A heated screed moves. It expands when the system runs and contracts when it cools, and over a heating season it does that hundreds of times. Underfloor heating expansion joints, more precisely called movement joints, give it somewhere to go.
Leave them out and the screed finds its own relief. It cracks, usually at the point of greatest restraint, and the crack telegraphs up through the tile or the grout. Repairing it means lifting the floor covering and sometimes re-screeding, which costs several hundred times what the foam strip would have cost at the time.
The part most guidance gets backwards is what actually sets the joint positions. It is not the floor covering. It is the screed bay, and the limits for a heated screed are specific numbers in EN 1264-4. This article sets out those limits, how the floor covering fits around them, and the two rules that cause most of the cracking we see: how pipes cross a joint, and how soon the system is first heated.
What the Screed Bay Limits Actually Are
Start here, because every other decision follows from it.
For a heated screed finished with a rigid covering such as stone or ceramic tile, BS EN 1264-4 sets three limits on a single bay, and all three apply at once:
| Limit | Figure | Why it matters |
|---|---|---|
| Maximum bay area | 40 m² | Caps the total thermal movement any one bay has to absorb |
| Maximum side length | 8 m | Movement accumulates along the length, so a long run fails before a square one |
| Maximum aspect ratio | 2:1 | A narrow bay cracks across its width even when the area is within limits |
| Cut joint depth | One third of screed thickness | Deep enough to control where the crack forms, shallow enough to keep the bay intact |
The three limits together are the point. A corridor 3 m wide and 13 m long is 39 m², which passes the area test comfortably. It fails the 8 m side length test and it fails the 2:1 ratio test, and it needs intermediate joints. Judging a layout on area alone is the most common planning error.
Note also what the standard is addressing. These are limits for screeds under stone and ceramic finishes, which are rigid and have no capacity to absorb movement themselves.
Screed Type Changes the Answer
Not all screeds are governed by the same figures, and this gets left out of most articles.
Sand and cement screed is the case the 40 m² and 8 m figures are written for. It shrinks as it cures, and that drying shrinkage adds to the thermal movement.
Calcium sulphate, or anhydrite, flowing screed behaves differently. It shrinks far less on curing, which allows considerably larger bays, and some products permit areas many times the cement figure. The limit comes from the screed manufacturer’s own specification rather than from the general guidance, so get it in writing for the product actually being poured. Designing an anhydrite floor to cement screed rules means cutting joints the floor did not need, which is a cosmetic and practical cost for no benefit.
Industry guidance is candid that there is no single definitive table for joint frequency. The sensible approach is the stricter of the standard and the manufacturer’s specification.
Where Movement Joints Have to Go
Position is as important as spacing, and some positions are not optional.
Around the full perimeter of every heated area. This is the perimeter or edge insulation strip, typically a compressible foam strip run vertically against every wall before the screed is poured. It must be continuous with no gaps, and it must stand above the finished screed level so it can be trimmed flush afterwards. A perimeter strip that stops short at one wall gives the screed a rigid point to push against, and that is where it will crack.
At doorways and room transitions. A doorway is a narrow neck between two larger areas, so movement from both sides concentrates there. Joint it.
At internal corners and in L-shaped layouts. An L-shape has a re-entrant corner where two directions of movement meet, and it concentrates stress reliably. Joint across the leg of the L rather than trying to treat the whole shape as one bay.
At columns, structural elements and changes in floor level. Anything the screed cannot move past needs isolating. Where columns and insets exist, use them to define bay boundaries rather than working around them.
At changes in floor covering. Two coverings with different expansion behaviour need a joint at the interface, regardless of what the screed below is doing.
The Floor Covering Sits On Top of This, Not Instead of It
Once the screed bays are set, the floor covering’s own movement joints have to coincide with the screed joints. A tiled joint that lands 300 mm away from the screed joint beneath it protects nothing, because the screed will still move at its own joint and crack the tile above it.
Beyond coinciding, each covering then has its own intermediate requirements:
Ceramic and porcelain tile. Rigid, and with no capacity to absorb movement. Intermediate movement joints across the tiled field, in addition to those over the screed joints, with flexible sealant rather than rigid grout at every joint position. Perimeter gaps left open and sealed flexibly after the adhesive has cured. Flexible adhesive rated for underfloor heating throughout.
Natural stone. Treated like tile but less forgiving. Stone is rigid, often large format, and visibly unforgiving of any movement. Tighter intermediate spacing than ceramic, and the screed bay limits applied strictly.
Engineered wood and laminate, laid floating. These are a different mechanism and the terminology causes confusion. A floating floor does not have mid-field expansion joints in the screed sense. It has a perimeter expansion gap, typically 10 to 15 mm, concealed behind skirting or beading, plus T-profiles at doorways and on long continuous runs per the manufacturer’s limit. The board expands and contracts with humidity as much as with temperature, which is why the perimeter gap matters more than anything in the middle of the floor.
Vinyl sheet and LVT. Thin flexible finishes are normally laid without movement joints, because the material absorbs the movement the screed transmits. What they do need is the screed joint below treated properly before the covering goes on, and adhesive suitable for the temperature range. Specifying intermediate joints in a vinyl floor is a solution to a problem it does not have.
In every case, confirm against the covering manufacturer’s instructions and the applicable local standard, since climate and product formulation both affect the figures.
How Pipes Cross a Movement Joint
This is the rule most often missed, and it is the one that actually cracks screeds.
A common instruction is to keep the pipes away from the joint positions. That is impossible, and following it literally produces a worse outcome. The flow and return tails have to travel from the manifold to each loop, and on any floor with more than one bay some of them must cross a joint.
What EN 1264-4 requires:
- Only connecting pipes may cross a movement joint. The heating loop itself stays within its own bay. Design the layout so each loop serves one bay, with only the tails crossing.
- They may cross in one level only. No stacking pipes at the crossing point.
- Each crossing pipe is sleeved in a flexible tube of about 0.3 m, centred on the joint, so the pipe is debonded for roughly 150 mm either side. The sleeve lets the pipe move with the screed instead of being gripped by it.
Without the sleeve, the screed bonds to the pipe either side of the joint and the joint cannot open. The movement goes somewhere else, usually into a crack a short distance away, and the pipe itself takes a shear load it was never designed for.
This is why joint positions have to be marked and agreed before the loop layout is finalised rather than after. Our guide to underfloor heating pipe layout covers loop planning, and the manifold specifications page covers the port count you will need once the bays determine how many loops there are.
Installation Sequence
The order is what makes the detail work. Each step depends on the one before it.
1. Mark the bays first. Before any pipe is laid, set out the joint positions from the screed bay limits and the fixed positions above. This is a design decision, not a site decision.
2. Lay insulation and perimeter strip. Insulation boards below, then the perimeter strip run continuously against every wall, standing proud of the finished screed level. See insulation boards for underfloor heating and types of floor insulation.
3. Install intermediate joint profiles. Foam strips or proprietary profiles at the marked internal positions, fixed so the pour cannot displace them.
4. Lay the pipe, one loop per bay. Sleeve every tail that crosses a joint. Use oxygen barrier floor heating pipe to DIN 4726 and connect to the manifold.
5. Pressure test, and keep it pressurised through the pour. Typically 6 bar held for 24 hours before screeding, with the pressure maintained during the pour so any damage from the screeding shows immediately rather than in year three.
6. Photograph every loop before the screed goes down. This costs nothing and it is the most valuable document the building will ever have. Anyone who later needs to drill, fix or repair the floor will need to know where the pipes and the joints are.
7. Pour, cure, then commission on the curve below. Not before.
8. Lay the covering, with its joints coinciding with the screed joints. Leave the joint gaps open during tiling and seal them flexibly once the adhesive has cured.
The Heat-Up Curve That Prevents Cracking
A floor can have every joint in the right place and still crack, because it was heated too early or too fast. This step gets skipped under programme pressure more than any other.
Curing before first heat. A sand and cement screed should not be heated for at least 21 days after laying, per EN 1264-4. An anhydrite screed should not be heated for at least 7 days, subject to the manufacturer’s own figure.
Then the commissioning schedule:
- Water entering the circuits at no more than 25 °C, held for at least three days.
- Raised to the maximum design flow temperature and held for a further four days.
- Then cooled down, but not below 15 °C, before the floor covering is laid.
Record the dates and temperatures. Many floor covering manufacturers require evidence that this was done before they will honour a warranty, and in a dispute about a cracked floor it is the first document anyone asks for.
Do not improvise this to save a week. Driving a green screed to design temperature is a reliable way to crack a correctly jointed floor.
Electric and Hydronic Floors Differ, But Not For The Reason Usually Given
Published guidance often says electric systems are more flexible than hydronic ones and therefore need fewer joints. That is the wrong explanation.
The real difference is screed thickness and thermal mass. A hydronic floor embeds pipe in 65 to 75 mm of screed, which is a substantial mass that moves slowly and significantly. An electric mat often sits in a thin layer of adhesive or levelling compound over an existing substrate, with very little mass of its own, so the movement it generates is correspondingly smaller and the substrate below governs instead.
That changes which rules apply, not whether rules apply. A thin electric system over an existing concrete slab inherits the slab’s existing movement joints, and those must be carried through the new build-up rather than bridged. If you are deciding between the two systems, our comparison of electric and hydronic underfloor heating covers the wider trade-offs.
Maintenance and Inspection
Movement joints need very little, but they are not maintenance free.
Check that sealant at the joint positions has not hardened, split or pulled away from one edge. A sealant that has lost its flexibility is no longer a movement joint, it is a rigid filler, and the floor behaves as though the joint were not there. Reapply where it has failed.
Keep the joints clear. Debris packed into a joint restricts it in exactly the way the joint exists to prevent.
Watch for the early signs: hairline cracking in grout lines, particularly running in a straight line across a room rather than randomly; a slight ridge or hollow sound along one line; a board lifting at the centre of a wood floor. A straight crack usually marks where a joint should have been. Finding it early is the difference between re-sealing one line and relaying the floor.
Legom Underfloor Heating Components
Jiaxing Legom Technology Co., Ltd. manufactures hydronic underfloor heating components from our facility in Jiaxing, Zhejiang, supplying more than 90 countries.
The bay layout decided by the joint positions determines how many loops the floor needs, and therefore the manifold. Our manifold range covers stainless steel and brass bodies in 2 to 13 circuit configurations with integral flow meters, alongside thermal actuators for zone control, oxygen barrier floor heating pipe to DIN 4726, floor and room sensors, and the mixing valves that set the flow temperature the commissioning curve depends on.
One loop per bay is the design rule that comes out of this article, and it tends to increase the circuit count on a complex floor. Where a standard manifold does not fit the layout, our OEM and ODM service covers non-standard port counts, connection types and cabinet dimensions. Send us the floor plan with the bay layout and we will work through the manifold specification with you.
For cost and installation context across the whole system, see wet underfloor heating cost and installation considerations.
“Installers underestimate the relationship between the pipe layout and the joint positions. The joints have to be set out before anyone decides where the loops run, because only the flow and return tails may cross a joint and they have to be sleeved where they do. Without that sleeve the screed grips the pipe either side and the joint simply cannot open, so the movement goes into a crack somewhere else and the pipe takes a load it was never designed for. We have seen floors relaid twice over a missing perimeter strip at one doorway. Getting it right during installation costs a few metres of foam.”
— Maggie Shen, Director of Legom
Frequently Asked Questions
What are underfloor heating expansion joints and why are they necessary?
They are compressible strips and gaps built into the screed and the floor covering to absorb the thermal expansion and contraction a heated floor goes through every time the system cycles. Without that space, the screed pushes against walls and rigid elements until it cracks, and the crack transmits up through the tile or grout above. The joint gives the movement somewhere to go.
How big can a heated screed bay be?
For a heated screed under a rigid covering, EN 1264-4 limits a bay to 40 m² in area, 8 m in any side length, and a 2:1 length to width ratio. All three apply together. A 3 m by 13 m corridor is only 39 m² but breaks both the 8 m and the 2:1 limits, so it needs intermediate joints. Anhydrite screeds permit larger bays, to the screed manufacturer’s specification.
Where exactly do the joints go?
Continuously around the perimeter of every heated area, at doorways and room transitions, at internal corners and across the leg of any L-shape, at columns and changes in floor level, and at every change of floor covering. Then intermediate joints as needed to bring every bay within the limits above.
Can heating pipes cross a movement joint?
Only the connecting pipes, and only in one level. Each crossing pipe must be sleeved in a flexible tube of about 0.3 m centred on the joint, so it is debonded roughly 150 mm either side and can move with the screed. The heating loop itself should stay within one bay, which is why the loop layout has to be planned after the joint positions, not before.
Do the floor covering joints have to line up with the screed joints?
Yes. A joint in the tiling that does not sit over the screed joint beneath it protects nothing, because the screed still moves at its own joint and cracks the tile above. The covering joints coincide with the screed joints, plus whatever intermediate joints the covering’s own standard requires.
Does vinyl flooring need expansion joints?
Generally no. Thin flexible finishes such as vinyl sheet and LVT absorb the movement the screed transmits and are normally laid without movement joints. What matters is that the screed joint below is detailed correctly and the adhesive suits the temperature range.
How soon can I turn the heating on after the screed is poured?
Not for at least 21 days on a sand and cement screed, or 7 days on anhydrite subject to the manufacturer’s figure. Then run water at no more than 25 °C for at least three days, raise to maximum design flow temperature for four days, and cool to not below 15 °C before laying the covering. Heating a green screed will crack a correctly jointed floor.
What happens if the joints are omitted?
The floor cracks, usually in a straight line at the point of greatest restraint. In tiled floors the grout goes first, then the tiles. In floating wood or laminate the boards lift at the centre of the room. The damage runs down into the screed and in severe cases reaches the pipe. Repair means lifting the covering and sometimes re-screeding, against a material cost at installation of almost nothing.
Reviewed by Maggie Shen, Director at Legom, on October 15, 2026. This article was reviewed for technical accuracy, including the EN 1264-4 heated screed bay limits of 40 m², 8 m and 2:1, the one third cut joint depth, the requirement that only connecting pipes cross a joint and only when sleeved in a flexible tube of about 0.3 m, the distinction between screed movement joints and floating floor perimeter expansion gaps, the normal omission of joints in thin flexible finishes, and the 21 day and 7 day curing periods with the subsequent commissioning heat-up schedule.