Kerdi band waterproofing joints 5 x 323 ceramic tile & flooring kerdi band waterproofing for joints 5 x 323. Kerdi-band is a waterproofing strip used to seal butt joints, end joints, and floor/wall connections with kerdi and ditra mattings. Great product which is 4 mil thick, half the thickness of kerdi so that it has a minimum profile when waterproofing seasm in kerdi and ditra.