Last Spring I built 50 screened bottom boards.  Even though I keep my hives a foot or so above the ground on timbers, quite a few of my old boards were beginning to fall apart.  

The bottom boards are built out of 1X4 lumber, so they were not expensive.  I did have some difficulty finding the right mesh screen, but soon discovered it at a local hardware store.  I am able to slide a 1/4" piece of plywood under the screen when needed to block off the ventilation in the winter and spring.

I am currently wintering hives in an un-heated, insulated shed.  Each hive went into the shed as a strong single with a candy board on the top.  The screened bottom board is covered with plywood except for a 3x5" hold cut in the center for ventilation.  We have long winters here in Minnesota. The bees went in the shed on November 4 and won't come out until April 1st or so.  As this is the first year I have tried this method I am anxious to see how it works, or doesn't work.

Any advise, ideas or questions?

Mark in Northern Minnesota