Structure and support · Floor Structures
Ground-Bearing Floor Slab System
This entry explains how a ground-bearing floor is organised as interdependent layers, and why the position of the thermal layer relative to the slab reorganises the barrier below it, the perimeter around it and the finish above it.
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 ground-bearing slab is
A ground-bearing floor slab is the construction that lets a building's lowest floor rest on the ground rather than span over it. Ground that has been prepared and consolidated takes the floor loading directly, and the layers above it exist to interrupt moisture and soil gas, to place the floor at a chosen thermal position, and to give the finish something flat and stable to bear on.
The boundary against a raft foundation system is settled by looking at what lands on the slab. If the load-bearing walls, columns or piers of the building bear on the slab itself, the slab is a foundation and belongs with the foundation systems. If those elements bear elsewhere, on footings or a frame, and the slab carries only floor loading into the ground immediately beneath it, this is the right entry.
What separates the assembly from its ingredients is that the layers are not interchangeable in position. Concrete, insulation and a barrier sheet each mean something different depending on where in the stack they sit, and the same set of materials in a different order produces a floor with different behaviour, different edge details and different limits on what may be laid over it.
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.
- slab on grade
- ground-supported slab
- slab on ground
- oversite slab
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.
- Transfer floor loading into prepared ground beneath the building rather than spanning it between supports.
- Interrupt the upward passage of moisture and, where the ground demands it, of soil gas into the occupied space.
- Hold the thermal layer in a defined position relative to the slab so the floor's response to heat input is deliberate.
- Present a flat, stable and continuous base that the intended floor finish can be laid on.
- Keep the slab separated from the surrounding structure at its edges so movement is accommodated rather than transmitted.
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 7. Order is meaningful.SubstrateCompacted sub-base
The prepared granular layer that receives the floor loading and spreads it into the ground below. Its consolidation is what the slab's behaviour rests on, and any softness or unevenness left in it tends to reappear in the slab as differential movement.
- Layer 2 of 7. Order is meaningful.SubstrateBlinding over the sub-base
A regularising layer over the sub-base, giving the barrier a surface it is less likely to be punctured on and closing the coarse voids the fill leaves open. Where a slab is cast with nothing between it and the fill, a separation sheet laid directly under the concrete is a different layer in a different position, and the two are readily confused.
- Layer 3 of 7. Order is meaningful.Control layerDamp and gas barrier
The sheet that interrupts moisture rising from the ground and, in some ground conditions, the route soil gas would take. Its behaviour depends entirely on continuity, so laps, service penetrations and the turn-up at the perimeter are where it is decided.
- Layer 4 of 7. Order is meaningful.Thermal layerFloor insulation layer
The board layer placed beneath the slab or above it. Below the slab the concrete sits inside the insulated envelope; above it the slab is outside and the floor responds more quickly to heat input. The position reorganises everything around it.
- Layer 5 of 7. Order is meaningful.Primary supportStructural slab
The concrete plate that distributes loading over the prepared ground and provides the plane everything above is set out from. It is the layer the rest of the build-up is positioned relative to, not simply the heaviest part of it.
- Layer 6 of 7. Order is meaningful.Edge and terminationPerimeter isolation and edge upstand
The compressible strip and insulation upstand at the junction with surrounding walls. It keeps the slab from bearing hard against the structure and keeps the thermal layer from stopping short at the very place the floor meets the outside.
- Layer 7 of 7. Order is meaningful.Finish surfaceScreed or finish interface
The levelling or wearing layer that receives the floor covering. Whether it is bonded to the slab, laid on a separating membrane or floated changes what the slab has to be like and what movement the covering above must tolerate.
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 ordering question runs through everything. Placing insulation under the slab puts the concrete on the warm side, so the slab stores heat and the barrier normally sits lower in the stack; placing it over the slab puts the concrete on the cold side, moves the barrier's likely position, and means the layer above has to carry loading across something compressible.
Under a ground-bearing floor the barrier is settled at the places nobody sees again: laps that end up buried under the pour, the turn-up at the perimeter, and the collars around pipes and ducts fixed before any concrete arrives. A sheet stopping at the slab edge relocates the moisture path into the wall junction instead of interrupting it, which is why it is lapped into the damp course in the wall rather than finished against it. Nothing here is damp-proof by itself; the completed and correctly detailed assembly is what matters.
Edge isolation and thermal continuity are the same detail seen from different sides. The compressible perimeter strip exists so the slab can move without loading the wall; the upstand insulation exists so the thermal layer does not stop where the floor meets the structure. Where either is omitted or squeezed out, the floor gains a hard connection and a cold path in one move.
The finish depends on decisions taken far below it. A slab finished flat may take a covering directly; one that is not needs a levelling layer, and where the insulation sits above the slab that layer stops levelling and becomes an unbonded screed carrying loading across something compressible. Heating embedded at this level then binds the covering choice to the thermal layer position beneath it.
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
These families are commonly encountered in the slab and in whatever reinforces it. Whether a particular concrete or reinforcement arrangement suits a given floor is a matter for the structural designer and the supplier's documentation.
Control layer
Barrier sheets of these kinds are commonly encountered beneath and around ground floors. Which applies, and whether ground gas protection is called for at all, follows from ground investigation and the designer rather than from the floor type.
Thermal layer
Rigid boards of these families are commonly encountered in ground floor build-ups. Whether a board can be loaded beneath a slab, and how it behaves against ground moisture, is determined by the manufacturer's documentation and the designer.
Finish surface
These are commonly encountered between slab and covering, or as the wearing surface itself. Compatibility with adhesives, coverings and embedded heating belongs with the manufacturer's information and the designer.
Edge and termination
Edge and joint products of this kind are commonly encountered at slab perimeters and at planned joint lines. What a given floor needs at its edges is set by the structural and waterproofing design, not by the product range.
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.
Junction with the load-bearing wall foundation
The slab meets a foundation it takes no load from. The detail has to leave the slab free to move, carry the thermal layer past the junction and lap the floor barrier into the wall's damp course, because that corner resolves structure, moisture and heat loss at once.
Read about Strip Foundation →Continuity with ground gas protection
Where ground investigation calls for gas protection, the floor barrier becomes part of a wider protective envelope rather than a damp measure. Every lap and every penetration then belongs to that envelope, and verifying it is a specialist matter.
Read about Ground Gas Protection →Perimeter meeting with the external wall
At the wall, the floor's insulation and the wall's insulation are trying to be continuous around a corner that also carries a damp course and often a change of material. Where the two thermal layers fail to meet, the shortfall shows at the perimeter first.
Read about Twin-Leaf Cavity Wall →Rising services and penetrations
Drainage, incoming water and conduits pass through the slab and the barrier below it. Each is a hole in a layer whose whole value is continuity, so their positions are a design matter fixed before the slab rather than a fitting matter afterwards.
Read about Penetration Sealing →Floating screed laid over the slab
Where insulation sits above the slab, the layer over it becomes a floating screed rather than a bonded topping, and this entry stops at the slab surface. The two then describe the lower and upper parts of one construction, meeting at the separation layer.
Read about Floating Screed Build-Up →Ground water beneath and around the floor
How wet the ground under the slab stays is largely decided outside the building line, by land drainage and by the way surface water is taken away. A floor detailed for wet ground is a different floor from one detailed for a well-drained site.
Read about Land Drainage →
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
- Rising moisture, construction water held in the slab and moisture trapped under an impermeable covering are separate problems meeting in this floor. Whether a covering can be laid, and when, is judged by testing and by the covering manufacturer's documentation rather than by appearance.
- Thermal
- The position of the insulation decides whether the slab sits inside or outside the insulated envelope, which changes how quickly the floor responds to heat input and where a cold path at the perimeter would run. This is a design decision rather than a product decision.
- Movement
- The slab shrinks as it cures and moves with temperature afterwards. Where joints are placed, and whether the perimeter lets the slab move at all, determines whether that movement is taken up in planned lines or expressed as random cracking through the finish.
- Buildability
- Everything below the slab is buried by it. Barrier continuity, insulation joints and service positions cannot be revised once concrete is placed, which puts unusual weight on inspection beforehand rather than remediation afterwards.
- Interfaces
- The perimeter carries structure, damp control and thermal continuity in the same corner, and is where drawings from different disciplines have to agree. Most of the difficulty in this floor is concentrated there rather than in the field.
- Documentation
- Slab flatness, moisture condition and the barrier arrangement all have to be recorded, because later trades take decisions from them. A floor covering dispute is usually a documentation dispute about what the base was when it was handed over.
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.
- Is the slab carrying only floor loading, or is any part of the building's structure bearing on it?
- Where is the thermal layer intended to sit relative to the slab, and what does that settle about the finish above?
- Does the ground investigation call for gas protection as well as damp control on this site?
- How will the barrier be carried through the perimeter and lapped into the surrounding wall construction?
- Which services penetrate the slab, and are their positions fixed before any concrete is placed?
- Is the floor covering being chosen early enough for the base beneath it to be specified around it?
- Where are movement and construction joints intended to fall in relation to the room layout?
Boundaries
Commonly misunderstood points
Distinctions that are easy to blur, and the places where a familiar term means something narrower than it sounds.
- A slab resting on the ground is often assumed to be a foundation; it becomes one only when the building's structure bears on it.
- The barrier is thought to be made effective by its material, when a sheet lapped short, punctured or stopped at the slab edge simply relocates the moisture path.
- Insulation above the slab is treated as insulation moved upward, when it turns the screed into a load-carrying layer and alters the perimeter detail.
- A flat-looking slab is taken for a base ready to receive a covering, when readiness is judged by testing rather than by eye.
Conversations
Questions for qualified professionals
Bring these to the relevant qualified professional, manufacturer or authority. Requirements vary by project and location.
- What ground conditions and investigation findings has this floor build-up been designed around?
- Which layer here is relied on for moisture control, and how is its continuity maintained at the perimeter?
- Has ground gas been assessed, and does the design rely on this floor as part of a protective envelope?
- Where does the insulation sit relative to the slab, and what were the reasons for that arrangement?
- How is the slab isolated from the surrounding structure, and what is expected to happen as it shrinks?
- What condition does the base have to be in before the intended floor covering can be laid on it?
What this page does not do
- No single layer in this floor is damp-proof on its own; moisture control is a property of the completed and correctly detailed assembly.
- Ground gas protection is a specialist design and verification matter, and nothing here indicates whether a particular site needs it.
- Slab thickness, reinforcement and joint layout are structural decisions for a qualified engineer and sit outside the scope of this entry.
- Cutting or coring a slab after it is cast can affect both the structure and the barrier below it, and is not a finishing operation.
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.
Alternatives
Different systems answering the same need. Listing them together is not a comparison and does not suggest one is better — which, if either, suits a project is a design decision.
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.
Browse all floor structures entries →