Waterproofing both the walls and floors is a must in bathrooms nowadays, especially with power showers, which throw out a lot of water at high pressure. However, there's another consideration for floors -- not only do the tiles need to be waterproof, they must also be protected against movement. Otherwise they could crack or become debonded from the substrate.
Because the sheet used on walls is purely to waterproof, it is equipped with an anchoring fleece on both sides. Therefore it is mainly used for wall tiling, and is also ideal for protecting surfaces such as renders, plasters, plasterboard and mature screed. But to protect floor tiles from movement -- by uncoupling them from the substrate -- as well as ensuring they're waterproof, it would be better to look at alternative solutions.
While there are types of brush-applied liquid which offer a degree of waterproofing protection, the British Standards Institution recommends using impervious membranes. BS 5385 states: "Tiles and bedded finishes, even when the joints are filled with impervious grout, cannot be guaranteed to eliminate entirely the passage of liquids downwards.....in the case of suspended floors water passing downwards may cause dampness on walls and ceilings below, and in the worst cases leads to flooding."
It says the most satisfactory method of preventing this is by: "Providing a membrane between the base and the tiling...the membrane material should be impervious...and be sufficiently flexible and strong enough to resist movement in the structure, and loads, without rupturing."
A universal underlayment such as Schlüter-DITRA which is a polyethylene membrane with the anchoring fleece laminated just to the underside, serves as a waterproofing layer, as well as protecting the tiles against movement. In order to protect the tiles from movement its upper surface consists of a grid structure of square cavities which allow any stresses that occur between the substrate and the tiles to be neutralised evenly, in all directions.
And it is not only suitable just for inside use such as bathrooms. As a universal underlayment for uncoupling, supporting, waterproofing and protecting tile and stone, it can be used for outside applications such as balconies, terraces and patios, just as effectively as bathrooms and other indoor installations.
In a nutshell, after ensuring that the substrate is even and load-bearing, the bonding adhesive is applied with a 3mm x 3mm or 4mm x 4mm notched trowel. While the type of bonding adhesive depends on the type of substrate, for most substrates a dry-set tile adhesive is suitable.
The fleece on the underside of the membrane needs to be solidly embedded into the adhesive so that its entire surface is bonded.
Work the membrane into the adhesive, in just one direction, using a float or screed trowel. The tiles can be laid on top immediately afterwards, by anchoring the adhesive into the square cavities.