Durock cement board doesn't need waterproofing in dry tile applications that won't be getting wet. However, when using Durock in showers or bathtub situations, you want waterproof the cement boards. There are two ways to do this. First, you can put a layer of plastic sheeting behind the cement board on the studs and follow up with 100% over screw holes. Alternatively, you can choose to paint two layers of RedGard waterproofing membrane on top of the cement board before tiling.