Quality with FransWhat it is
Debonding is when the tile, the adhesive or the substrate lets go of each other. It starts quietly. First you hear a dull note when you tap the tile, then the grout cracks, then the tile rocks, pops or lifts. When restrained tiles have nowhere to expand, they can push each other up into a ridge. That is "tenting".
Hollow does not always mean failed. A US tile consultant writing in TileLetter, and Ardex in its technical papers, both point out that tiles can sound hollow and still be held. Small voids, some membranes and acoustic mats, and some substrates give a hollow note over a tile that is still bonded. What matters is how much of the tile and the floor is hollow, where it is, and whether there is a pattern.
Defect ID table
| Symptom | Likely cause | Check |
|---|---|---|
| Hollow at corners and edges, solid in the middle | Spot bonding ("dot and dab"), or ridges of adhesive not collapsed | Lift one tile: look for five blobs or untouched trowel lines on the back |
| Whole tile hollow, adhesive stuck to the screed only | Adhesive skinned over (open time exceeded), dusty or dirty tile back | Lift a tile: a clean tile back with combed lines still sharp on the floor |
| Adhesive stuck to the tile, screed surface came with it | Weak surface or laitance not removed, no primer, screed too wet | Scratch test the screed; check moisture test records and primer used |
| Tiles lifting off a calcium sulphate (anhydrite) screed | Cement adhesive on an unprimed or wet anhydrite screed | Moisture records against the adhesive maker's limit; primer type and coats |
| Tenting ridge or popped tiles in a long run | No perimeter or field movement joints, tiles tight to walls, thermal or moisture movement, new slab still shrinking | Lift the skirting: is there a gap? Measure joint spacing against the drawing |
| Cracked grout lines following a slab joint | Slab or screed joint not carried up through the tiling | Compare tile joint layout with the structural and screed joints |
| Hollow and cracked near sunny glazing or heated floors | Thermal movement, adhesive not deformable enough | Check the adhesive class (e.g. S1) on the bag and data sheet |
| Lippage and rocking on large tiles | Uneven substrate, adhesive bed too thin, not back-buttered | Straightedge on the substrate and tiles; lift one large tile |
Main causes, in plain terms
- Not enough adhesive contact. Industry guides agree: spread with a notched trowel, press and slide the tile across the ridges so they collapse, and back-butter large or deep-keyed tiles. Spot fixing with a few dabs is called out as wrong practice by The Tile Association, a US consultant and Ardex.
- Poor substrate. The Tile Association asks for backgrounds that are dry, stable, strong and clean before tiling, with laitance and curing agents removed.
- Screed too wet or too new. New concrete keeps shrinking while the tiles do not, which Laticrete links directly to tenting. On calcium sulphate screeds, UK adhesive makers (Palace and Instarmac) work to below about 75% RH with a hygrometer and ask for laitance removal and a primer. Palace warns that cement adhesive on a damp anhydrite screed can react and lift the tiles.
- Missing movement joints. CTaSC and Laticrete both name this as a leading cause of tenting, usually combined with a weak bond.
- Thermal and moisture movement. Sun through glazing, underfloor heating and porous tiles taking up moisture all make the tiling move. Laticrete says rooms like this may need to be treated as exterior.
- Wrong adhesive class. Under EN 12004, C1 is a normal and C2 an improved cement adhesive. S1 (deformable, 2.5 to 5 mm in the test) and S2 (highly deformable, 5 mm or more) handle more movement. An S rating will not rescue a bad floor, as an Ardex trainer points out in Tile & Stone Journal.
How much coverage? The sources do not agree
| Source | Dry / general | Wet / exterior |
|---|---|---|
| US (ANSI A108.5 / TCNA, as explained by CTEF and TileLetter) | 80% average contact, edges and corners supported | 95% for exterior and interior wet areas. Some US consultants also ask 95% for commercial floors |
| UK (BS 5385, as explained by The Tile Association and BAL) | Floors solidly bedded, inside or outside. Dry internal walls: ribbed bed with at least 50% contact; tiles of 0.1 m² or more, or heavy for the background, solidly bedded (2018 edition) | Solid bed in showers, pools and all external locations |
| Australia (AS 3958, as quoted by Ardex and HIA) | 65% internal residential walls, 80% residential floors, 90% commercial floors | 90% for wet areas, external walls and floors |
| Ardex (own guidance) | 85% minimum as a general rule | 90% or more for exposed or high-load areas; 100% where submerged |
So for a UK floor the target is a solid bed, not a percentage. For US natural stone, CTEF also says no single void should be bigger than about 13 cm² (2 sq in), and none within 50 mm (2 in) of a corner. Your specification decides which applies.
On hollow area, I found no UK or US standard that sets a limit. The 2007 Australian standard, as quoted by Ardex, treats a tile that is more than about 20% hollow as suspect over the long term, adjusted for location and traffic. A US consultant called 25% of a restaurant floor sounding hollow a red flag. Agree an acceptance rule in the ITP before work starts, not after.
How to measure and record
- Tap every tile with a steel ball bearing or a tapping rod. On big floors, drag a chain and listen for the change in note. Compare with tiles you know are solid.
- Mark hollow tiles on a floor plan, using a grid reference for each one. Note if the hollow area is at the edges, the whole tile or a cluster.
- Work out the percentage: hollow tiles divided by tiles tested, per room or bay. For single tiles, estimate the hollow part of the face (Ardex suggests a simple grid, about 10% per square).
- Lift tiles from both a hollow area and a solid area, as TileLetter recommends. That is the only way to see the real cause. CTEF says ANSI checks at least three tiles. Photograph the tile back and the substrate with a scale.
- Record the evidence: adhesive batch and class, trowel notch size, moisture test results, primer, movement joint positions, and the photos. Raise an NCR when it falls outside your agreed rule.
Prevention and repair
Prevention is a hold point before tiling: substrate dry, flat, clean and primed; moisture readings on record; movement joints on the drawing (TCNA says the designer must show them); the right adhesive class for the background and exposure. During tiling, lift a freshly laid tile now and then to check coverage. CTEF and Ardex both recommend this.
For movement joints, the TCNA detail EJ171 (as summarised by CTaSC and The Construction Specifier) gives perimeter joints of at least about 6 mm, field joints every 6 to 7.6 m indoors, and every 2.4 to 3.7 m outdoors or in sun or wet. The 2024 BS 5385-3, as reported by The Tile Association, gives 3 to 5 m for external floors. Follow your specification first.
Repair starts with the cause. Loose, cracked or tented tiles need to come up, the substrate made good, and the tiles re-laid in a full bed with joints added where missing. For tiles that sound hollow but are still held and undamaged, resin injection through small holes in the grout is offered by Ardex and tool suppliers. Ardex calls it a last resort, because it does not fix the original cause.
Why this matters for QA/QC
Tiling is a finish, so it gets judged late, often at snagging when the floor is clean and the client is walking it. That is the worst time to find that half the corridor was spot bonded. Hollow tiles crack under point loads, let water into voids in wet areas and can end in lifting and trip hazards.
As an inspector, you can catch most of this early. The causes are visible before the tiles go down (moisture, primer, joints) or in the first hour (coverage on a lifted tile). If you only tap at handover, you can only argue about the result. Agree with the contractor, in writing, how hollow tiles will be tested, how many lifts, and what is acceptable.
Check on your site tomorrow
- Ask for the screed moisture test results and the primer used before the next floor is tiled.
- Lift one freshly laid tile in each active area. Is the back fully wetted, with corners and edges covered?
- Lift a skirting or trim at the wall: is there a perimeter gap, or are the tiles tight to the wall?
- Check the adhesive bags against the specification: C1 or C2, and S1 or S2 where required.
- Tap one finished room and mark any hollow tiles on a plan. Start the record now.
Same habit, different trade: check the work against what was approved before it gets covered up. The 31-point checklist I use before every concrete pour.
Download the checklist
Sources: The Tile Association: Brief guidance on the use of adhesives · The Tile Association: Changes to BS 5385-1:2018 · BAL: Changes to BS 5385-1 · The Tile Association: External floor tiling and movement joints · CTEF: Insufficient mortar · TileLetter (D. Pompo): Hollow-sounding tiles · CTaSC: Why tile tents · The Construction Specifier: Movement joints for tile and stone · Laticrete: TDS-1252 Movement joints · Ardex: TP002 Adhesive coverage · Ardex: TB252 Resin injection under drummy tiles · HIA: Updates to AS 3958 · Palace Chemicals: Tiling on anhydrite screed · Instarmac: Tiling to calcium sulphate screeds · Tile & Stone Journal: S1 and S2 adhesives · Karl Dahm: Tile injection resin. Summary in my own words; your specification and the adhesive manufacturer's instructions apply.