Shiplap bathroom. How to Use Shiplap in the Bathroom: The material used for the walls is a 1x8 poplar ship lap that was pre-primed on all faces before installing. You do this for two reasons. First, to help reduce moisture absorption during the humid part of the year which in turn keeps the wood more stable. Second, so when the boards are installed and gaped to 1/8" between each board, you won't see the raw wood.