The flatness of a substrate is measured against industry standards, such as those provided by the TCNA. When a floor is not flat, tiles cannot sit flush with one another, resulting in 'lippage'—where one edge is higher than the next—which creates tripping hazards and uneven surfaces. Furthermore, unevenness creates voids under the tile where mortar fails to make full contact. These voids are susceptible to stress and impact damage. Proper substrate preparation, including grinding high spots or filling low spots, is mandatory to ensure uniform load distribution across the entire tile installation surface.

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