What Sits Between the Structure and the Tile

The structure comes first and it is assessed rather than assumed, because nothing above corrects a deck that deflects. Levelling comes next where it is needed, since the layers above follow rather than correct the plane. Then a substrate board or a graded former where the arrangement calls for one. Then waterproofing where the room needs it. Then an uncoupling layer where movement is expected. Then bedding, tile and jointing.

Some of these are alternatives and some are additional, and that is where most of the confusion lives. A board and a membrane are not the same kind of thing at all: one is a rigid sheet that adds thickness and stiffness, the other is a thin layer that adds almost nothing to either.

An uncoupling membrane sets a deliberate discontinuity between substrate and tile, so that a screed maturing, a timber deck moving or a heated floor cycling does not send that stress into the tile and the grout. It carries load down through its studs, ribs or bonded contact areas while allowing limited movement below.

It follows the surface it is laid on rather than correcting it, so any deviation in the substrate reappears in the tiled surface above. The substrate has to be as flat as the tiling already requires before it goes down, and the layer is often bought precisely to avoid doing that work.

A backer board gives tiling a flat, stable and moisture-tolerant base in place of materials that would not cope. Moisture tolerance means the board itself is not damaged by water, which is a different thing from the assembly keeping water out. Some boards are described by their makers as part of a water-managing system when used with that maker's own jointing components and details.

The same applies to an uncoupling membrane carrying a waterproofing claim: that claim belongs to a named product installed with its own tapes and collars, in the manner its maker sets out. Mixing product families on site quietly removes the compatibility the documentation described.

A wet room floor former is manufactured with the fall already in it, sloping towards an aperture that receives the outlet or channel. That is the opposite of a backer board, which is manufactured flat. Trying to create a fall by building it up in adhesive or levelling compound under a flat board is the usual reason a finished floor drains towards the wrong place.

It is one component in a wet room floor. The watertight layer is a separate designed system bonded across the former and detailed at the outlet, at the wall junctions and at every penetration, and the sequence runs from the drainage upwards.

Cement-based grout is porous, the joints are numerous and water moves through and around it. Where water has to be controlled that is done by a separate designed layer behind the tiling and by flexible sealant at movement junctions, never by the grouting. Tiled surfaces pass water through their joints as a matter of course.

Most complaints about tiling turn out to concern joints, sealant and junctions rather than tiles. Joints can be raked out and re-grouted and sealant lines renewed, which is why so much of the upkeep of a tiled room is joint work.

A movement joint in the construction has to be carried through the tiling on the same line. A joint stopped at the screed and tiled over usually reappears as a crack in the tiling, and the crack is rarely where anybody would have chosen it.

Edge trims sit in the same category. The anchoring leg is bedded in the adhesive as the tiling proceeds, its depth matched to the tile plus the bed, so the profile has to be chosen before the tiler begins. Adding one afterwards means taking tiles off.

Once tiled, every layer below is inaccessible without breaking out the tiling. Any service, fixing ground or detail that needs to pass through has to be resolved before tiling starts. Grab rails, screens and shelves fixed later pass through whatever is behind the tile, including a waterproofing layer if there is one.

That irreversibility is why the compatibility questions are asked in writing rather than assumed, and why the build-up is worth recording somewhere the next owner will find it.