Roof deck works alone. There is no concrete, so the steel profile and its gauge carry the load unaided, and depth is therefore the dominant specification. Composite floor deck acts with concrete and serves as reinforcement in the finished slab. The two look broadly similar but are engineered for different conditions and are not interchangeable, either structurally or in fire.

Because it spans further, which reduces the amount of secondary steelwork underneath. Fewer purlins, or wider frame spacing, generally saves more than the deeper deck costs, and it also reduces erection time and the number of connections. Very deep long-span profiles can span directly between main frames, removing the purlin layer entirely, which is where the largest savings occur.

The sheeting, when properly fixed at its supports and along its side laps, acts as a stiff plate in the plane of the roof. That plate can carry horizontal forces and provide restraint and stability to the frames beneath, sometimes removing the need for separate bracing. It has to be designed and specified deliberately, because it depends entirely on the number and type of fixings.

Fixings into the supporting steelwork at a specified frequency across the sheet width, and stitching fasteners along the side laps between sheets. The specification is driven by wind uplift, which is greatest at eaves, ridges and corners, and by whether diaphragm action is being relied on. Uplift failures almost always trace to insufficient or incorrectly installed fixings rather than to the deck itself.

Typically a vapour control layer, insulation, and then a waterproofing system, whether single-ply membrane, built-up felt or a standing-seam metal roof. Each layer has fixings that pass through into the deck, so the deck must be able to accept them, and the combined build-up determines the thermal and acoustic performance. The deck is the structural base for all of it rather than a finish in itself.