Uncoupling, decoupling membranes matting should be used on difficult surfaces.
The principle -
To separate, uncouple, decouple your tiled area from sub-bases which may be subject to tension across their surface.
Eg - Screeds with heating pipes in will expand and and contract during the heating up and cooling down process, this can cause cracks in the surface
Floors over boarded with ply wood. Wood will expand and contract as it warms and cools separating at joints.
New screeds as they dry will shrink up and or expand expand
All the above are instances where you might use a membrane, they are all instances where movement can crack your finished tile surface.
How it works -
Uncoupling mats are manufactured in 2 layers, the top layer to which you fix your tiles is a plastic form, the bottom layer which fixes to the floor is a fleece type membrane. These two layers are sandwiched together.
If and when the floor beneath the membrane moves and cracks the fleece layer tears but leaves the plastic layer above in tact and the tiles sound.
Schluter, Durabase, PCI, Homelux, all supply uncoupling membranes in differing forms for use under tiles and stone on unstable areas.
Uncoupling membranes will prevent cracking across a surface they will not combat up and down movement eq springy floors.................
Hope this helps
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