Which material cannot be used on floors that are sloped to drain?

Cementitious backer units are designed for use as a substrate beneath tiles and other floor finishes, providing a solid, flat, and moisture-resistant base. However, they are not suitable for use on sloped floors that are intended for drainage. This is primarily because cementitious backer units are not designed to manage water drainage effectively. In a sloped environment, water can pool or create water damage issues if not properly channeled, making these backer units inappropriate for such installations.

On the other hand, wood framing, concrete pavers, and sheetrock each have their specific applications and limitations. Wood framing can be used in certain conditions but requires adequate waterproofing measures. Concrete pavers are well-suited for sloped surfaces as they can channel water away effectively. Sheetrock is generally not used on floors at all, as it is designed primarily for wall applications; however, it is not used in areas where water exposure is expected, like sloped drains. Therefore, the issue with cementitious backer units in this context is their inherent inability to support a drainage system.