Schluter® KERDI Waterproofing Membrane 3'3" WideKERDI is what actually keeps water out of your shower walls. Cement board alone is not waterproof it's moisture resistant at best. KERDI bonds directly to the substrate with thin set and creates a continuous waterproof barrier behind your tile. Modified polyethylene (PEVA) core with non woven polypropylene fleece on both sides the fleece anchors into thin set mortar, bonding the membrane directly to cement board, drywall, or KERDI BOARD 33" wide rolls