Metal Decking and Composite Floor Slabs: A Practical Guide

Metal decking is profiled steel sheet laid across steel beams to form the base of a floor slab. Once concrete is poured on top, the deck acts first as permanent formwork and then, in a composite slab, as the tension reinforcement for the finished floor. It is the standard floor system for steel-framed offices, car parks, industrial buildings and many mixed-use developments, valued for speed, light weight and the way it turns each floor into an immediate working platform.

This article explains how composite slabs on metal deck work, the main profile types, how the deck interacts with the steel frame, and the design and site issues a structural engineer pays attention to. It builds on our guides to composite construction and types of concrete slabs.

What metal decking is

A metal deck is a cold-formed sheet of galvanised steel, typically around 0.7 mm to 1.2 mm thick, rolled into a ribbed profile that gives it stiffness. The sheets are laid across the top flanges of steel beams, lapped or butted at their ends, and fixed down with shot-fired pins, screws or through-deck welded shear studs.

The deck plays different roles at different stages:

During construction: it is a working platform and safety deck, and it carries the weight of wet concrete, operatives and equipment as permanent formwork.
In service, as a composite slab: embossments or indentations in the profile create mechanical bond with the hardened concrete, so the steel sheet carries tension in the slab’s span.
In a non-composite slab: the deck acts only as formwork, and conventional reinforcement carries the slab loads.

Most building floors use the composite form, where the deck’s contribution to strength reduces the reinforcement needed and allows a thinner, lighter slab.

Profile types

Metal decking profiles fall into two broad families:

Re-entrant (dovetail) profiles: narrow ribs that close in toward the top, giving a nearly flat soffit and good mechanical key. They allow easy hanging of services from the ribs and provide good fire performance, but are generally limited to shorter spans.
Trapezoidal profiles: deeper, open ribs that give greater stiffness and allow longer spans between beams. They use less concrete for a given overall depth because the ribs displace volume.

Deep-deck profiles, much deeper than standard trapezoidal sections, are used in slim-floor or integrated-beam systems where the deck sits on the bottom flange of an asymmetric beam, keeping the floor zone shallow. Profile choice depends on span, required fire resistance, slab depth, service integration and whether the soffit will be exposed.

How the composite slab and beam work together

A composite floor in a steel frame usually involves two levels of composite action:

Composite slab: the deck and concrete act together to span between beams, the deck acting as reinforcement in tension.
Composite beam: shear studs welded to the beam’s top flange, often through the deck, connect the slab to the steel beam. The slab then acts as a large compression flange, significantly increasing the beam’s bending capacity and stiffness.

Through-deck welding of studs is common, but it depends on the deck’s coating, thickness and the condition of the beam flange; dry, clean surfaces and proper welding parameters are essential, and studs are typically tested on site by bending a proportion of them. The orientation of the deck ribs relative to the beam affects stud resistance, which is reduced when ribs run perpendicular to the beam. Composite slabs in Singapore are designed to the Eurocodes, with composite steel and concrete structures covered by EN 1994, adopted locally as SS EN 1994. Our article on steel grades in construction covers the steels involved.

Spans, propping and the construction stage

The construction stage often governs metal decking design. The deck alone must carry wet concrete, the construction load and the deck’s own weight without excessive deflection. If the span is too long for the profile and gauge chosen, temporary props are needed at mid-span or at intervals until the concrete reaches adequate strength.

Unpropped construction is faster and cleaner, so engineers often choose a profile and beam spacing that avoid propping. When the deck deflects under wet concrete, ponding adds extra concrete at mid-span, increasing load and slab thickness beyond the design. This must be accounted for in the self-weight and in level tolerances.

Other construction-stage points include:

Bearing lengths at supports, which must meet the manufacturer’s and designer’s minimums.
Edge trims and closures at slab edges and around openings, to contain concrete.
Avoiding concentrated loads, such as stacked materials, on the unconcreted deck.
Correct fixing, so sheets do not lift under wind before concreting.

Fire resistance

A composite slab must meet the fire resistance period required for the building. In a fire, the exposed steel deck heats rapidly and loses strength, so the slab’s fire resistance depends on the concrete thickness above the ribs (for insulation and integrity) and on additional reinforcement, either mesh in the topping or bars placed in the ribs, which takes over the tension role when the deck is weakened.

The steel beams beneath may need fire protection such as intumescent coatings, boards or sprayed systems, unless a performance-based fire engineering approach demonstrates otherwise. Openings, voids and the junction with protected beams need careful detailing. Our guide to fire protection of structures sets out the broader principles.

Advantages and limitations of metal decking

The benefits explain the system’s popularity:

Speed: decks are lifted in bundles and laid quickly, with little formwork and fewer props.
Weight: composite slabs are lighter than equivalent solid slabs, reducing column and foundation loads.
Safety: each laid floor becomes a working platform and protects trades below.
Services: re-entrant profiles allow simple hangers for ceilings and services.

The limitations are equally real. Spans between beams are limited compared with post-tensioned slabs. Vibration and acoustic performance need attention in lightweight floors, particularly in offices and residential uses. The galvanised deck is exposed on the soffit and may corrode in aggressive environments such as car parks or coastal sites if not protected. Later modifications, such as cutting openings, need an engineer’s check because the deck may be a primary reinforcing element.

Frequently asked questions

What is metal decking used for?

Metal decking forms the base of concrete floor slabs in steel-framed buildings. It acts as a working platform and permanent formwork during construction and, in composite slabs, as tension reinforcement once the concrete has hardened.

Does a composite slab still need reinforcement?

Usually, yes. Mesh or bars are typically included to control cracking, distribute loads, provide continuity over supports and give fire resistance when the exposed deck loses strength. The amount depends on the span, loads and fire rating required.

Does metal decking need propping?

It depends on the span, profile and gauge. Many floors are designed to be unpropped for speed, but longer spans or heavier slabs may need temporary props until the concrete gains strength. The designer and deck manufacturer specify propping requirements.

Can openings be cut in a composite floor slab?

Only with an engineer’s approval. Because the deck and any added bars may be acting as reinforcement, cutting an opening can reduce the slab’s capacity. Small openings are often accommodated with trimming bars, but larger ones may need steel trimmers or beam modifications.

What thickness is metal decking?

Structural metal decking for composite slabs is commonly in the range of about 0.7 mm to 1.2 mm in steel thickness, depending on the profile and span. Thicker gauges allow longer unpropped spans and greater construction-stage capacity.

Designing a composite floor or planning openings in an existing metal deck slab? Talk to our consultancy or get in touch.