Metal decking is a structural building material made from profiled steel sheets, used as the base layer for roofs and floors in commercial and industrial construction. You’ll see it in nearly every steel-framed building, from office towers to warehouses, where it serves as either a standalone roof surface or the foundation beneath a concrete floor slab. The corrugated, ribbed sheets span between steel beams and joists, creating a platform that supports loads while adding rigidity to the entire structure.
How Metal Decking Works
Metal decking starts as flat steel coil that gets cold-formed into a repeating pattern of ribs and valleys. Those ridges aren’t decorative. They give the thin sheet its strength, the same way corrugated cardboard is far stiffer than a flat piece of paper. Once the panels are placed across structural steel framing, they distribute weight across the span and transfer it down into the beams below.
In floor applications, the steel deck acts as a stay-in-place form for poured concrete. Rather than building temporary wooden forms, removing them after the concrete cures, and discarding them, the metal deck stays permanently in place and becomes part of the finished floor system. This saves significant time and labor on a job site. In many designs, the deck also contributes to the slab’s overall load-bearing capacity, meaning the finished floor is stronger than the concrete alone would be.
Roof Deck vs. Floor Deck
The two main categories of metal decking serve fundamentally different purposes, and they’re designed accordingly.
Roof deck is typically used as a standalone structural layer. Insulation and roofing membrane go on top, but no concrete is poured over it. Its job is to span between roof joists, support the roofing system’s weight, and resist wind uplift forces. Because it doesn’t need to bond with concrete, roof deck panels have smooth surfaces without special texturing.
Floor deck (also called composite deck) is designed to work together with a poured concrete slab. The steel sheets have raised patterns, called embossments, stamped into their surface. These small bumps and ridges create mechanical interlocking between the steel and the concrete once it cures. Without them, the concrete would tend to slide horizontally along the steel surface under load, and the two materials wouldn’t share the structural work. The embossments prevent that slippage, allowing the steel and concrete to behave as a single, stronger unit. In addition to embossments, shear studs welded through the deck into the beams below further lock everything together.
Common Deck Profiles
Metal deck panels come in standardized profiles, each identified by a letter. The profile you need depends on how far the deck has to span between supports and how much load it needs to carry.
- Type B deck: The most common roof deck profile. It has 1.5-inch deep ribs, is 36 inches wide, and works well for low-slope roofs with moderate spans between supports.
- Type N deck: A deeper profile at 3 inches, covering 24 inches wide. The extra depth gives it significantly more spanning capability, making it the choice for long-span roofs where the designer wants fewer supporting beams.
- Type F deck: Also 1.5 inches deep but narrower at 27 inches wide, with tighter rib spacing. It’s less common today but still shows up in retrofits and specialty applications on older buildings.
Floor deck profiles are typically deeper, often 2 or 3 inches, to hold the concrete slab above. The deeper the profile, the more concrete it displaces, which can reduce the total weight of the floor system.
Steel Gauge and Thickness
Metal decking thickness is specified by gauge number, and the system is counterintuitive: lower gauge numbers mean thicker steel. Most metal decking falls in the 22 to 16 gauge range. A 22-gauge sheet is 0.028 inches thick, while a 16-gauge sheet is 0.065 inches thick, more than double. A 20-gauge sheet, one of the most common for roof applications, measures 0.035 inches.
The right gauge depends on the span length, the expected loads, and whether the deck is composite or non-composite. Longer spans and heavier loads call for thicker (lower gauge number) steel. Engineers specify the gauge based on load tables published by the deck manufacturer, which account for the profile shape, steel strength, and support spacing.
Galvanized Coatings for Corrosion Protection
Nearly all metal decking is galvanized, meaning it’s coated with a thin layer of zinc to prevent rust. The two most common coating grades are G60 and G90, named for the total zinc weight applied to both sides of the sheet.
G60 applies 0.60 ounces of zinc per square foot, resulting in a coating about 0.51 mils thick on each side (a mil is one-thousandth of an inch). G90 applies 0.90 ounces per square foot, yielding 0.77 mils per side. Since coating thickness directly determines how long the zinc protects the steel underneath, G90 lasts longer in corrosive environments. Interior floor decks that will be encased in concrete typically use G60, while roof decks exposed to moisture or decks in humid climates often call for G90.
How Metal Decking Is Installed
Panels are lifted onto the steel frame in bundles, then spread out and positioned by ironworkers. Each panel overlaps the one beside it by one rib to create a continuous surface. Attachment to the structural beams below has evolved considerably over the years.
The traditional method is puddle welding, where a welder burns through the deck sheet and fuses it to the beam flange below, creating a small weld “puddle” at each connection point. This remains common, but mechanical fasteners have gained significant ground because of their speed and consistency. Powder-actuated tools (essentially construction nail guns powered by a small explosive charge) and pneumatic tools drive hardened pins through the deck into the steel below. These methods are faster, don’t require a certified welder at every connection, and produce reliable results in a wider range of site conditions.
Side-lap connections between adjacent panels are typically made with self-drilling screws or crimping tools that mechanically lock the overlapping ribs together.
Industry Standards and Specifications
The Steel Deck Institute (SDI) publishes the primary design and installation standards for the industry. The current benchmark is ANSI/SDI SD-2022, which consolidated several older, separate standards for roof deck, composite floor deck, and non-composite floor deck into a single document. It covers design criteria for routine structural applications.
A companion standard, ANSI/SDI QA/QC-2022, addresses quality control during installation, covering things like fastener spacing, weld inspection, and proper panel alignment. For composite floor systems specifically, ANSI/SDI T-CD-2022 provides standardized testing procedures to verify the bond strength between the steel deck and concrete. Architects and engineers reference these standards in their project specifications, and building codes in the U.S. recognize them as the baseline for acceptable practice.