The flooring all by itself is completely waterproof. The joints are tongue and groove that fit snugly together which helps to keep water from seeping through. We have it installed in two bathrooms - one full bath with a tub and shower, the other a powder room and it is the best for these rooms. We have not seen any mold or mildew in the joints. Since the flooring is dense and durable vinyl, even if water lays on it for some time, the floors have not gotten damaged like hardwood would.