That the steel sheet acts as reinforcement in the finished slab rather than only as formwork. Shear bond between the deck and the concrete, provided by rolled embossments or by a re-entrant rib shape, transfers force between the two so they act as one section. The deck sits in the tension zone at the bottom of the slab and does the job that bottom reinforcement would otherwise do.

Trapezoidal profiles have ribs that widen towards the top and rely on embossments for shear bond; they are generally deeper and span further. Re-entrant profiles have an undercut rib that grips the concrete by shape, give a flatter soffit, and accept proprietary hangers dropped into the undercut for services and ceilings. Choose trapezoidal for span, re-entrant for soffit fixings and appearance.

Usually only anti-crack mesh in the topping, because the deck provides the bottom tensile reinforcement. Additional bars are commonly placed in the ribs for fire design, and over supports where the slab is continuous. That reduction from a full bottom mat to mesh is a substantial saving in fixing labour and is a large part of the economic case for composite construction.

The construction condition governs, because the deck must carry wet concrete and construction loading on its own before composite action exists. Unpropped construction allows the deck to span unaided, which keeps the floor below clear and speeds the programme but limits span. Propping supports the deck during the pour, allowing greater spans or thinner decks at the cost of temporary works and access below.

Principally through the concrete: its thickness gives insulation, and additional reinforcing bars placed in the ribs provide tensile capacity once the exposed steel deck has lost strength. The deck itself is not relied on for long fire periods. Manufacturers publish fire design tables giving slab depth and rib reinforcement against required fire resistance periods, and these should be used rather than general assumptions.