Structure and support · Floor Structures
Floating Screed Floor System
This entry describes floating as a relationship rather than a material, and sets out why the perimeter isolation and the compressible layer beneath govern what the screed above them has to be.
Educational reference entry. How this system behaves depends on climate, jurisdiction, loading, substrate, the adjacent systems it meets, how the building is used, the manufacturer's instructions and qualified professional design.
Overview
What floating screed build-up is
A floating screed is defined by what it is not touching. Instead of bonding to the base beneath, the screed is laid over a compressible layer, insulation or a resilient sheet or both, and separated from the walls around it at every edge. The result is a plate of screed that moves as its own element, supported by what is under it but restrained by nothing it sits against.
Its nearest sibling is the separating floor system, and the two are told apart by asking who is underneath. This entry is the build-up mechanics of an isolated screed, used for heated floors and thermal isolation as much as for sound, and it stays the right entry when there is nobody below at all. A separating floor is the whole floor and ceiling construction judged by what passes between two occupancies, flanking paths included.
Because the screed spans over something soft, what it has to be is dictated from below. The compressibility of the layer beneath, the presence of anything embedded within the screed and the completeness of the isolation at the edges all set what the screed must do, and none of that can be read from the screed material considered on its own. It is not the materials-library entry acoustic-floating-floor-system, which is a proprietary layer product rather than the relationship between screed, compressible layer and perimeter that this entry describes.
Terminology
Common names and aliases
These names describe the same assembly. The encyclopedia keeps one entry per system so that a trade term or a regional name never becomes a second, thinner page.
- floating screed
- screed over insulation
- isolated screed layer
- unbonded floating floor screed
Purpose
What this system is intended to do
The job the assembly exists to perform, conceptually. An intention is not a guarantee that any particular build achieves it.
- Separate the finished floor surface from the structural base so the two can move independently of each other.
- Provide a continuous, flat plane over a compressible layer for the floor covering above it.
- Hold insulation or a resilient layer in position beneath a solid surface that does not bear on the structure around it.
- Contain embedded heating elements where the floor itself is used to distribute heat into the room.
Composition
The roles this system is made of
What each part does in the assembly and what it depends on — never a product, a thickness, a fixing or a determination that anything is adequate for the role.
Order is meaningful in this system. The roles below sit in sequence, and moving one relative to another changes what the assembly does. The sequence is conceptual only: it states no thickness, no dimension, no fixing and no order of work on site.
- Layer 1 of 8. Order is meaningful.SubstrateStructural base beneath
The slab or deck the build-up sits on. Its flatness and its cleanliness govern how evenly the compressible layer bears, and any hump or debris left on it becomes a point where the screed above is locally unsupported.
- Layer 2 of 8. Order is meaningful.Control layerSeparation layer
The sheet laid to keep wet screed out of the joints of the layer beneath and off whatever is under it. Where it is torn, lapped short or left open at an upstand, screed can find its way down and harden into a rigid connection to the base, which is why its continuity matters more than the sheet considered alone.
- Layer 3 of 8. Order is meaningful.Thermal layerInsulation or resilient layer
The compressible layer the screed floats on. Whether it is there for heat or for isolation from the base, it is the component deciding how much the screed above has to do between the points where it is genuinely bearing.
- Layer 4 of 8. Order is meaningful.Edge and terminationPerimeter edge isolation
The strip separating the screed from every wall, column, threshold and pipe it would otherwise touch. It has to run the full depth of the screed and stay unbroken, because a single hard contact reconnects the floating layer to the structure.
- Layer 5 of 8. Order is meaningful.Primary supportScreed layer and its reinforcing role
The screed carrying loading across the compressible layer to the base. Over something soft it is acting as a spanning element rather than a levelling course, which is why its make-up is decided with the layer beneath rather than apart from it.
- Layer 6 of 8. Order is meaningful.Service zoneEmbedded heating or service elements
Pipes or cables cast into the screed. They occupy a defined position in its depth, they influence how it dries and how it moves, and once they are in, the screed is no longer something to be lifted or drilled casually.
- Layer 7 of 8. Order is meaningful.Jointing and sealingMovement joints within the screed
The planned lines dividing the screed, at doorways, at changes of shape and where heated areas are zoned. They exist because a floating layer will move, and their positions are decided with the finish rather than after it is chosen.
- Layer 8 of 8. Order is meaningful.Finish surfaceFloor covering interface
The surface laid over the screed. It has to tolerate the movement the build-up is deliberately allowing and the joints running through it, so covering and screed are chosen against one another rather than one after the other.
Interaction
How the parts work together
The reason this is a system rather than a list of parts: what depends on what, and what stops working when one part is changed.
The isolation is either complete or it is not there at all. A floating screed touching the wall at a doorway, bridged by a pipe sleeve, or pinned to the base by a fixing driven through it has ceased to float at that point, and the whole layer behaves differently for the sake of one contact. The isolation therefore has to be established and protected around the entire perimeter, including the places other trades pass through, before anything is poured against it.
The layer beneath sets the terms for the layer above. A screed floating over something compressible is spanning, so the softer and the more variable that layer is, the more the screed has to do. Change the insulation and the screed changes with it, which is the reverse of how the two are usually discussed, where the screed is chosen and the insulation treated as fill.
Embedded heating locks several decisions together. Pipes cast into the screed fix its depth and its drying behaviour, they mean the screed will expand and contract in service rather than only while curing, and they make the joint layout a heating question as well as a structural one. The covering above then has to be one that tolerates that regime.
The base below is not passive. Debris, a local high spot or an unfilled hollow all leave the compressible layer bearing unevenly, and the screed then bridges the low areas until something cracks. What is reported as a screed failure very often began as a base that was accepted without being checked.
Materials
Material families commonly met in each role
Commonly encountered, not recommended. Whether a material suits a given project depends on the whole assembly, the exposure, the manufacturer's documentation and qualified professional review.
Primary support
Screed families of these kinds are commonly encountered as the floating layer. Whether a given screed can span over a compressible layer, and what it needs within it, is set by the manufacturer's documentation and the designer.
Thermal layer
Boards of these families are commonly encountered beneath a floating screed. How much a board compresses under load, and whether it can be used beneath a screed at all, is determined by the manufacturer's information.
Control layer
Resilient and separating layers of these kinds are commonly encountered in this build-up. What any of them is relied on to do, and where it sits in the order of layers, is a matter for the designer rather than the installer.
Edge and termination
Edge and joint components of these kinds are commonly encountered at perimeters, doorways and joint lines. What a particular floor needs in those positions follows from the screed design and the covering above it.
Finish surface
These are commonly encountered as coverings over a floating screed. Whether a covering tolerates the movement and the drying regime of a particular build-up is a matter for its manufacturer's documentation.
Junctions
Where this system meets others
Interfaces are where most assemblies actually fail, so they are set out explicitly rather than left inside the prose. What resolves a junction is a detail designed for the specific building — not a rule of thumb.
Floating build-up over a ground slab
Where insulation sits above a ground slab, this build-up is the upper part of that floor and the slab entry is the lower. The separation layer is the line between them, and the barrier arrangement below decides how the screed can dry.
Read about Ground-Bearing Slab →Where the floor divides two homes
A resilient layer under a screed is often part of how impact sound is handled between occupancies, but performance belongs to the complete tested construction and its flanking junctions. That assessment sits with the separating floor entry.
Read about Separating Floor →Partitions built off or through the floor
Whether a partition stands on the structural base or on the finished screed changes both the isolation and the sequence. A wall built on the screed loads a floating layer; one built off the base interrupts it and needs the isolation carried round.
Read about Framed Partition →Doorways and level thresholds
A doorway is where the isolation is most often bridged and where the covering usually changes. The joint in the screed, the threshold detail and the edge strip all meet in a narrow line that several different trades pass through.
Wet rooms formed within the build-up
Where a bathroom or shower area sits on a floating screed, the tanking above has to accommodate a layer that moves and a screed that may take longer to dry than the programme assumed. The two are designed together or not at all.
Read about Wet Area Waterproofing →Floating layer over a suspended concrete deck
Over precast units the base is not flat in the way a power-floated slab is, so the compressible layer bears on a surface with its own camber and joints. Regularising that base is part of making the floating layer work.
Read about Beam and Block Floor →
Considerations
Topics worth discussing
Which topics genuinely apply to this system and what to raise about them. Measured values, classes and ratings come from a qualified professional's design for the specific building, not from a reference page.
- Moisture
- A screed contains water when it is laid and gives it up slowly, and a floating build-up can generally only dry upward. When a covering may be laid is judged by testing rather than by programme, and the answer depends on the layers below as much as the screed.
- Thermal
- Where the build-up carries heating, the floor's response is governed by what sits under the pipes as much as by the pipes themselves. Whether heat goes upward or into the structure is decided by the layer beneath, and that is a design matter.
- Acoustic
- A resilient layer under a screed is one part of how impact sound is dealt with, but the result is a property of the whole construction including its edges. Any performance claim belongs to a tested assembly and a qualified acoustic professional.
- Movement
- The layer is meant to move, so the real question is where. Joints at doorways, at changes of shape and at zone boundaries decide whether that movement happens along planned lines or turns up as cracking through the covering.
- Buildability
- The build-up is laid by different trades in quick succession and buried immediately. Perimeter strips get displaced, boards get walked out of position and debris gets left behind, and each of those is invisible once the screed is poured.
- Maintenance and access
- Anything embedded in the screed is effectively permanent. A leak in an embedded pipe is found by tracing rather than by inspection, so what goes in and how it is recorded matters more here than in a floor where services stay reachable.
Design
Questions the design has to answer
The decisions this assembly turns on. They are questions rather than answers, because the answer depends on the building.
- What is the compressible layer there for, whether heat, isolation from the base, or both together?
- How will the perimeter isolation be kept unbroken at doorways, columns and rising pipes?
- Is anything being fixed through the screed into the base, and has that been recognised as a bridge?
- Where will movement joints fall, and does the covering layout agree with those positions?
- Has drying been allowed for before the covering is laid, and who is carrying out the testing?
- Is anything embedded in the screed, and is its position being recorded for whoever comes next?
Boundaries
Commonly misunderstood points
Distinctions that are easy to blur, and the places where a familiar term means something narrower than it sounds.
- Floating is treated as a property of the screed material, when it is a relationship between the screed, the layer beneath and the perimeter.
- The perimeter strip is seen as a finishing item, when a single hard contact at the edge undoes what the whole build-up was arranged to do.
- Drying is estimated from the programme, when what a covering needs is judged by testing the base it is about to be laid on.
Conversations
Questions for qualified professionals
Bring these to the relevant qualified professional, manufacturer or authority. Requirements vary by project and location.
- What is the compressible layer beneath this screed, and how has the screed been designed around it?
- How is the perimeter isolation maintained at doorways, thresholds and rising services?
- Where are movement joints placed, and how does the intended covering deal with them?
- What has to be measured before the floor covering can be laid, and by whom?
- Is anything embedded in this screed, and will its position be recorded for the future?
What this page does not do
- Screed make-up and depth over a compressible layer are design matters, and depend on the layer beneath and the loading above.
- Acoustic behaviour is a property of a complete tested construction with its flanking junctions, and none is stated here.
- Fixing through a floating screed into the base changes how the build-up behaves and is not a fixing decision alone.
- Embedded pipes and cables cannot be seen once the screed is laid, and later drilling into such a floor carries obvious risk.
Related systems
How this system relates to others
Every link states what the relationship actually is, rather than leaving a bare list of related pages to be read as a suggestion.
Meets these systems
Systems this one physically meets. The junction is usually where the design problem lives, so these are worth reading together.
Commonly built alongside
Systems routinely present in the same building without necessarily touching this one.
Go deeper
Related Build Design Hub guides
Planning guidance behind the decisions this assembly involves.
Preparation
Related planning checklists
Owner-side preparation before the conversation where this system comes up.
Inspiration
Related Ideas Library pages
Design directions where this system commonly appears.
Floor Structure and Build-Up Systems
Horizontal decks read together with the stack placed on them, told apart by what spans, what is infill and what is only a topping over both.
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