A heating engineer called me to look at a floor where several of the anhydrite screed panels had cracked across the full width of the room. The UFH had been commissioned 10 days after pouring - much too early. The screed had cured sufficiently to bear foot traffic but had not dried enough for thermal cycling. Expansion and contraction as the heating ramped up too fast had introduced stress cracks that then propagated through the full floor.
The screed had to be removed and relaid. The cost exceeded the original screed contract.
Underfloor heating and screed work well together. But the installation sequence and commissioning protocol are non-negotiable.
Why screed depth matters for UFH
The screed over UFH pipes affects:
Heat output: A thick screed stores more heat (thermal mass) and takes longer to warm up. A thinner screed responds faster. For most domestic use, faster response is preferred - the occupant can turn the heating on and feel warmth sooner.
Structural integrity: Below the minimum depth, the screed is insufficient to bridge the pipe and distribute loads. Cracks can form along the pipe runs as the thin screed section between pipes behaves differently from the sections above pipes.
Cracking risk: Screed expands and contracts with temperature. Without adequate depth, the thermal gradient through the screed is too steep and the surface can crack.
Minimum depths by screed type
| Screed type | Minimum above insulation | Minimum above top of pipe |
|---|---|---|
| Sand:cement | 75mm | 65mm |
| Anhydrite (liquid screed) | 45-50mm | 30mm |
| Rapid-set | Follow manufacturer | Typically 65mm |
These are absolute minimums. Going slightly deeper (say 80-85mm for sand:cement) increases thermal mass but does not harm the system.
Why most UFH systems now specify anhydrite
The difference in minimum depth is significant in practice:
- Sand:cement: 75mm total above insulation = significant floor build-up
- Anhydrite: 45-50mm total above insulation = 25-30mm less floor depth
On a new extension with a 150mm concrete slab, this 25-30mm saving in screed depth directly affects window cill heights, door thresholds, step heights and DPC clearances. It is not trivial.
Beyond depth, anhydrite screed has:
- Better thermal conductivity than sand:cement (heat transfers more efficiently to the surface)
- Self-levelling application that flows completely around the pipes (no voids)
- Less cracking risk (it shrinks less on curing)
The downside: higher material cost, laitance must be removed before tiling, and the system requires specific UFH commissioning protocol before use.
The commissioning programme
This is where most problems arise. Rushing the commissioning process is the primary cause of screed cracking over UFH.
Minimum waiting time before commissioning:
- 28 days for sand:cement screed in normal conditions
- 21 days for anhydrite screed (though 28 is safer)
- 14 days absolute minimum in any circumstances (follow manufacturer if shorter)
The commissioning protocol:
| Day | Water temperature |
|---|---|
| 1-3 | 25°C |
| 4-7 | Increase by 5°C per day up to max operating temperature |
| 8-14 | Hold at maximum operating temperature |
| 15+ | Normal operation |
The purpose is to allow moisture to escape the screed gradually as temperature rises. Rapid heating from cold creates too steep a moisture gradient and induces cracking.
After the commissioning programme:
- Allow the screed to cool fully (24-48 hours)
- Then lay floor coverings
Expansion joints
Screed over UFH requires more careful attention to expansion joints than standard screed:
- All room perimeters must have an expansion gap (typically 10mm edge strip foam)
- At doorways and changes of direction
- At approximately 40m2 intervals in large open areas (anhydrite: 40m2, sand:cement: smaller)
The UFH loops must be continuous within each expansion zone - pipes should not cross expansion joints.
Floor coverings over UFH screed
Not all floor coverings are equally compatible with underfloor heating:
| Floor covering | UFH compatibility | Notes |
|---|---|---|
| Porcelain/ceramic tiles | Excellent | Best conductor, fastest response |
| Natural stone | Very good | High thermal mass, good conductor |
| Engineered wood | Good (with correct product) | Max water temp 55°C typically |
| LVT / vinyl plank | Good | Check manufacturer temp rating |
| Solid hardwood | Poor to moderate | Can dry and gap; check species |
| Carpet | Poor | Insulating effect - check TOG rating |
For underfloor heating, floor coverings with combined resistance below 0.15 m²K/W are preferred. Thick carpets with high thermal resistance significantly reduce UFH efficiency.
My tips on screed over underfloor heating
Get the UFH commissioning protocol in writing from the screed supplier. If there is a dispute about cracking, the commissioning record is critical. Note the dates, water temperatures and readings.
Pressure test the UFH before screed is laid. The pipes must be pressure-tested and left at working pressure while the screed is poured and cures. This confirms there are no leaks before they are buried and identifies any damage during screeding.
Do not lay the screed in winter without heating. Cold ambient temperature slows curing. If the building is unheated in cold weather, maintain a minimum of 5°C using temporary heating until the screed has cured (7 days minimum).
Agree the commissioning schedule with the flooring contractor. The floor covering installer needs to know when the commissioning programme ends and the floor is ready. Do not tile or lay wood flooring while the commissioning programme is running.
Use the Floor Screed Calculator to work out how many bags or tonnes of screed you need for your project.