force when the tiles are supported by just a few globs of mortar. Improperly bedded tiles that just rest on mortar ridges are next in line for failure under force. At approximateyly 50% coverage and minimal contact with the cement paste and polymer, tiles can’t resist as many of the nor- mal stresses compared to when the required continuous 3/32” minimum thickness is achieved. When installed in wet conditions, water trapped under a tile can freeze, or when exposed to heat and humidity, tiles expand and failure occurs. Too often, there’s the perfect storm – low coverage, a lack of movement joints, con- tamination and heavy loads. So what can we expect from a tile with proper mortar coverage when project application require- ments are met? The reality is that there are millions of tiles perform- ing well, lasting for decades, even centuries, with proper coverage! When they’re removed for a new style, it takes a lot of force. In a study of over 80 residential projects in the Southwest where a random tile cracked and/or loos- ened over post-tensioned concrete slabs, more than 85% of the tiles had low mortar coverage and over 50% showed substrate contamina- tion. Every adjoining tile with full coverage and without contamina- tion was intact. The exceptions were stone tiles with acceptable coverage, but that had cracked since they can’t resist movement stresses as well as porcelain tiles. On one memorable project, tiles with greater mortar coverage failed because the floors and walls cracked right down the middle of the home. The conclusion was that the more tile and substrate surface areas bonded at 200-400 psi strengths – and with polymer for deformability – provided more resistance to failure. Impact will also damage tiles, but as shown in the 2016 NTCA Trowel & Error video (see also Spanish version), a tile bond can withstand heavy forces with high mortar cov- erage compared to those without. TECHNICAL FEATURE ––––––––––––––––––––––––––––––––––––––––––––––– The bigger the tile, the more important the use of the proper mortar and coverage becomes. Effects of ridge collapse based on technique The bigger the tile, the more important the u Correct directional combing across the short The bigger the tile, the more im Correct directional combing acr Correct directional combing across the short side of the rectangular tile. 94 TileLetter | October 2018