Planning underfloor heating in your renovation is often all about the end game – invisible comfort, the uncluttered walls, the warm kitchen tiles on a chilly January morning. But beneath every successful underfloor heating system sits something far less glamorous, though just as important: the screed.
For most homeowners, screed is one of those building terms that appears briefly in quotations and disappears beneath the finished floor forever. Yet the type of screed used, its depth, its curing time, and its compatibility with the heating system can have a major impact on how well your underfloor heating system performs for years to come.
Dylan Sutton, a director at Cozia Systems, with 20 years of screeding experience under his belt, comments:
“Nobody ever fell in love with a floor because of its screed. And yet if you get it wrong, all manner of problems can arise.
“Cracking, slow heat response, uneven floors, refusal of flooring contractors to lay floor coverings or provide warranties on their products, and damage to expensive floor finishes down the line can all delay a project and cause huge inconvenience and expense.
“But get it right, and the floor feels solid, efficient and responsive for years to come. In reality, screed is less of a commodity and more of a science.”
First Things First: What Is Screed?
Before getting into the details, it helps to understand that “screed” isn’t one thing. The term covers a family of materials that behave quite differently and serve distinct purposes.
Screed is the layer laid over a structural subfloor (usually concrete or insulation boards) to create a smooth, level surface ready for the final floor finish.
In underfloor heating systems, screed plays another critical role: it surrounds the heating pipes and acts as a thermal conductor, helping distribute heat evenly across the floor.
Different screeds behave differently. Some dry quickly, some conduct heat more efficiently, some are stronger under heavy loads, and some are better suited to awkward renovation projects where floor heights are tight and thinner pours are required.
That’s why choosing the right screed isn’t simply about pouring concrete onto a floor. It’s about understanding the many screed products on the market – and various additives – and knowing exactly what will work best for each installation, balancing cost efficiencies along the way for homeowners.
Main Types of Screed
1. Traditional Sand & Cement Screed
The longstanding workhorse of the construction world, sand and cement screed is typically mixed on site using sharp sand and cement, then manually levelled by screeders, working their way gradually across the floor.
This is the screed most often specified on architect and building regulation drawings at around 75mm depth, as it’s suitable for domestic applications and generally a lower material cost. However it’s rarely used in combination with underfloor heating due to its lower thermal efficiency around UFH pipes compared to flow screeds.
2. Flow or Liquid Screed
Flow screeds have become increasingly popular for use with underfloor heating systems because of how effectively they wrap around heating pipes and the flat finish they leave.
There are two main families of flow screed – Anhydrite (calcium sulphate) and Cement-based – but they share many benefits:
- Excellent for Underfloor Heating – because the screed flows around the pipes rather than being compacted by hand, there are fewer voids or air pockets. This means, better heat transfer, more even floor temperatures and improved system efficiency.
- Very Flat Floors – flow screeds produce highly level surfaces (often to SR2 or SR1 tolerances) across large areas. That’s particularly useful for large-format tiles, engineered timber or vinyl and other sensitive floor finishes.
- Faster Installation – arriving by truck, ready mixed, flow screeds can therefore be pumped in quickly across large areas. A large open-plan ground floor that might take a team a full day in traditional screed can often be poured in a matter of hours using flow screed.
- Thinner Floor Build-Ups – flow screeds can be poured thinner than traditional sand & cement screeds, while still maintaining strength. That can help enormously in renovations where floor heights are restricted.
When choosing between anhydrite and cementitious screed options, cost and project timescales usually dictate the most appropriate solution:
- Cost – anhydrite screeds are generally the more cost-effective route, although these screeds dry with a film (laitance) which needs to be removed if the chosen floor finish is to be adhered to the screed. This can add time and extra expense to the final bill so should be factored in.
- Project Timescales – cementitious screeds offer faster drying formulations and don’t require laitance removal. Whilst both screed types can accept foot traffic within 24-48 hours (so trades can get back on site to carry on work), the cure time is key as this dictates when enough moisture has escaped to allow floor coverings to be installed.
Anhydrite generally cures at a rate of 1mm per day for the first 40mm and 0.5mm per day thereafter, so a 50mm deep pour would take approximately 60 days (with ideal drying conditions) to accept floor coverings. In contrast, a 50mm deep cementitious screed would cure in 21 days – a fraction of the time. Even faster drying formulations than that, with additives, are available if project timeframes demand.
For homeowners working to tight renovation timelines, screed choice often becomes a key consideration.
3. Self-Levelling Compounds
Homeowners frequently hear “self-levelling screed” used interchangeably with flow or liquid screed, but they’re not the same product.
Self-levelling compounds are usually much thinner, used to correct or smooth minor floor imperfections in existing substrates, skim surfaces before final flooring installation or go over underfloor heating overlay panels in a thin layer of circa 10mm depth, as per industry standards.
Self-levelling compounds generally come in bagged powder form, to be mixed and poured on site rather than being delivered pre-mixed.
It’s Not Just the Screed That Matters
Ultimately, underfloor heating is only ever as good as the floor built around it. The pipes may do the heating, but it’s the screed, and everything beneath it, that determines how efficiently, evenly and reliably that warmth is delivered over time.
From damp proof membranes and insulation boards to perimeter edge strips, expansion allowances and correctly primed surfaces and ambient temperature, the unseen groundwork beneath the finished floor is where many renovation projects are quietly won or lost.
Cozia’s Dylan Sutton says:
“For homeowners navigating renovations, extensions or self-builds, the technical detail can feel overwhelming, particularly when multiple trades are involved. That’s why an integrated approach works best.
“Bringing underfloor heating, subfloor preparation and screeding together under one roof can help avoid delays, reduce costly mistakes and ensure every layer of the floor performs exactly as it should. Because while nobody ever sees the screed, you’ll feel the difference every single day.”
if you’d like to know more about how to look after a newly screeded floor, download our Guide to Screed Aftercare.
If you have a project you’d like to discuss with our expert team, get in touch with us on 01283 759085.
