When is a tiling installation like a wristwatch? When its water-resistance qualities are mistaken for being waterproof.
Just as a watch which is only water-resistant should never be submerged, water-resistance will not protect a tiled installation from long-term water penetration. Waterproofing will protect the watch, just as it will protect the tiling installation.
Unfortunately, it is a very common misapprehension that water-resistance is the same as being waterproof. And this mistaken belief is not just amongst tilers -- many instances of troubleshooting where water has seeped into the fabric of buildings through tiled floors, have shown that architects, specifiers, and builders had thought the water-resistant proprietary tiling systems they used in the installations would serve as adequate waterproofing. But in a ceramic tile or stone application "waterproof" and "water-resistance" do not perform the safe function.
"Waterproof": Not permitting the passage of water; is impervious to water.
"Water-Resistant": Does not break down in water. Able to resist water penetration to a small degree.
When proprietary tile backing systems such as boards or paint-applied, are used instead of plywood or plasterboard, it is essential to find out if they are waterproof or merely water-resistant. If they're water-resistant, it means they are unaffected by the water themselves, but they don't stop it eventually seeping through into the fabric of the building, causing long-term damage.
When waterproofing - or tanking - between the tile and the substrate, a brush-applied liquid can offer some waterproofing protection, but the British Standards Institution (BSI) recommends using impervious membranes which stop the water from going any further, protecting the installation.
BS5385 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."
According to BSI, the most effective waterproofing underlayment for floors includes universal matting systems comprising a polyethylene membrane with an anchoring fleece laminated to the underside, such as Schlüter-DITRA.
In many cases where a water-resistant system is used, these universal waterproofing systems have also been installed. In fact, some manufacturers of water-resistant backing systems have actually recommended using this type of waterproofing as well, to fully waterproof the installation.
So if you're looking for a proprietary tile backing system that needs to be fully waterproof, only a fully waterproof system will do the job by itself. If a water-resistant system is chosen, it will need additional waterproofing to protect the installation.