Why does the floor above the insulation board crack or buckle

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Why does the floor above an insulation board crack or warp – a complete technical analysis

We deal with this problem with customers regularly. They come with photos – here a cracked joint in the tiling, there a wave in a floating floor, somewhere laminate lifting at the wall. And it almost always starts with the same phone call: "We did it exactly according to the instructions, but something happened." When we then look deeper into the installation details, a whole range of mistakes appears, each of which alone might not cause a problem – but combined, the result looks exactly like this.

The floor above a system insulation board is not just a matter of aesthetics. Cracking, warping and bulging are symptoms of mechanical, thermal or moisture stress. This article will explain why these phenomena occur, how to diagnose them and – most importantly – how to prevent them in your next installation.

Basic mechanics: what happens in the floor structure

Before we get into specific causes, we need to understand what forces act within the floor structure above the insulation board. A modern floor structure with underfloor heating typically consists of several layers with different mechanical properties – and it is precisely this difference in properties that causes the problems.

In this structure, each layer behaves differently when temperatures change. Cement screed expands differently than EPS (expanded polystyrene) insulation board when heated. Tiles glued to the screed have yet another coefficient of thermal expansion. And if there is no proper separation or expansion joint between the layers, a mechanical conflict arises – that is, cracking, bulging or warping.

Cause No. 1 – Missing or insufficient expansion joints

This is by far the most common cause of cracking in a floor above a system board. Cement screed expands linearly when underfloor heating heats it up. The coefficient of thermal expansion of concrete is roughly 10–12 × 10⁻⁶ /K, meaning that a 10-metre screed will elongate by 2–2.4 mm when the temperature rises by 20 °C. That sounds like little, but if the screed has nowhere to expand, stress accumulates – and eventually gives way somewhere, usually at the point of least resistance.

In practice, it looks like this: the installer pours the screed, forgets to make expansion joints at the walls and in large areas (typically every 25–40 m² or when an area exceeds 6 m in one direction). The first winter everything is fine because the floor is not yet in operation. When the heating is switched on for the first time in autumn, the screed heats up from 15 °C to 35 °C – and cracked tiles or lifted laminate appear at the wall in several places.

The highest-risk locations are:

Transitions between rooms (doors without a threshold strip / expansion profile)
Long corridors where the screed has no lateral expansion joint
Places where the floor passes around a fixed structure – a column, staircase, masonry partition
Corner areas in large open-plan spaces
Any place where the installer forgot to insert an edge expansion strip

What thickness of edge expansion strip is needed?

An edge expansion strip (usually made of mineral wool or PE foam, 8–10 mm thick) must be inserted along all walls, columns, corners and fixed structures, along the entire height of the screed – not just at the base. A very common mistake is that the strip reaches only to the top level of the screed but does not extend under the wear layer – and it is precisely at the wall that the pressure then causes the tiles to lift. The strip must be left protruding 10–15 mm above the wear surface and only cut off after the covering has been laid.

Cause No. 2 – Moisture in and beneath the screed

The second major problem is moisture – in two forms. The first is residual moisture in the screed that was not sufficiently dried before the wear layer was laid. The second is condensation moisture, which can form at the bottom surface of the screed under certain conditions.

Residual moisture in the screed

Cement screed needs considerably more time to dry than most installers assume. The basic rule states: 1 mm of thickness = 1 day of drying, at normal temperature and ventilation. For a 65 mm thick screed, that means at least 65 days. Anhydrite screeds dry faster (by roughly 20–30 %), but still need at least 5–7 weeks. If the wear layer is laid too soon, the residual moisture has nowhere to go – and starts acting upward (bulging) or towards the insulation board (soaking the EPS, loss of rigidity).

Measuring the screed's moisture content before laying is not a luxury but a necessity. The recommended maximum values are:

Cement screed: CM ≤ 2.0 % (carbide method), or ≤ 1.8 % before gluing tiles
Anhydrite screed: CM ≤ 0.5 %
Under PVC / vinyl (resilient covering): even stricter – CM ≤ 1.5 % for cement

We have seen cases where a customer laid a laminate floor 3 weeks after pouring the screed "because it seemed dry". Half a year later they called to say the floor was warping and bulging. The moisture was released during the first seasonal heating, the wood absorbed the vapour and expanded.

Condensation moisture and the role of the separation foil

Less obvious, but equally dangerous, is the opposite direction of moisture movement – from the bottom upward through the insulation board. If the floor lies on a concrete slab above ground (ground floor without a basement), ground moisture rises capillary-wise. If a vapour barrier is not properly placed under the system board, moisture passes through the board and condenses on the bottom surface of the screed. Result: the EPS gradually softens, loses load-bearing capacity, and the screed begins to sag locally.

The Separation foil 601001H is used for separation between the screed and the system board – a PE foil that also prevents cement slurry from mechanically penetrating the system board, thus protecting the retention pins and maintaining the geometric accuracy of the laying grid. This foil also performs the function of separating movements – the screed can freely slide over the insulation board without creating mixed stress between them.

Cause No. 3 – Insufficient rigidity and load-bearing capacity of the polystyrene

Not every polystyrene is suitable under a wet screed with underfloor heating. EPS used in floor structures must meet the STN EN 13163 standard and must have sufficient compressive strength. For underfloor heating with screed, EPS marked Cs(10) ≥ 100 kPa is used, meaning the stress at 10 % deformation must be at least 100 kPa.

What happens when a substandard or incorrect type of EPS is used? Under load (people, furniture), the screed sags locally, the system board deforms plastically, and furniture stands on an uneven surface. Tiles crack diagonally because the subsurfaces beneath them do not have the same rigidity.

For wet systems (screed) we recommend:

System insulation board UHP55 (STIROTERMAL BASIC) – a universal system board for common residential loads, with retention pins for a 50 × 50 mm grid
System insulation board UHP51 (STIROTERMAL DUO 11) – a combined board with an aluminium cladding for systems requiring a better heat distributor within the board

For dry systems (without screed) there is a specific alternative – Polystyrene for dry underfloor heating UHPD (STIROTERMAL DRY), where the screed is eliminated entirely and the pipe is laid into grooves directly in the polystyrene with an aluminium distributor. This system has different requirements for the top layer (dry board, not screed), which is why the mechanical behaviour of the floor is also different.

Cause No. 4 – Poor-quality or improperly laid boards

System insulation boards are mostly laid with tongue and groove or with a mutual interlocking profile. If the boards are laid without interlocking with each other, gaps arise between them – and when the screed is poured, cement slurry leaks into these gaps. This alone need not be a problem, but the gaps create weak spots in the insulation (thermal bridges) and also lines along which the boards can move independently of each other. Result: the screed above the gap cracks along the joint line.

Another classic problem is an uneven subsurface under the system boards. When the concrete slab or screed base is not sufficiently level (the permitted deviation is 5 mm over a 2-metre straightedge), the system board does not follow the subsurface across its whole area, but only on the high points. In the middle of a "sagging" area, the board "hangs" – and when load is applied to it, it bends. With screed, this leads to the screed cracking along the edges of the boards.

Therefore a strict rule applies: before laying system boards, the subsurface must be cleared of protrusions (grinding, chipping), depressions greater than 5 mm must be filled with a self-levelling compound, and flatness must be checked with a 2-metre straightedge. This applies equally to both wet and dry systems.

Cause No. 5 – Thermal bridges and uneven heating

Cracking of tiles is not always caused by mechanical movement of the entire screed. Local cracking, especially along the pipe runs, can indicate thermal bridges. When a pipe is laid without proper contact with the system board, or when the grid is too sparse (e.g. 20 cm instead of 10 cm in edge zones), the surface of the screed is thermally uneven. It is 5–8 °C warmer above the pipe than between the pipes – and this temperature difference causes local differences in expansion.

With tiles glued to the screed, this leads to cracking along the tile joints, since these are precisely the areas with the smallest cross-section for stress distribution. With parquet or laminate flooring, bulging (the so-called "tenting effect") appears exactly above the pipe rows, where the subsurface is warmer and the material expands more.

The solution is proper placement of the pipes in the system board – the retention pins of the system board ensure the correct position of the pipe as well as proper separation from the neighbouring loop. We also recommend reading the topic How to correctly lay pipes in a system board without thermal bridges in this Knowledge Centre, where the grid spacing distances for different room types are discussed in detail.

Cause No. 6 – Problems specific to dry systems (without screed)

In dry underfloor heating systems, warping and cracking manifest differently, but no less unpleasantly. Here there is no screed, so the wear layer lies directly on the dry board (e.g. OSB, fibre-gypsum board) and beneath it is polystyrene with an aluminium distributor and pipe.

Moisture-related movement of the dry board

Dry boards made of wood-based materials are hygroscopic – they change dimensions according to the relative humidity of the air. If they are installed in summer (high humidity) and the space begins to be heated in winter (low humidity), the board shrinks. If there are no expansion joints between the boards (at least 3 mm), movement arises that is transferred to the wear layer. Vinyl or laminate can lift, tiling can loosen at the joints.

Aluminium foil and distributor spacing

In dry systems, the Aluminium foil for dry underfloor heating is also important, serving as a heat distributor and ensuring even distribution of heat across the polystyrene surface. If this foil is not properly placed or is missing in places outside the groove with the pipe, the subsurface under the wear layer is thermally uneven – and this again leads to differential expansion and visible surface warping.

Cause No. 7 – Premature loading of the screed

Another problem we see especially in residential construction under deadline pressure: the screed is poured, and even before it has hardened sufficiently, people start walking on it, carrying materials, or even placing heavy objects on it. Cement screed reaches the strength needed for normal walking after 24–48 hours, but needs at least 7 days for furniture loading or anchoring door frames – and reaches its full design strength only after 28 days.

If the screed is loaded prematurely, microcracks can form in the structure that are invisible to the eye but weaken the screed enough that under later thermal stress (underfloor heating) they crack visibly. In addition, premature loading of a system board made of soft EPS can deform the retention pins – and the pipe changes position within the grid, worsening the evenness of heating.

Cause No. 8 – Incorrect screed thickness above the pipe

The minimum screed thickness above the top surface of the pipe is set by the STN EN 1264 standard at 45 mm for cement screed and 40 mm for anhydrite screed. This thickness is not random – it ensures sufficient mechanical rigidity of the screed so that it does not crack above the pipe, and at the same time sufficient heat distribution so that no heat striping occurs on the surface.

In practice we see two extremes: either the screed is too thin (the installer saves on material), and then it cracks right above every pipe. Or it is too thick (over 80 mm), and then the heating system is inert – it reacts slowly to control, and sudden heating can cause cracking from thermal shock.

The correct values depend on the pipe diameter:

Pipe 16 × 2 mm: minimum 45 mm of cement screed above the top of the pipe, total thickness with the UHP55 system board roughly 65–70 mm
Pipe 20 × 2 mm: minimum 45 mm above the top, total thickness roughly 70–75 mm
Anhydrite screed: minimum 40 mm above the pipe

We cover the topic of thicknesses in more detail in the article What thickness of polystyrene is needed under underfloor heating in this Knowledge Centre.

Diagnostics – how to find out where the problem is

When a customer comes with damaged floors, we proceed diagnostically. First we ask exactly where the damage is located – at the walls, in the middle of the room, along rows, at transitions between rooms. Each location tells us something different:

Cracking at walls and corners: almost always a missing or insufficient expansion strip
Warping in the middle of a large area: missing central expansion joints, or moisture in the screed
Cracking along parallel lines: thermal bridges – the lines are probably above the pipe rows
Local sagging: insufficient load-bearing capacity of the EPS or air pockets under the board (uneven subsurface)
Bulging at the transition between rooms: missing expansion profile at the transition
General lifting of the wear layer: moisture in the screed, premature laying of the covering

To verify the screed's moisture content, we always recommend the carbide method (CM meter), not just a surface condensation probe – that only gives indicative results. If there is doubt about the EPS load capacity, a simple test can be performed: place a steel straightedge against it and measure the deviation from the plane.

Repairing a damaged floor – what is and isn't possible

The honest answer is that most repairs of cracked screed above a system board are only cosmetic unless the cause is removed. Injecting cracks with epoxy resin will stop the cracking at a specific spot, but if the expansion problem is not resolved, a crack will appear next to it – the screed will always find a place where it can give way.

A real repair requires:

Identifying and eliminating the cause (expansion, moisture, load capacity)
For extensive damage: milling expansion joints into the screed, inserting flexible sealant
In extreme cases: complete removal of the screed, inspection of the condition of the system board, new installation following the correct principles

Restoring the system board is one of the most costly repairs in the entire underfloor heating project. That is why it always pays to invest in proper installation – including the use of quality system boards with certified parameters and a correctly chosen separation foil.

Preventive measures – checklist before pouring concrete

From dozens of jobs in practice, we have compiled a checklist that we recommend using before actually pouring the screed:

✔ The subsurface is level – max. deviation 5 mm / 2 m, protrusions are ground down
✔ A vapour barrier is placed under the system board (for ground floors above grade)
✔ The system boards are laid interlocked with each other, without uncovered gaps
✔ An edge expansion strip is placed along all walls, corners, columns and fixed transitions
✔ Expansion joints are milled / prepared for areas over 25 m² or longer than 6 m in one direction
✔ The pipes are fixed in the retention pins of the system board, with no loose loops
✔ A hydraulic pressure test is performed on the pipes before pouring (min. 6 bar, checked after 24 h)
✔ The separation foil is placed on the system board before pouring
✔ The screed thickness above the top of the pipe is at least 45 mm (cement) or 40 mm (anhydrite)
✔ The screed was dried to the prescribed moisture content before laying the covering (CM meter)
✔ The underfloor heating was commissioned gradually (not jumped straight to full output)

The last point is important and often overlooked: new underfloor heating must be commissioned using a gradual start-up protocol. On the first day, the maximum flow temperature is set to 25 °C, increased by 5 °C each day until the operating temperature is reached (typically 40–45 °C). Screed curing/firing is carried out in the same way after pouring – slow heating before final drying.

Frequently Asked Questions (FAQ)

Why does the floor crack only at one wall, but not at the others?

This almost always means that the edge expansion strip is missing at that particular wall, or was inserted too thin. The screed expands evenly in all directions when heated – but it gives way exactly where it hits a fixed obstacle sooner than a flexible strip. The other three sides may have a correct strip, or the screed may be under less stress there (e.g. shorter distance from the centre). The solution is to mill a joint at the damaged wall and fill it with a permanently flexible sealant.

The screed looks dry, but the floor is bulging. What's happening?

The surface of the screed may be dry, but moisture may remain deep inside (in the middle of the thickness) for several more months. When a vapour-tight wear layer (vinyl, PVC, laminate with an AL foil) is laid over such a screed, the residual moisture has nowhere to escape and creates upward pressure. Bulging is visible precisely in the places where the moisture is highest – typically in the middle of the room, where the screed dried slowest, from the edges inward. The only reliable test is the carbide method (CM meter) – not a visual check of the surface.

Can I use ordinary construction polystyrene instead of a system board?

No, for several reasons at once. Ordinary facade EPS (type EPS 70 or EPS 100) has no retention pins for guiding the pipe, so the pipe cannot be reliably fixed – the loops will move when the screed is poured and can change position. Furthermore, ordinary polystyrene may not have sufficient compressive strength for floor loading and lacks surface treatments (studs, laminate) that protect the EPS from mechanical damage during laying. That is why we always choose a system board certified for underfloor heating – such as UHP55, UHP51 or UHPD, depending on the type of system.

How many years does it take for screed problems to show up?

Most problems appear in the first or second heating season. Expansion problems appear during the first significant heating (a temperature rise of more than 15–20 °C). Moisture problems appear later – sometimes only after the second winter, when the floor stabilises. EPS problems (load capacity), on the other hand, develop slowly – at first only a slight drop, then after 3–5 years a significant sagging under constant load (heavy furniture). So don't forget to carry out a warranty inspection before the end of the warranty period after installation.

What is the difference in cracking risk between laminate and ceramic?

Ceramic tiling is rigid and brittle – it always cracks when the stress in the screed reaches the limit. Laminate and vinyl are more flexible and tend to warp or lift rather than crack. The risk of the screed itself cracking is the same for both – only how the problem looks on the surface differs. With tiling, the crack is more dramatic and visually obvious. With laminate flooring, warping develops gradually and the customer overlooks it for longer. That's why it's important to follow all the principles of expansion and screed drying even with laminate flooring.

Is a separation foil needed even when the floor is not above ground, but above a heated space?

Yes, the separation foil is important even above a heated space – not primarily because of moisture (which is not a problem there), but because of separating movement. The separation foil between the system board and the screed prevents these two layers from being mechanically bonded, which would otherwise transfer expansion stress directly into the EPS and cause it to deform. In addition, the foil prevents cement slurry from penetrating the grooves and between the retention pins of the system board, which could restrict the movement of the pipe. Using the Separation foil 601001H is therefore recommended always, regardless of the floor's position.

Conclusion

Cracking and warping of the floor above a system insulation board is not a matter of chance or bad luck – it is always the result of one or more specific mistakes in the design or installation. The most common are missing expansion joints and edge strips, premature laying of the wear layer on an insufficiently dried screed, an uneven subsurface, and insufficient load-bearing capacity of the polystyrene. Each of these problems can be foreseen and eliminated with proper preparation.

A quality system insulation board with certified compressive strength, the correct separation foil, a carefully placed expansion strip, and patience while drying the screed – these are the four basic conditions for your floor to last decades without problems. You will find further related topics in the articles Installing a system insulation board under underfloor heating step by step and Common mistakes when laying polystyrene and system boards under underfloor heating, where these principles are elaborated in even more detail.

Do you have a question on this topic?

Can't decide, or are you dealing with a specific situation in your home? Write to us – we'll be happy to help.