Liquid Screed Over Underfloor Heating: Depths, Drying Times and Commissioning
Quick answer
Liquid screed suits underfloor heating because it fully encases the pipes and conducts heat well at thinner depths. A floating flow screed is typically poured at 45–55mm total, giving 25–30mm of cover. Leave a cement-based screed seven days before commissioning, then raise the flow temperature 5°C per day through a controlled heat-up cycle.
Underfloor heating lives or dies by the screed around it. The pipes are the easy part — what determines whether the floor heats quickly, runs efficiently and never cracks is the material poured over them, the depth it’s poured at, and the discipline of the drying and commissioning that follows. Most UFH problems we’re called to investigate trace back to one of those three things, not the heating system. As approved installers for Cemfloor and Tarmac Topflow pouring floors across Oxfordshire every week, here’s how to get it right.
Why liquid screed and UFH belong together
A flow screed is poured as a liquid, so it wraps every pipe completely — no voids, no air pockets. That full contact matters: air is an insulator, and every void between pipe and screed is heat that never reaches the room. Liquid screeds also conduct heat better than conventional sand and cement and can be laid thinner, so the heating responds faster and runs at lower flow temperatures — which is exactly what heat pumps want. Our complete guide to liquid floor screeding covers the fundamentals; this piece focuses on the UFH-specific decisions.
Anhydrite or cement-based flow screed?
| Cement-based flow screed (e.g. Cemfloor) | Anhydrite (calcium sulphate) screed | |
|---|---|---|
| Drying | Fast — foot traffic in 24–48 hours, UFH commissioning from day 7 | Slower — drying typically runs to weeks, depth-dependent |
| Surface preparation before flooring | Minimal | Usually needs the surface laitance sanding off before adhesives |
| Wet areas | Suitable | Moisture-sensitive — needs care in bathrooms and wet rooms |
| Where it wins | Programme speed, wet rooms, renovation timetables | Large open pours; well-proven with UFH where time allows |
Both work well over UFH when specified and installed properly. The honest answer is that the right screed depends on the building, the programme and the floor finish going on top — which is why we talk through all three before quoting.
Screed depth over UFH pipes: the number that matters
The non-negotiable rule is that every pipe is fully covered with enough screed above the crown to distribute heat evenly and protect the pipe — plus enough total depth to cover any services crossing the floor. On a typical domestic UFH job we’re pouring a floating liquid screed at around 45–55mm total depth, giving roughly 25–30mm of cover over the pipe — but the governing figure is always the manufacturer’s datasheet for the specific product, system and insulation build-up, and depths carry a tolerance to absorb unevenness in the base.
For comparison, conventional sand-and-cement screed over UFH typically needs 65–75mm or more — one reason flow screeds respond faster and suit renovations where build-up height is precious. Cemfloor’s guidance on screed depth and underfloor heating is a good reference for how insulation type and pipe fixing systems change the calculation. Too thin risks cracking and pipe damage; too thick adds thermal mass that slows the system down and weight the structure doesn’t need.
Two details that get missed and cause real problems: pipes must be properly fixed down so they can’t float when the liquid screed goes in, and the base needs preparing correctly first — our guides to priming before screeding and floor preparation cover both.
Drying: the stage nobody wants to wait for
Drying is where good floors get ruined by impatient programmes. Using Cemfloor Therm as the worked example (the protocol comes from the manufacturer’s UFH guidelines):
- First 24–48 hours: the room is sealed — windows and doors closed — while initial hardening completes (24 hours in summer, 48 in winter). The floor will take foot traffic after this.
- Then: ventilate during the day to assist drying, closing up overnight.
- Drying continues until the screed reaches the moisture content the floor finish requires — for Cemfloor, 2.5% CM, confirmed by testing, not guesswork.
Expansion joints and bay sizing are part of the same conversation — get them wrong and the heating cycle finds out for you. We’ve covered the rules in our guide to expansion joints and bay sizes.
Commissioning the UFH: the heat-up cycle that protects the floor
You don’t just switch underfloor heating on. The screed needs taking through a controlled first heating cycle — and skipping it is the single most reliable way to crack a new floor. The Cemfloor protocol, which is typical of the genre:
- No heat for the first 7 days after the pour.
- From day 7, commission the system at a low flow temperature — around 25°C to start.
- Increase the flow temperature by 5°C per day up to the datasheet maximum (40°C for Cemfloor Therm), keeping room air below 25°C.
- Run until the screed has dried to its required moisture content, then ramp back down by 10°C per day to 20°C before switching off and allowing the floor to cool.
The commissioning should be done by the UFH contractor and, ideally, recorded — flooring manufacturers increasingly ask for evidence of the cycle before honouring warranties. Used properly, the UFH itself becomes the drying tool, pulling weeks out of the programme without risking the slab.
Moisture testing before the floor finish goes down
Whatever the screed, the flooring fitter will (or should) demand a moisture test before laying tiles, timber or vinyl — and “it’s been down six weeks, it’ll be fine” is not a test. Trapped moisture under an impermeable finish is how adhesives fail and timber cups. We test, we document, and the finish goes down when the numbers say so. If you’re choosing finishes, our guide to liquid screed finishes explains what works over heated floors.
The failures we get called to fix
The same handful, every time: pipes that floated because they weren’t fixed; no edge isolation strip, so the slab had nowhere to expand; missing or misplaced joints across doorways; heating switched on full in week one; finishes laid on wet screed. Every one of them is preventable at the design and installation stage — which is the argument for having the screed and the UFH treated as one system by people who do it constantly. It’s also why we invest in the kit: our Euromair X-PRO D80 pump puts consistent, properly mixed material down at pace, on programme.
Liquid screed and UFH — FAQs
How deep should screed be over underfloor heating pipes?
Deep enough to fully cover every pipe with sufficient cover to spread heat evenly — typically 45–55mm total for a floating liquid screed on a domestic job. The manufacturer’s datasheet for the specific product governs the exact figure.
How long before you can turn on underfloor heating after screeding?
With a cement-based flow screed such as Cemfloor, no heat for 7 days, then a controlled commissioning cycle starting around 25°C and rising 5°C per day. Conventional screeds need considerably longer before first heat.
What’s the best screed for underfloor heating?
Liquid flow screeds — cement-based or anhydrite — outperform conventional sand and cement over UFH because they fully encase the pipes and conduct heat better at thinner depths. Which type wins depends on the programme, the rooms and the floor finish.
Can you walk on liquid screed the next day?
Usually yes — cement-based flow screeds take foot traffic after 24–48 hours. That’s foot traffic, not point loads, trades and materials storage.
Specifying screed for an underfloor heating project? Send us the floor build-up and we’ll tell you what we’d pour, at what depth, and what the programme genuinely needs to allow. Liquid screed in Oxford · Contact us · 01865 416811.
