I would like to cover the ceiling a porch with something. What is
there now are 2X4 appearance "beams" spaced 16 inches apart. I like
the look of bead board, but am thinking that the 1/4" thickness isn't
substantial enough. It may end up sagging.

So what I thought I might do is cover the area with plywood first,
then attach the bead board to the plywood. Or, attach (glue?) the
bead board to the plywood first then install over the ceiling.

Good idea, bad idea?