Structure and support · Floor Structures
Precast Concrete Plank Floor System
This entry treats a plank floor as a plate rather than a row of units, and shows why bearing, jointing and tying are stability details for the structure beneath as much as support details for the floor.
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 precast plank floor is
A precast plank floor arrives complete. Each unit is made in a factory, brought to site and set down on its supports, and the floor takes shape as the units are placed side by side. What turns that row into a floor is what happens afterwards: the longitudinal joints are grouted, reinforcement ties the units to each other and to the structure, and in many arrangements a topping is cast over the whole.
Against a beam and block floor the distinction is what carries the span. A plank spans on its own across its full width, and the grouted joint between neighbours ties them together rather than completing the surface. In a beam and block floor the beams span and the blocks fill between them, so the infill exists precisely because the beams cannot make a surface by themselves.
The reason the arrangement matters more than the units is that the floor is also part of the building's stability. Once tied, it acts as a plate at that level, gathering horizontal forces and taking them into the walls, frame or cores. Bearing and tying are stability details, and a plank floor placed but not yet tied is not the floor that was designed.
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.
- hollowcore floor
- prestressed plank floor
- precast slab floor
- hollow core units
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.
- Span between supports with factory-made units placed quickly and with little site work beneath them.
- Act as a single plate once the joints are grouted and the units are tied to each other and the structure.
- Carry horizontal forces at that level into the supporting walls, frame or cores.
- Present a soffit that can be left exposed or lined, and a surface a topping or finish can be built on.
- Accommodate planned openings by trimming and by units made short, rather than by cutting on site.
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 detail
The line where the units land on walls, beams or corbels. It is designed rather than adjusted, because bearing controls both how load leaves the floor and how the floor holds the structure together at that level.
- Primary supportPrecast spanning units
The factory-made elements, often with voids running through them to reduce weight. Each spans on its own, which is why the pattern of units and the position of openings are settled before manufacture rather than during the work.
- Jointing and sealingGrouted longitudinal joints
The narrow joints between neighbouring units, filled so that load applied to one unit is shared with those beside it. Left unfilled, or filled into a dirty joint, the floor behaves as a row of separate elements under a concentrated load.
- AttachmentTying and continuity reinforcement
The steel placed at supports, in joints and around the perimeter to connect the units to each other and to the structure. It is what makes the floor a plate, and it is often the least visible part of the whole design.
- SubstrateStructural topping where used
A reinforced layer cast over the units in many arrangements. Whether it is present, and whether it is structural, changes what the floor can do in gathering horizontal forces, and it is not interchangeable with a levelling screed.
- Edge and terminationOpenings and trimming
Stairwells, risers and service holes formed by trimmer arrangements or by units made short. Because each unit spans on its own, an opening is a design and manufacturing matter rather than something cut once the floor is down.
- Service zoneService routing provision
The voids within the units and the zone beneath the soffit used for distribution. What may be used, and where a fixing can be made into a unit containing prestressing steel, is defined by the manufacturer and the designer.
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.
Grouting the joints is what converts neighbours into collaborators. Until the joints are filled, a load placed on one unit is carried by that unit alone; afterwards the units either side take a share and the floor responds as a surface. This is why a joint left short, contaminated or dry is a structural omission rather than an untidiness.
Tying and bearing are the same conversation from opposite ends. The reinforcement around the perimeter and over the supports is what lets the floor gather horizontal forces and pass them into the walls or frame, so the detail that appears to be about holding the floor up is also what holds the structure together. Alterations at the bearing therefore reach a long way past the floor.
The topping, where used, changes what the rest has to do. With a structural topping, plate action can be developed through the cast layer; without one, everything depends on the joints and the tying steel alone. Substituting a levelling screed for a structural topping looks like a finishes decision and is not, and the difference is invisible once a covering is down.
Openings interrupt units that cannot be cut freely, because the steel within them runs the length of the unit. Trimming an opening redistributes load onto the neighbouring units and is settled with the manufacturer before delivery. A hole agreed on site, after the floor is placed, is a different kind of decision entirely.
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 as the units and the tying steel. What a given floor requires, including the prestressing arrangement within a unit, is determined by the manufacturer's design and the structural engineer.
Jointing and sealing
Cementitious materials of these families are commonly encountered in the grouted joints. What is used, and how the joint is expected to behave, is defined by the unit manufacturer's information rather than chosen on site.
Substrate
These are commonly encountered as the topping over the units. Because the planks already span, a topping that carries no reinforcement leaves plate action resting on the grouted joints and the tying steel alone, and which of those the design assumed is a question for the structural engineer.
Finish surface
These are commonly encountered as the surface built over such a floor. Whether a covering can be laid on a topping, and when, depends on moisture testing and on the covering manufacturer's documentation.
Protection
Materials of these kinds are commonly encountered where penetrations through the floor are closed or where the soffit is lined. What any of them is relied on to achieve is a matter for the fire strategy and the relevant authority.
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.
Connection into precast walls
Where the walls are also precast, floor and wall are designed as a jointed structure and the connection between them carries stability as well as support. Site jointing is where a set of factory-made parts becomes a building.
Read about Precast Panel Structure →Floor acting as a stabilising plate
The tied floor is how horizontal forces at that level reach the walls, cores or bracing. That is why the tying reinforcement is designed with the stability system rather than treated as a detail belonging to the floor.
Read about Lateral Stability →Bearing onto masonry
Units bearing on walls deliver their load along the line of support rather than at a point, and that line has to be complete and level before anything lands on it. The wall in turn is restrained by the floor once it is tied, so the two constructions are sequenced around one another.
Read about Load-Bearing Masonry →Isolated layers over the units
Where a resilient layer and screed are laid over the floor, that build-up belongs to the floating screed entry, and this system's involvement ends at the surface it presents for the layers above.
Read about Floating Screed Build-Up →Where the floor divides occupancies
In flats the same construction is judged by what passes between homes, including the paths running around its edges. That assessment belongs with the separating floor entry and with a qualified acoustic professional.
Read about Separating Floor →Distribution below the soffit
Services are usually hung beneath rather than buried within, so the soffit becomes a fixing plane. What may be drilled into a prestressed unit is defined by its manufacturer and is not a site judgement.
Read about Service Void →
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
- Units arrive by crane to a placing sequence, so access, standing and the readiness of the supports govern the programme. Once the units are down, changes are constrained by what the factory made rather than by what would be convenient.
- Movement
- Precast units carry a built-in curvature from manufacture and continue to change shape after placing. That is why the surface presented is not necessarily flat, and why the levelling layer above is planned rather than assumed away.
- Fire
- A floor between storeys usually forms part of the fire strategy, and penetrations, joints and any lining beneath all bear on it. Performance belongs to tested arrangements and the relevant authority, and none is stated here.
- Acoustic
- What is heard through a dense floor depends on the complete construction, including anything laid over it and the flanking walls around it. This is assessed by a qualified acoustic professional rather than inferred from the mass of the units.
- Documentation
- Where the prestressing strand sits inside a unit is recorded by its maker and nowhere else, and there is nothing on the soffit or the surface that reveals it. A proposal to core or to fix into the floor rests on retrieving that record rather than on inspection.
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 structural, and what is this floor relied on to do for the stability of the structure?
- Where are openings needed, and have they been agreed with the manufacturer before the units are made?
- How are the units tied to each other and to the supporting walls or frame?
- What is the soffit going to be, whether left exposed, lined, or hung with services?
- How flat is the surface expected to be, and what levelling layer has been allowed for above it?
Boundaries
Commonly misunderstood points
Distinctions that are easy to blur, and the places where a familiar term means something narrower than it sounds.
- The grouted joint is treated as a filler, when it is what lets neighbouring units share a load placed on any one of them.
- A plank floor is thought complete once the units are placed, but until it is tied it is not the floor that was designed.
- Drilling into a unit is assumed to be like drilling into a slab, when steel running the length of the unit changes what is permissible.
Conversations
Questions for qualified professionals
Bring these to the relevant qualified professional, manufacturer or authority. Requirements vary by project and location.
- What is this floor relied on to do for the stability of the structure at this level?
- Is the topping structural, and what happens to the design if it is changed or left out?
- How have the openings been arranged, and were they designed before the units were made?
- What may be fixed into or drilled through these units, and on whose information does that rest?
- What surface tolerance should be expected, and what levelling has been allowed for above?
- How do the bearing and tying details relate to the walls or frame beneath this floor?
What this page does not do
- Unit selection, bearing, tying and any opening are structural matters for a qualified engineer and the unit manufacturer.
- Cutting or coring a prestressed unit can sever steel the unit depends on, and is not a decision to take on site.
- Craneage, temporary support and the placing sequence are temporary works matters for competent people.
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 →