It usually happens without warning and often with an audible crack. A run of floor tiles or stone slabs debonds from the bed and lifts into a ridge along a line — a hospital corridor, a mall concourse, an airport pier, a large lobby. The tiles themselves are undamaged. The adhesive is frequently still sound on both faces. Nothing has failed in the sense that a materials test would recognise.
The floor has run out of room.
What is actually happening
A tiled or stone floor is a thin, stiff, brittle plate bonded to a much thicker one. Both change dimension, and they do not change together:
- The finish expands. Stone and tile respond to temperature, and fired ceramic products additionally take up moisture from the air over months and years and expand irreversibly as they do. That expansion never reverses.
- The substrate shrinks. The screed and the slab beneath keep drying and shortening for months after the pour — the same drying shrinkage that governs joint spacing in the frame.
The two effects act in opposite directions, so the differential across the bond line is the sum of them, not the difference. If the finish has nowhere to go — no perimeter gap, no field joints, tiles hard up against columns and walls — the only place that strain can be resolved is as compressive stress in the tile layer itself.
Stone and ceramic are strong in compression, so they do not crush. What gives way is the weakest link in the system, which is the bond. Once a few tiles release, the ridge propagates along the line in seconds, because every tile that lifts hands its share of the stored strain to the next one.
Tenting is not a defect in the tile. It is stored strain finding the only exit it was left.
The four places the joint should have been
1. Directly over every structural movement joint — the non-negotiable one
If the structure has a movement joint, the finish must have one in exactly the same place, of at least the same width, cut cleanly through tile, adhesive and screed to the full depth, with nothing bridging it.
Tiling across a structural joint is the single most reliable way to destroy a floor, and we still see it regularly. The gap below is going to move by the full amount the structural engineer calculated. A 10 mm tile bed is not going to stop it. What tiling over the joint achieves is to guarantee that the tiles crack along a ragged line a little way off the joint instead of moving cleanly at it — and the crack is then in the middle of a tile rather than at an edge where it could have been designed for.
The corollary matters too: the movement joint in the finish must be continuous with the structural one, not near it. A finish joint offset by 200 mm because it landed better with the tile module is a finish joint that does nothing.
2. Around the perimeter, at every abutment
The most commonly omitted joint, and the cheapest. The floor needs a compressible gap wherever it meets something that will not move with it — walls, columns, kerbs, thresholds, staircase strings, lift openings. Typically 6 to 10 mm, left free of mortar and covered by the skirting rather than filled with it.
Columns are the usual failure. A floor is laid with a perimeter gap at the walls, and then tiled hard against every column in the middle of the space, because the column reads as an obstruction rather than as a restraint. It is a restraint. Tenting often starts within a metre or two of one.
3. At intervals across the field
Large floors need intermediate movement joints even where nothing is moving underneath, simply to keep the length of any continuous run short enough that the accumulated strain stays small. International tiling practice — BS 5385 and the TCNA guidance are the usual references — puts these at roughly 6 to 8 m internally, and considerably closer, of the order of 3 to 4 m, where the floor is external or in direct sun through glazing.
The second figure is the one that gets missed in India, and it is the one that matters most here. An atrium floor under a glazed roof, a shopfront zone behind full-height glass, an entrance apron, a podium deck — these are external floors as far as the physics is concerned, whatever the drawing calls them, and they need external spacing.
4. Wherever the floor changes its mind
At changes of plane, at doorways and thresholds, where the substrate changes from slab to screed or from concrete to steel deck, and where the finish changes material. Each is a discontinuity in stiffness, and stress concentrates at every one of them.
Three things that make it worse
- Laying on green concrete. Programme pressure pushes tiling forward, and a slab tiled at 28 days has most of its drying shrinkage still ahead of it — all of which is now happening underneath a bonded, stiff finish. Time is the cheapest mitigation available and the first one traded away.
- A rigid bed. A thick sand-cement mortar bed transmits substrate movement into the tile far more directly than a cementitious adhesive with a deformability classification does. On large-format stone over a young slab, the bed choice is a structural decision.
- Filling the joint solid. A movement joint packed with grout, mortar or a hard filler is not a movement joint. This happens at fit-out, after the joint was correctly formed, because an open line looks unfinished to whoever is closing out the works. It is worth writing into the specification that these joints stay compressible and are covered, not filled.
What to do about a floor that has already tented
Two things, in this order, and the sequence is what people get wrong.
First, find out why. Lifting the ridge and re-laying it is not a repair — the strain that lifted it is still in the floor, and it will lift again somewhere else, usually within a year. Establish whether the cause is a missing structural joint, a missing perimeter gap, a missing field joint, or a floor laid too early, because the answer changes the fix entirely.
Then cut in the joints that should have been there, and only then re-lay. Retrofitting a movement joint into an existing floor is straightforward — saw-cut the full depth of the finish and bed, clear the line, and cover it with a proprietary movement joint profile sized to the movement rather than to the width of the saw blade. Tiles that have already debonded have to be replaced; tiles that have not can usually stay.
The reverse sequence — re-lay first, joints later — is how the same floor gets repaired three times.
Where this meets what we make
The joint over a structural movement joint in a finished floor is a manufactured product, not a caulked line. It has to sit flush with the finish, take the traffic the floor takes — trolleys, beds, pallet trucks, occasionally vehicles — stay watertight where there is a membrane below, and keep working across the full movement cycle without lifting or rattling.
For floor-to-floor joints in trafficked interiors that is usually a MegAtec 340 or a MegAtec 370; where the run has to stay sealed, a FlexAtec 1200; and where the joint turns up a wall — which it always does eventually — a MegAtec 360. The full range is on the products page.
The intermediate field joints in the tiling are a different and much lighter product, and are usually the tiling contractor's scope. Both need to be on the drawing, and they need to be told apart — which is the subject of expansion, control and construction joints.