We didn't technically do a true shiplap, we lapped it just like exterior siding. Each piece laps over the last piece. We sealed the Hardie Boards with concrete waterproofing on every side/end (available at Sherwin Williams), then caulked and painted it with an exterior paint. We also put Hardie Backer on the wall before the siding. 6 months later, it is still performing wonderfully and looking great. I clean it with vinegar.