Structure and support · Floor Structures
Composite Metal Deck Floor System
This entry explains composite action itself: how a sheet that begins as formwork ends as reinforcement, and how a separate decision about connectors brings the supporting beams into the same section.
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 composite metal deck floor is
A composite metal deck floor begins as profiled steel sheets laid across a grid of beams. In that state the sheet is formwork, carrying wet concrete and the people placing it. Once the concrete has gained strength the relationship inverts: the sheet is locked into the underside of the slab and works as its tension reinforcement, and the floor spans as a single element that neither part could form alone.
Against a precast concrete plank floor, the question to ask is whether the concrete is structural. Here the topping and the sheet act as one and neither works without the other, so take the concrete away and there is no floor at all. Over precast planks the concrete is added to units that already span, and a plank floor without a topping is still a floor.
That difference is why this entry is not a page about profiled decking. A sheet has a profile, a coating and a way of being fixed. Composite action is the arrangement in which that profile keys into the concrete; connecting the slab to the beams is a second arrangement of the same kind, made or not made on its own merits. It is the arrangement rather than the sheet that decides how the finished floor behaves.
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.
- composite slab
- metal deck and concrete floor
- steel-concrete composite floor
- profiled deck 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.
- Provide a working platform and the permanent formwork for the concrete during construction.
- Act with the concrete as one spanning element once the finished floor has gained its strength.
- Act with the supporting beams as one section where connectors are provided, and work as a plate carrying horizontal force at that level.
- Offer an underside that services and ceilings can be hung from at each level.
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.Primary supportSupporting beam grid
The beams the sheets land on. Their layout sets the deck span and the direction the sheets run, and once the floor is composite with them, beams and slab are behaving as a combined section rather than as separate elements stacked on each other.
- Layer 2 of 8. Order is meaningful.SubstrateProfiled steel deck
The shaped sheet spanning between beams. Its profile is what keys into the concrete and what carries the wet weight before the slab has any strength, so the same component is doing two entirely different jobs at different moments.
- Layer 3 of 8. Order is meaningful.AttachmentShear connection to the beams
The connectors joining the slab to the beams beneath it. The sheet already works with the concrete through its own profile; what these add is a further decision, tying beam and slab into one deeper section instead of leaving a slab that rests on a frame bending separately beneath it.
- Layer 4 of 8. Order is meaningful.Primary supportConcrete topping
The cast layer becoming the compression part of the section. It is not a screed or a levelling course, and its make-up, placing and curing are structural matters rather than finishing ones however ordinary the pour looks.
- Layer 5 of 8. Order is meaningful.Primary supportReinforcement zone
The mesh and bars within the topping dealing with shrinkage, continuity over the supports and the areas the sheet alone cannot cover. It occupies a defined position in the depth that other trades cannot borrow for their own purposes.
- Layer 6 of 8. Order is meaningful.Edge and terminationEdge trim and penetration framing
The trims closing the deck at slab edges and the framing around holes. They contain the concrete while it is placed and afterwards define the edges, and every planned opening interrupts sheets that were spanning across it.
- Layer 7 of 8. Order is meaningful.Service zoneSoffit as a services attachment plane
The underside of the deck, used to hang services and ceilings from. The ribs give fixing positions and also constrain where a hanger may go, so services layouts and deck direction are related whether or not anyone coordinated them.
- Layer 8 of 8. Order is meaningful.ProtectionApplied protection to the exposed steel
The coatings or boarded encasement applied to deck and beams where the design calls for it. What is required follows from the fire strategy and the exposure, and the presence of concrete above settles neither of those questions.
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 sheet changes job halfway through the floor's life, and that is the whole idea. While the concrete is wet the profile carries it; once it has hardened the same profile is locked into the underside and acts in tension. Anything done to the sheet in between, a hole cut for convenience, a damaged rib, a lap left unfixed, takes something away from both roles at once.
Two different collaborations are at work here and they are easy to run together. Within the slab, the profile of the sheet keys into the concrete so that sheet and topping behave as one spanning element. Between slab and beam, connectors are what turn two elements resting on each other into a single deeper section. A decked floor can have the first without the second, and reading one as evidence of the other is where later alterations go wrong.
Openings are more disruptive here than in a floor of discrete units. A sheet spans continuously, so cutting through it interrupts the tension reinforcement of the slab as well as the formwork, and the framing has to reinstate both. That is why penetrations are located before the deck is set out rather than agreed with the services trades afterwards.
The soffit is doing structural and coordination work at the same time. Its ribs give a hanging surface for services and ceilings, and its exposure decides what protection the design calls for. Fixings driven into the underside meet reinforcement and, in places, the connection zone above, so what looks like a free surface to hang from is constrained by the section it belongs to.
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.
Substrate
Sheet families of this kind are commonly encountered as the deck. Which profile and coating a floor requires, and how it behaves before the concrete gains strength, is determined by the structural designer and the manufacturer's information.
Primary support
These families are commonly encountered in the beams, the topping and the reinforcement. What each has to be in a given floor follows from the structural design, and none of it is settled by the choice of deck profile.
Protection
Protective coatings and encasement boards of these kinds are commonly encountered on exposed steel and soffits. What applies to a particular building is set by the fire strategy, the exposure and the relevant authority.
Finish surface
These are commonly encountered above the structural slab as levelling, wearing or raised layers. Whether a covering can be laid on the topping, and when, is a matter for testing and the covering manufacturer's documentation.
Service zone
Ceiling families of this kind are commonly encountered hung beneath the deck. How anything is fixed to a composite soffit is constrained by the section above it and belongs with the structural designer.
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 the steel frame
Floor and frame are designed together, and they meet in two separate ways. Connectors between slab and beam make those two act as one section, while the slab behaving as a plate, tied at its edges and around the perimeter, is what delivers horizontal force into the stability system. A change to either arrangement reaches the frame.
Read about Steel Frame →Floor acting as a stabilising plane
Because the finished slab ties the frame at each level, it forms part of how horizontal forces reach the bracing or the cores. Openings and slab edges are therefore stability details as much as spatial ones.
Read about Lateral Stability →Services hung from the soffit
Distribution below the floor is fixed to the deck. Deck direction, rib positions and the reinforcement above all constrain where a hanger may go, and coordination is far easier before the deck is set out than after.
Read about Service Void →Slab edge meeting the facade
The facade is commonly supported from and sealed to the slab edge. The edge trim, the concrete profile along that line and the movement expected of the floor all become part of a junction asked to keep water and air out.
Read about Curtain Walling →Isolated layers laid over the slab
Anything isolated set over the topping begins from a surface finished for structural reasons rather than for what sits on it, and the structural floor stops at that concrete face. Perimeter isolation, joint positions and drying all belong to the build-up above rather than to this entry.
Read about Floating Screed Build-Up →
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.
- Fire
- Steel exposed on the soffit and on the beams below usually falls within the building's fire strategy, and what is required belongs to tested arrangements. Concrete above the sheet does not by itself settle what the underside needs.
- Durability
- Coated steel in a dry interior behaves differently from steel that moisture or aggressive conditions reach, as over a pool hall or in a car park. The exposure the deck will actually see is a design input rather than an afterthought.
- Acoustic
- A dense floor deals with airborne and impact sound differently, and what reaches the space below depends on the whole construction including any raised floor and ceiling. Performance is a matter for a qualified acoustic professional.
- Buildability
- The floor is built through stages that behave differently: a propped sheet, a slab being placed, then a composite element. Loading imposed during that period is a temporary works matter and is not the finished floor's capacity.
- Interfaces
- Slab edges, penetrations and the connection to the frame carry most of the design content, and they involve the facade, the services and the structure along the same lines. Coordination is either done early or paid for later.
- Documentation
- Nothing about the sheet acting as reinforcement can be read from the soffit once a ceiling is up, and a rib pattern gives no indication of profile, gauge or lap. The record made when the floor was designed is the only place any of it survives.
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 concrete on this floor structural, or is it a topping over units that already span on their own?
- Where are penetrations required, and have they been located before the deck is set out?
- What exposure will the deck and the beams beneath it actually see once the building is in use?
- How are services intended to be hung from the soffit, and does the deck direction suit that layout?
- What is this floor expected to do for the stability of the frame at each level?
- What protection does the design call for on the exposed steel, and who is determining 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 deck is thought of as permanent formwork only, when in the finished floor it is also acting as reinforcement.
- Concrete over steel is assumed to settle the question of protection to the underside, which remains a matter for the fire strategy.
- Openings are treated as a services matter, when cutting the sheet interrupts something the finished slab is relying on.
Conversations
Questions for qualified professionals
Bring these to the relevant qualified professional, manufacturer or authority. Requirements vary by project and location.
- How is composite action between the deck, the slab and the beams being achieved in this floor?
- What temporary support does the deck need before the concrete has gained its strength?
- Where may services be fixed to the soffit without affecting the section above them?
- What role does this floor play in the stability of the frame it forms part of?
- What protection is required to the exposed steel, and on what basis has that been decided?
- What information will be recorded so that future openings in this floor can be assessed?
What this page does not do
- Deck spans, propping, connection arrangements and reinforcement are structural matters for a qualified engineer.
- Forming a new opening cuts an element that is acting as reinforcement, and is not a services alteration.
- Fire protection to the soffit and the frame is a property of tested arrangements and is governed by the relevant authority.
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 →