Can I tile over cedar wood? I am doing a tub to shower conversion?

and my shower pan is only 30" wide. I've added another six inches to the width by laying down a 6" x 4" x 5' cedar board. The board is only a little to high to add cement board to and then ceramic tile. If I skip the backer board, the tile would then be flush with the shower pan. Can I lay the ceramic tile directly on the cedar board?