A bunkie board is used on a bunk bed if you don't want to use a box spring, but want the extra support. Made from 1/2" plywood that is sanded with three cross supports underneath. You can also opt to use cross slats instead, which offers more breathability for the mattress. 

(Note, you will need two of these, one for the top and one for the bottom)

Bunkie Board

$65.00Price