Tiles and grout are not waterproof. The cement board does not crumble when exposed to water. But like a concrete walkway, it can absorb water. It can also dry out and look perfectly fine.
However, when a product is characterized by being water resistant, it means that it is waterproof, capable of repelling water. Most of the concrete backing plates on the market today are not water resistant, but they are all waterproof. Concrete or wonder board is a cement-based product designed as a cladding material for walls and floors. It is used as a submaterial behind tiles in the shower, bathroom or kitchen.
The cement board comes in sheets, usually 3'x5′, and is secured to the heels with special screws. There is a misconception that cement board is fully waterproof because it is designed for wet areas, but it is not. When installed, cement board is waterproof, not waterproof. This means that the cement board needs waterproofing.
Before installing any tile, all joints, cracks, screws and corners must be properly sealed and waterproofed. Some builders even recommend sealing all large flat areas as well. This ensures that no water can penetrate through the cement board and into the frame, which can eventually cause rot, mold and deterioration. Most cement tiles will need to be applied with a waterproofing system when used in a shower.
Cement plates are very porous and will allow water to pass through them. The application of waterproofing will protect the cavities of the subfloor and the wall from exposure to water. James Hardie has a cement board product that they claim is 100% waterproof, which they say eliminates the need to add or apply an additional waterproofing system when used in a shower. The waterproofing used can be in the form of a vapor barrier placed behind the cement board or with the application of a waterproofing membrane or sealant applied liquid to the outer surface.