Bentonite Waterstop 20mm x 10mm x 9m Roll. Bentonite waterstop tapes are used for waterproofing vertical and horizontal structural joints, including those with complicated geometric shapes. Bentonite waterstop is a swellable tape used for the sealing of structural joints which are exposed constantly or temporarily to water. Bentonite is a natural clay that has the ability to grow in volume quickly in response to contact with water.