Schluters Ditra would be ideal on a substrate like that....
Bringing History Up To Date - Understanding The Tiling Shift
Tiles debond from the substrate because bonding the tiles directly to the building structure means there is no "give" between the two separate parts of the assembly, and movement and stress cracks in the substrate are transferred to the surface, damaging the tiles. While the methods of installing ceramic tiles and stone have changed over the years, the physical dynamics of the assembly have not. The old traditional methods, used for centuries, addressed these dynamics by uncoupling the tile from the structural base through the use of a forgiving shear interface, in the form of a layer of sand.
Following the development of the thin-bed method of bonding the tile directly to the substrate with a very strong adhesive, problems occur because the rigid tiled surface moves at a different rate from the substrate.
An installed tile surface can be compared to a large sheet of glass. In addition to being a hard, brittle material, the tile also expands and contracts in reaction to environmental changes, but at a different rate from the substrate.
So, when the tile is strongly bonded directly to the substrate, it results in what is known as a "force transfer assembly," where this differential in the movement manifests itself as cracks in the tiled surface. One of the major causes of tiling installations failing is where stresses from the substrate are transferred in this way, into the finished covering of stone or ceramic tiles.
The solution is to install an uncoupling system - which utilises modern technology to bring the "sandwich" technique bang up to date. (This sandwich, used by ancient builders, comprised a structural base and a layer of sand, then a mortar bed - a mixture of sand, cement or other binder, and water - was laid, with the tile adhered on top).
Neutralising stresses by uncoupling the building structure from the tile is particularly important with the use of today's thinner, larger-format tiles and lightweight building materials.
The up-to-date method of uncoupling uses the Schlüter-DITRA polyethylene membrane, which safeguards installations over any even and load-bearing substrate.
Tiles will move independently from the screed because of different thermal expansion and contraction. Schlüter-DITRA neutralises this differential movement, preventing stresses being transferred to the tile covering. The system supports applied loads by transferring them directly to the load bearing substrate.
Schlüter-DITRA is a polyethylene matting with a fleece laminated to a grid-type structure of square cavities on the top. The fleece is adhered to the substrate with standard adhesive, and the adhesive used to stick the tiles actually anchors into the dovetailed configuration, mechanically locking the tiles onto the top. The cavities allow any stresses that occur between the substrate and the tiles to be neutralised evenly in all directions.
The matting itself has four main functions -- uncoupling, waterproofing, vapour diffusion over green screeds, and it bridges cracks such as timber board joints and cracks in screeds.
It protects the surface covering over a wide variety of substrates, including green concrete; green cement-based screeds, gypsum-based screeds, poured bituminous screeds, plywood and pressed wood, existing tile coverings, solid vinyl coverings or coatings, mixed substrates, gypsum plaster and plaster block, mixed masonry, and metal.
Today's tile installers have an array of materials and installation systems at their disposal to meet the UK's growing demand for this ideal surface covering. Setting materials are available in ever-increasing numbers to address the variety of substrates and tiles commonly used. Underlayments continue to be developed to protect tiles and guarantee a successful installation. Other developments include drainage, waterproofing and uncoupling membranes for use in both interior and exterior applications.
It's crucial that today's installers understand the physical dynamics of the entire tile assembly in order to achieve consistently successful results. Accepted wisdom regarding tile installation is now shifting away from the direct-bond philosophy, back to an understanding of the need for a forgiving shear interface to absorb stress.
This shift, with its resulting successful installations, represents a great deal of potential for the professional tiler.
