Structure and support · Floor Structures
Beam and Block Floor System
This entry explains how beams, infill and topping combine into a single deck, why the topping is rarely just a finish, and why a floor made of concrete still depends on the void beneath it being ventilated.
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 beam and block floor is
A beam and block floor is assembled rather than cast. Precast beams are set out across the supports, blocks are dropped between them to close the gaps, and a grout, levelling or structural topping is worked over the surface. The result spans a void as a cast floor would, but it arrives as separate pieces and becomes a floor only once those pieces are working together.
The test against a precast concrete plank floor is what is doing the spanning. Here the beams span and the blocks are infill sitting between them, so lifting out a block leaves the floor still spanning; nothing was relying on that block to reach across. Lift out a plank in the sibling system and there is a hole, because each unit spans across its own full width unaided.
The reason this belongs in a systems library rather than a materials one is that no part of it is a floor. A beam is a spanning member, a block is a filler, a topping is a layer. The arrangement turns them into a deck, and the arrangement also decides where the thermal layer can sit and what has to happen along the perimeter.
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.
- block and beam floor
- precast beam and infill floor
- suspended concrete ground floor
- precast beam floor
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.
- Carry floor loading across a span into the supporting walls without depending on the ground beneath the deck.
- Leave a ventilated void beneath the deck so moving air removes ground moisture, with any soil gas role belonging to a protective scheme established by ground investigation.
- Produce a level and continuous deck from factory-made units placed and connected on site.
- Give the thermal layer a defined position above or below the structural deck rather than an incidental one.
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 supportBearing and support interface
The wall, ground beam or foundation the units land on. How far each beam is supported and what it is bedded on decide whether the floor behaves as designed, and this is fixed by the structural design rather than adjusted as the work proceeds.
- Primary supportPrecast spanning beams
The members reaching from support to support and carrying everything above them. They are laid to a set-out that the infill widths depend on, so beam positions decide whether the blocks drop in without cutting or fettling.
- SubstrateInfill blocks
The units dropped between the beams to close the surface. They fill, and they also hold the beams apart and steady while the floor is being built, which is why the infill is not simply a way of avoiding formwork.
- SubstrateGrout, levelling or structural topping
The layer worked over the assembled units. Depending on the design it may only fill the joints, or it may be a reinforced layer making the floor act as one plate for the walls beneath. The two are not interchangeable, and they look alike.
- Thermal layerInsulation above or below the deck
The thermal layer, laid over the units before a screed or held beneath them within the void. Its position changes what the topping has to do, what the perimeter looks like, and how quickly the floor responds to heat input.
- Cavity or voidVentilated void at ground level
The space between the units and the ground below, kept ventilated so ground moisture and, where present, soil gas are carried away rather than accumulating against the underside of the floor and the walls around it.
- Edge and terminationPerimeter damp and thermal continuity
The edge where the floor meets the external wall, carrying the damp course, the junction between floor and wall insulation, and the ventilation openings serving the void. Several jobs resolve along one line here.
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 topping is the component most often mistaken for a finish, and the mistake matters. Where the design treats the floor as a plate bracing the walls, the topping and its reinforcement are what join separate units into that plate. Treat it as levelling and substitute something else, and the structural assumption behind the floor quietly goes with it.
Beams and blocks depend on each other in opposite directions. The beams carry the blocks vertically; the blocks hold the beams steady sideways until the topping is on. That is why the assembly is vulnerable in a particular window, after the units are placed and before they are tied together, and why the beam set-out and the infill dimensions have to agree before anything is delivered.
The void beneath is doing a job the deck itself is indifferent to. Concrete does not need ventilation the way timber does, so the openings exist for the ground rather than for the floor, and their role is to let moving air carry ground moisture away from beneath the deck; any soil gas role belongs to a protective scheme established by ground investigation rather than to the void. That is why filling the void, or letting external levels close its openings, has a consequence even though nothing above appears to change.
Insulation position reorganises the perimeter. Above the units, the thermal layer sits under a screed and the edge needs isolation and an upstand; below them, the units fall outside the insulated envelope and the continuity problem moves down to the bearing. Either way the wall junction has to carry the thermal line past the point where the floor stops.
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
Precast elements and reinforcement of these families are commonly encountered as the spanning members and the tying steel. What a floor requires is determined by the structural design and by the manufacturer's information for the units.
Substrate
Block families of this kind are commonly encountered as the infill between beams. Whether a particular unit can be used as infill, and how it behaves under a topping, is a matter for the manufacturer's documentation and the designer.
Finish surface
These are commonly encountered as the grout, levelling or structural topping over the units. In one floor the layer does little more than close the gaps left between beam and block; in another it is the reinforced course that makes the whole deck act as a plate. Which of the two a given design intends is settled by the structural engineer.
Thermal layer
Boards and quilts of these families are commonly encountered above or below the units. Whether a product can be loaded, and how it is retained where it sits beneath the deck, follows from the manufacturer's documentation and the designer.
Control layer
Barrier products of these kinds are commonly encountered at the perimeter and, where ground gas has been assessed, within the build-up. The need for gas protection is established by ground investigation rather than by the floor type.
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.
Bearing onto the supporting walls
The units land on walls that are also carrying the building. Bearing, bedding and the tying of the floor into the wall are structural details, and where the topping is relied on to brace those walls, the connection along this line is what makes it work.
Read about Load-Bearing Masonry →Ventilated void and ground gas measures
A ventilated void is one of the arrangements used where soil gas has been identified, but a floor is not gas protection in itself. Whether the void forms part of a designed protective scheme, and how that is verified, is a specialist matter.
Read about Ground Gas Protection →Perimeter junction with the external wall
The floor edge meets a wall carrying its own insulation and damp course. The two thermal layers have to meet around the corner, and the void's ventilation openings pass through the same wall without disturbing the cavity's own drainage.
Read about Twin-Leaf Cavity Wall →Base presented to an isolated screed above
Beams and blocks present a surface that steps between units and carries the camber the beams were made with, so a compressible layer laid over it does not bear evenly until that surface has been brought together. Whether the topping is asked to do the regularising, or the build-up above allows for it, is settled between the two designs.
Read about Floating Screed Build-Up →Support off the foundation line
At ground level the units usually bear on foundation walls, so floor level, ventilation openings and the damp course are all settled by decisions taken in the foundation work long before any beam arrives on site.
Read about Strip Foundation →
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.
- Buildability
- The units are heavy and arrive to a set-out. Access for lifting, the readiness of the supports and the sequence of placing govern the programme, because adjustment is limited once the load is on the ground and the crane has gone.
- Moisture
- Ground moisture is dealt with by the void rather than by the deck, so the openings serving that void are part of the floor's moisture strategy even though they sit in the walls. Blocking them moves a problem indoors without changing the floor.
- Thermal
- Because the deck is dense, where the insulation sits decides whether that mass is inside or outside the heated space. That in turn changes how the floor responds to heating and where the perimeter would run cold, and it is a designer's judgement.
- Interfaces
- Almost everything difficult about this floor is at the bearing and the perimeter, where support, tying, damp control, thermal continuity and ventilation all arrive at the same line and are drawn by different people.
- Documentation
- Whether the topping was designed as levelling or as the layer bracing the walls is recorded in the structural information and cannot be told apart by looking at the finished floor. That distinction is the one a later alteration turns on.
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 topping on this floor a levelling layer or a structural one, and what is it relied on to do?
- Where is the insulation intended to sit relative to the units, and how is it retained if it goes below them?
- How are the void's ventilation openings arranged, and will external ground levels leave them clear?
- Has ground gas been assessed, and does the design depend on the void as part of a wider scheme?
- Do the beam set-out and the infill units agree, and who is confirming that before delivery to site?
Boundaries
Commonly misunderstood points
Distinctions that are easy to blur, and the places where a familiar term means something narrower than it sounds.
- The topping is assumed to be levelling, when in many designs it is what makes the separate units act together as one plate.
- A concrete floor is thought not to need ventilation beneath it, but the void is ventilated for the ground's sake rather than the deck's.
- The infill is treated as filler alone, when it also holds the beams steady before the floor has been tied together.
Conversations
Questions for qualified professionals
Bring these to the relevant qualified professional, manufacturer or authority. Requirements vary by project and location.
- Is the topping in this design structural, and is the floor relied on to brace the walls beneath it?
- What bearing arrangement has been designed at each support, and how is the floor tied into the walls?
- Where does the insulation sit, and how is thermal continuity maintained around the perimeter?
- How is the underfloor void ventilated, and what will keep those openings clear over time?
- Has the ground been investigated for gas, and does this floor form part of any protective measures?
What this page does not do
- Beam layouts, bearing details and topping design are structural matters for a qualified engineer and are not indicated by this entry.
- A ventilated void is not in itself ground gas protection, and whether protection is needed follows from ground investigation.
- Cutting or forming openings in a precast floor changes how it spans and is not a matter to be settled on site.
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.
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 →