Tiling over underfloor heating — joints, clearances and commissioning
A heated floor moves every heating season: it expands as it warms and contracts as it cools, cycle after cycle for its whole life. Every figure in this guide serves one idea — giving that movement somewhere to go other than the back of your tiles.
The three figures everyone asks for
Before the detail, the direct answers:
| Where | Figure | Why |
|---|---|---|
| Between tiles | 3 mm minimum | Absorbs each tile's expansion against its neighbour |
| Last tile to wall (perimeter) | 8–10 mm | The whole field expands outward; the skirting hides it |
| Field division joints | Flexible joint every ~40 m², no side over 8 m | Large fields are divided so their parts move independently |
| Over the screed's own joints | The joint always carries through the tiling | Two heating loops move to two different rhythms |
The adhesive: S1 is the floor, not a luxury
Plain C1 adhesive is out over heating: the repeated thermal shear between a near-steady tile face and a cycling substrate will break a non-deformable adhesive within a few seasons. The floor is C2 with S1 deformability (≥2.5 mm); for formats over 60 cm or thin slabs, S2. Grout flexibly too — a deformable adhesive between brittle joints is pointless.
Before tiling: commissioning the screed is mandatory
A heated screed is commissioned before tiling, not after: run the heating up gradually to its maximum working temperature, hold it for days, then back down — that cycle forces the first thermal shrinkage out of the screed while you are still looking at bare screed rather than cracked tiles. Any cracks the cycle opens are repaired before bonding.
Then switch the heating off (or below 20 °C) 48 hours before tiling and keep it off while you work: adhesive curing on a warm bed loses its water too fast to build strength. A heated screed's moisture must not exceed 1.8% CM.
The tiling itself
Everything in the general tiling guide applies, with two points tightened: coverage ≥90% is mandatory — an air pocket over heating is a small insulator that makes a cold spot and a stress differential above it; the route there is double-buttering everything over 30 cm. Comb in one direction (no swirls) so air escapes at the end of the pass.
After grouting: the heat-up schedule
Keep the heating off for 7 days after grouting — adhesive and grout build their strength in those days, and heat during them dries the chemistry out before it completes. Then raise the temperature 5 °C per day to the working level, and step down the same way at the first shutdown. Thermal shock on first firing is the last chance for hidden mistakes to announce themselves — do not give it one.
The underfloor-heating mistakes specifically
- Plain C1 "because it is indoors" — S1 deformability is the requirement, not the room.
- Tiling a never-commissioned screed: its first thermal shrinkage happens under the tiles.
- 1–2 mm joints because "it looks cleaner" — tenting by the first winter.
- A tile hard against the wall with no 8–10 mm perimeter.
- Firing the heating to full a week later in one step instead of 5 °C a day.
- Rigid grout across an otherwise deformable assembly.
Frequently asked
Hydronic or electric — do the figures change?
Same rules: S1, 3 mm joints, 8–10 mm perimeter, gradual heat-up. Electric mats buried in a thin levelling layer need extra attention to adhesive coverage because the element sits closer to the tile.
Do 120 cm porcelain slabs work over heating?
Yes, on its terms: S1 minimum and preferably S2, double-buttering with a 10–12 mm notch, ≥90% coverage verified by lifting tiles, and joints never under 3 mm however rectified the slab.
When can I switch the heating on after tiling?
Seven days after grouting, then 5 °C a day. Rushing here saves days and gambles the whole floor.
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Last updated: 2026-08-09