Deflection: the number nobody mentions until it goes wrong
Timber floors flex under load. Tile and grout do not. When a joist deflects more than roughly L/360 of its span under a point load, the rigid tile skin above it cracks or debonds even when the adhesive was applied perfectly.
On suspended timber floors we check joist size, spacing and span before quoting tile, and where deflection is borderline we specify overboarding with 18-22mm plywood or a cement particle board screwed at close centres to stiffen the structure.
Chipboard flooring is a common culprit. It swells at joints when damp and moves under load in a way tile adhesive cannot bridge, so it is either overboarded or replaced rather than tiled onto directly.
Decoupling membranes and why they earn their keep
A decoupling or uncoupling membrane is a thin studded sheet, typically 3-6mm, bonded to the substrate that allows minor movement between the floor structure and the tile bed without transferring stress upward. Products like Schluter DITRA or Ditra-equivalent membranes are our default over timber floors and screeds with hairline movement.
Over new sand and cement screeds, membranes also isolate residual curing shrinkage from the tiles above, which matters because a new screed can keep moving for weeks after it feels dry to the touch.
They are not needed everywhere. A solid, well-cured concrete slab in good condition with no history of movement can often be tiled directly with the right primer, and adding a membrane there is an unnecessary cost.
Screed and levelling: getting the base flat first
Floor tile adhesive is not a levelling compound. Anything above about 3mm of variation across a metre needs correcting with a self-levelling compound before adhesive goes near it, or you end up lippage-chasing tile by tile.
New sand and cement screeds need proper curing time, roughly one day per millimetre of thickness up to about 40mm, before they are dry enough to tile. Rushing this traps moisture that later pushes tiles or blows the adhesive bond.
On older concrete floors we check for surface laitance, old adhesive residue and damp with a meter before deciding whether a primer, a levelling skim or a full damp-proof membrane is required.
Movement joints: the detail that stops cracking
Tiled floors need movement joints at perimeters, at every 40m2 internally (less where there is direct sunlight or underfloor heating), and wherever the substrate changes, such as at a doorway between a timber floor and a concrete slab.
A perimeter gap of around 5-8mm, hidden under skirting or a trim, allows the whole tiled field to expand slightly without pushing against walls and cracking grout lines.
Underfloor heating floors need this taken more seriously still, since the thermal cycling is greater. We use flexible movement joint profiles rather than relying on grout alone to absorb it.
Adhesive and grout choice for floors
Floors generally need a deformable, flexible adhesive rated C2TE S1 or S2, particularly over timber, membranes or underfloor heating, because the S-classification is specifically about accommodating movement.
Grout on floors should be CG2 cementitious for most residential jobs, or an epoxy grout in wet rooms and shower floors where stain resistance and total waterproofing matter more.
| Substrate | Typical issue | Usual treatment |
|---|---|---|
| Suspended timber | Deflection, movement | Overboard, decoupling membrane, C2TE S1 |
| Chipboard | Swelling at joints | Overboard with ply or cement board |
| New screed | Curing shrinkage | Full cure time, then membrane or primer |
| Old concrete slab | Laitance, residual damp | Prime or level, check damp meter first |
| Over underfloor heating | Thermal cycling | Flexible adhesive, extra movement joints |
Where to read next
Questions we get asked most
Can I tile straight onto floorboards?
No. Floorboards move too much individually. We always overboard with plywood or cement board first, screwed down at close centres, to give a rigid enough base.
Do I need a decoupling membrane on every floor?
Not always. A solid, well-cured concrete slab in good condition often does not need one. Timber floors, new screeds and underfloor heating usually do.
How long does a new screed need to dry before tiling?
As a rule of thumb, roughly one day per millimetre of thickness up to 40mm, checked with a moisture meter rather than by eye or feel.
What happens if a floor is tiled without movement joints?
Grout lines and tiles crack, usually in a straight line following where the movement was trying to happen, most commonly at doorways and room perimeters.
Is floor tiling much more expensive than wall tiling?
The tile cost is similar, but floor prep, levelling and membranes add labour and material cost that wall tiling rarely needs, which is why floor quotes are usually higher per square metre.