping I have replaced bottom boards every
three to five years or so.  Usually as a result of carpenter ant damage.
 
One thing I have found makes the entire hive last longer is keeping the wooded
components off the ground entirely.  I place four cement blocks partially
buried in the soil (to anchor them) and sit the hive stand and bottom board on
these.  This serves to raise the hive up about 10 to 12 inches, protecting the
hive from the constant contact with the moisture in the soil. It also provides
more ventilation around and under the hive to help keep it dry.  The cement
blocks also serve to position the supers up a bit higher so you can work in
them better without bending over quite so much.
 
jeff young    One backyard hive in Upton, MA
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