Checking for coverage

When installing ceramic tile, particularly on floors, the bonding material not only adheres the tile to the substrate, it also provides support for the tile. Voids underneath tile are weak spo ts where cracks can occur due to point loads and impact. Common culprits include wheeled loads like hospital equipment that travels room to room. Women walking in high-heeled shoes can also cause damage in both commercial and residential applications. Couches, tables and chairs are also point loaders.

Floor tile is only required to withstand 250 lbs of force under a point load in order for the tile to be compliant with ANSI A137.1, the manufacturing standard for ceramic tile. Most floor tiles withstand about twice as much point load, but tiles are not intended to make up for installation shortfalls like lack of coverage. ANSI installation standards require at least 80% mortar coverage in dry areas and 95% in wet areas. Additionally, the coverage must be evenly distributed, with edges and corners well supported. 

Although mortar bags give notch trowel size recommendations, there is unfortunately no chart or formula that can guarantee good coverage. The tile size, the flatness of the tile, and the flatness of the substrate have enormous effect on coverage. Additionally, the troweling technique of the individual installer makes a difference. Some installers “pick-up” too much mortar by troweling too hard. A lighter combing of the mortar leaves “more on the floor” and achieves better coverage.

The only way to guarantee good coverage is to periodically pick up tiles to check for coverage while installing. Experienced installers do this constantly, adding mortar under individual tiles as necessary. Less-experienced installers learn the practice-the easy way, by watching-or the hard way, by going back to jobs to replace cracked tiles, often at their own expense.