Structure and support · Foundations & Ground
Raft Foundation System
A reference account of the raft as a stiffened plate that is simultaneously foundation and ground floor, covering how its structural, damp, gas and thermal duties compete inside one element and what has to be settled with the design team before it is cast.
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 raft foundation is
A raft spreads the building onto the whole footprint at once. Rather than each wall or column finding its own patch of ground, the loads are collected by a single plate stiff enough to move as one, so local weakness in the ground is bridged rather than followed within the range the plate was proportioned for. Bending in a stiff plate follows the worst ground beneath it rather than the average, so how much variation a raft can accept is a design determination made from ground investigation. That is a different idea from a strip or a pad, which answer poor ground by growing wider or reaching deeper.
Many rafts are not flat plates of uniform depth. Where load arrives along wall lines or at columns, the plate is commonly stiffened locally - thickened at the edge, downstanding beneath load-bearing lines, deepened where a column lands - and that is how such a raft obtains the stiffness it needs to bend as one element rather than follow the ground beneath each part of it. Stiffening is a means of reaching that stiffness rather than the test of what a raft is; a plate of even depth carrying reasonably even loading is a raft as well.
The boundary against the nearest sibling, the ground-bearing floor slab, is settled by asking what bears on it. If the load-bearing walls and columns stand on the slab itself and it takes them into the ground, it is a raft. If they are carried by separate foundations beside or beneath it, and the slab carries only the floor loading applied directly to it, it is a ground-bearing floor slab whatever the drawing calls it.
Because a raft is the ground floor as well as the foundation, decisions that would be sequenced in other constructions collapse into one element. Barriers, any insulation below the plate, the reinforcement, the service entries and the finish above all have to be agreed before casting, since almost none of them can be revisited afterwards without breaking the structure.
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.
- mat foundation
- raft slab
- mat 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.
- Collect the loads of the whole building into a single plate so that no individual wall or column has to find its own bearing.
- Bridge local weakness or variability in the ground, within the range the plate was designed for, by being stiff enough for the footprint to move together.
- Serve as the ground floor structure at the same time, receiving the floor build-up and the services that rise through it.
- Give the damp, gas and thermal layers beneath the building a continuous plane to occupy under the whole footprint.
- Reduce the number of separate excavations where working space, groundwater or ground variability makes them difficult.
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 not meaningful in this system. These roles interact as parts of one whole rather than stacking up in a sequence, so the list below is not a build-up and nothing should be read into the order it appears in.
- Primary supportStructural plate
The continuous slab that spans the footprint. It is designed to bend rather than to bear evenly, so its behaviour is governed by stiffness and continuity rather than by the area it happens to cover.
- Primary supportEdge thickenings and downstands
Local deepenings at the perimeter and beneath load-bearing lines and columns. They are the source of the plate stiffness where load concentrates, not a detail added for the wall, which is why interrupting one is a structural change.
- Primary supportReinforcement arrangement
Steel laid so the plate acts as one element in both directions and around the points where load is applied. Where it is lapped, stopped or interrupted determines where the plate can and cannot be treated as continuous.
- SubstrateSub-base and separation layer
The prepared and compacted material beneath the plate, and the layer that separates the concrete from it. It provides a working surface, an even support during casting, and a defined interface for whatever the design places under the plate.
- Control layerDamp and gas barrier plane
The continuous barrier beneath or within the plate that has to remain unbroken across the whole footprint. Every rising service, edge upstand and construction joint is a place where its continuity has to be re-established rather than assumed.
- Thermal layerSub-slab insulation in insulated variants
Where the raft is insulated beneath, boards under the plate become part of the load path as well as the thermal envelope. Which arrangement suits a given building depends on the construction, the climate and the designer rather than on preference.
- SubstrateWall and column bearing zones
The areas of the plate that receive structural load from above. Their position relative to the stiffenings underneath is the crux of the design, because bearing that does not line up with the stiffening is bearing on the flexible part.
- Edge and terminationPerimeter edge and movement provision
The exposed rim of the plate, where the wall arrives, external ground meets it, and thermal and damp layers turn. It is also where the plate is closest to the weather and to anything later excavated alongside the building.
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.
Stiffness is what makes the plate a raft, and the stiffenings are where it comes from. A downstand under a load-bearing line is the depth the plate needs exactly where load concentrates. Reduce it, interrupt it for a drain, or move the wall off it during setting out, and the plate is asked to do the same work with less depth in the place it can least afford to lose it.
Reinforcement is what makes separate pours behave as one plate, so every place the steel stops is a place the plate stops being continuous. Construction joints, wide openings and changes in plate depth are all such places, and at each of them the joint detail, the barrier crossing it and the reinforcement around it are a single decision - a break in one usually means a break in the others.
The buried layers compete for the same position. Insulation placed beneath the plate has to carry the plate; the barrier has to be continuous under everything, so every rising service pierces it; and the concrete will shrink as it matures and move with temperature, which any finish bonded to its upper face has to tolerate. Reordering these layers changes what each of them is being asked to do.
Ground beneath a raft matters where it is worst, not where it is average. Because the plate spans over softer areas, a backfilled trench, a pocket of made ground or an abandoned service run under the middle of the footprint can put a stiff plate into a shape it was never designed to take. The same applies at the edge, where the plate is deepest and the ground closest to the outside.
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
The plate and its reinforcement are commonly encountered in these families. Whether fibres, bars or a combination belong in a particular raft is a structural design matter, and the mix itself is governed by the supplier documentation and the designer.
Control layer
Barriers beneath rafts are commonly encountered in these families. Which is relevant depends on the ground gas and groundwater assessment for the site; no membrane makes a floor resistant to water or gas on its own, only the completed and correctly jointed assembly.
Thermal layer
Where insulation is placed beneath the plate it is commonly encountered in these families, because it has to remain in position under load and in wet ground. Whether any product can be used in that position is a matter for its documentation and the designer.
Jointing and sealing
Construction and movement joints in the plate are commonly detailed with components from these families. What a particular joint requires depends on what the joint is doing structurally and on what the barrier beneath it has to achieve.
Finish surface
Where the plate is also the visible floor, these families are commonly encountered on its upper face. A finish bonded to a structural element shares that element movement, which is a matter for the flooring designer and the product literature.
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.
The boundary case
Raft and slab look alike once cast and are frequently confused on site. What separates them is whether the load-bearing structure stands on the element itself, and that answer changes the reinforcement, the stiffenings and what an alteration through the slab means later.
Read about Ground-Bearing Slab →Gas barrier and venting beneath the plate
A raft leaves no accessible space beneath it, so any venting element has to be built into the make-up below the plate before casting. That makes the gas strategy a decision taken with the foundation rather than after it.
Read about Ground Gas Protection →Continuity at the perimeter upstand
The barrier beneath the plate has to be connected to the damp control of the wall at the edge, which is the one place a buried plane becomes reachable. How the turn-up is protected during construction decides whether it survives to do its job.
Read about Damp-Proof Continuity →Walls bearing on the plate
Wall lines have to sit over the stiffenings that were designed to receive them, so raft design and wall setting out are the same drawing. A wall moved during design development can leave a downstand carrying nothing and a thin area carrying a wall.
Read about Load-Bearing Masonry →Perimeter of the insulated envelope
The edge of a raft is where the plate, the insulation beneath it and the wall insulation have to be reconciled around a continuous structural element. What that junction should be is a whole-envelope question rather than a foundation one.
Read about Thermal Bridging Control →Build-up over the plate
Anything laid over the raft has to accommodate the movement and moisture of a structural concrete element beneath it. Whether a floating build-up, a bonded screed or a direct finish is appropriate belongs to the flooring designer.
Read about Floating Screed Build-Up →Drainage and services through the plate
Every drain and incoming service passes through a structural element and a continuous barrier at the same point. Positions have to be fixed before casting, because a hole cut afterwards damages the plate and the barrier together.
Read about Buried Drainage Network →
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
- The plate is in permanent contact with ground moisture on its underside and carries a floor finish on its upper face, so the barrier arrangement and the drying behaviour of the concrete both matter. How they are assessed and tested belongs with the flooring and structural designers.
- Thermal
- Placing insulation above or below the plate changes what the concrete does thermally and what the insulation has to carry structurally. Which arrangement is appropriate depends on the construction, the use of the building and the climate, and is decided by the designer.
- Movement
- A raft moves as one element, which is its advantage, but it also shrinks, curls at free edges and responds to temperature. Where joints are placed, and what they are asked to do, is set by the structural design rather than by convenience of pouring.
- Durability
- The exposed perimeter is the part of the plate most at risk from splashing, freezing, later excavation and ground raised against it. Protecting that edge, and keeping external ground at a level consistent with the design, are recurring long-term issues.
- Buildability
- A raft concentrates a great deal of decision-making into a single operation, which is an advantage on constrained sites and a risk where information is still changing. Everything buried has to be right before the concrete arrives.
- Interfaces
- Because the plate is foundation, floor and barrier plane at once, its junctions serve several trades simultaneously. A detail drawn for the structure alone can break the barrier, and a detail drawn for the barrier alone can interrupt the reinforcement.
- Documentation
- As-built information on service positions, joints and barrier arrangement is what makes later alteration possible. Cutting into a raft without it risks damaging reinforcement and the continuity beneath in the same operation.
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.
- Do the wall and column positions sit over the stiffenings designed to receive them, and has that been checked after design changes?
- Where is insulation placed relative to the plate, and what does that mean for what it has to carry?
- Which barriers run beneath the raft, and what is the ground gas and groundwater assessment they respond to?
- Where are construction and movement joints proposed, and how does each barrier cross them?
- Has every drainage and service penetration been fixed before the reinforcement is set out?
- What floor finish is intended, and is it compatible with a structural element that will shrink and move?
- How is the exposed edge protected, and what external ground level does the design assume against it?
Boundaries
Commonly misunderstood points
Distinctions that are easy to blur, and the places where a familiar term means something narrower than it sounds.
- A raft is not simply a thicker floor slab, because what defines it is that the load-bearing structure stands on it rather than beside it.
- Spreading load over the whole footprint does not remove the need for ground investigation, since a stiff plate is sensitive to what lies under the middle of it.
- Insulating a raft beneath the plate is not automatically better than insulating above it; the arrangement changes what each layer has to do and is a design decision.
- Casting a raft in a single operation does not make it continuous, because continuity comes from reinforcement and joint detailing rather than from timing.
Conversations
Questions for qualified professionals
Bring these to the relevant qualified professional, manufacturer or authority. Requirements vary by project and location.
- What ground conditions across the footprint led to a raft rather than separate foundations?
- How stiff is the plate assumed to be, and what variability in the ground has that assumption allowed for?
- Which drawing shows the barrier arrangement, and how is it maintained at joints, upstands and penetrations?
- What is the intended sub-slab make-up, and does anything in it have to carry the plate?
- How will the finished floor be laid, and what condition does the concrete have to reach first?
- What record will exist of service positions and joint locations once the floor is complete?
What this page does not do
- A raft is a structural element and its design belongs to a qualified engineer working from ground investigation for the specific site.
- This entry states no thickness, reinforcement, insulation position or barrier specification, because each depends on ground, use and climate.
- Cutting or coring a completed raft affects the structure and the continuity of anything buried within it, and should not be done without design advice.
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.
Foundation and Ground Interface Systems
Assemblies where a building first meets undisturbed ground — strip, pad, raft and piled foundations, pile caps, ground improvement and ground gas protection.
Browse all foundations & ground entries →