Uncoupling Revisited By

Centuries ago, European builders developed a reliable means of installing tile for high-traffic, high-use applications which relied on the laying of a sand strata between the structural substrate and the mortar-bed/tile composite top layer. Due to space (height) requirements and other concerns, the sand strata method for tile installation is, for all practical purposes, extinct. Modern theory, however, explains that the sand strata "uncouples" the tile from the structure, allowing structural movement without damage to the tile layer. Approximately fifteen years ago, a modern analog of the sand strata system was developed in which a thin, polyethylene sheet membrane with a grid structure of square, cutback cavities and an anchoring fleece laminated to its underside, functions as the uncoupling layer. This article will provide the basis for the claim that a configured membrane allows the normal loading forces exerted on the tile surface to be widely distributed through a forgiving shear plane, similar to that which would be expected in the sand strata. These results contradict other work and theories which suggest that extremely strong bonds are necessary between the tile and substrate to maintain a crack-free tile surface. In fact, it is proposed that a weak interface is more forgiving, allowing substantial movement in the substrate without any evidence of cracking in the tile or the grout joints. This system also allows differential expansion and contraction between the tile and the substrate and can be used on a wide range of substrates which have traditionally been viewed as problematic; including plywood, OSB, post-tension concrete slabs, green concrete, radiant heated floors, and gypsum underlayments. To understand how this system accommodates stress within a tile assembly, the stress-strain relationship for materials under load will be reviewed, followed by a ceramic matrix composite analogy that illustrates how stress buildup and stress reduction are possible within composite layers. The shortcomings of directly bonding tiles, or setting tiles in a force-conductive assembly, will be evaluated in the context of large-scale tests performed on tile directly bonded to concrete slabs. Finally, it will be demonstrated that an effective means of ―uncoupling‖ the tile covering from the substrate, fundamental to successful tile installations, can be achieved in a contemporary thin-bed application.