Tile adhesive

Frequently asked questions

Why does coverage behind the tile matter more than adhesive thickness?

Because an unsupported void behind a tile is where it cracks, and notches are a means to coverage rather than an end. The trowel's job is to put ridges of adhesive down that collapse when the tile is pressed and slid into them, so the material spreads and fills the space. What the specification actually asks for is a PERCENTAGE of the tile's back in contact — commonly around eighty percent for internal dry areas and full coverage for floors, wet areas and anything external, where a void will hold water. Notch size is how you get there: a bigger tile, a less flat substrate or a deeply keyed tile back needs a bigger notch, and a large tile on a fine notch leaves hollows however much adhesive is used. The way to check is destructive and worth doing anyway — lift one tile early in the day and look at the back. Tapping for a hollow sound finds the failure later, when the floor is finished.

Why does adhesive go off on the wall before the tile is on it?

Because it is a cement mortar setting chemically, and its clock starts when water is added rather than when it is exposed. Two different intervals matter and are routinely confused. POT LIFE is how long the mixed material stays usable in the bucket. OPEN TIME is how long the combed ridges on the wall stay able to wet the back of a tile pressed into them — and it is much shorter, shortened further by heat, by a dry wind, and by a porous substrate drawing water out. Once the surface has skinned, a tile pressed into it will look bedded and will have bonded to almost nothing. The fixes are all about area: comb only as much as can be tiled in a few minutes, prime a porous or dusty background so it does not drink the water, keep sun and draught off the work, and never retemper stiffening adhesive with more water, which converts a chemical failure into a weaker mix as well.

What does modified or flexible actually add?

Polymer, and what it buys is adhesion to difficult surfaces plus tolerance of movement. Plain cement mortar bonds well to concrete and sand-cement render and poorly to almost everything else; adding a polymer — dry in the powder or as a liquid admixture — improves the grip on plywood, plasterboard, existing tile, and the low-porosity backs of porcelain, which absorbs almost no water and so cannot form a mechanical key by suction. The same polymer lets the set mortar deform slightly without cracking, which is what the European S1 and S2 deformability classes measure and why underfloor heating, timber floors and large-format tiles call for it. Two cautions travel with it. It is not a substitute for movement joints, which still have to pass through the tiling wherever the substrate has one. And it cures by both hydration and drying, so a modified mortar sandwiched between two impermeable layers — a porcelain tile over an uncoupling membrane — can take a very long time to develop strength, which is exactly why some membrane manufacturers specify the UNMODIFIED grade instead.