When reviewing steel deck specifications or purchasing orders, you will encounter the designations G60 and G90 — sometimes written as A60 or A90 in older documents. These refer to the zinc coating weight applied to the steel sheet during the galvanizing process, and they directly affect the deck's long-term corrosion resistance. Here is what the designations mean and how to specify the right one for your project.
What Do G60 and G90 Mean?
The "G" designation comes from ASTM A653, the standard specification for hot-dip galvanized steel sheet. The number indicates the total zinc coating weight on both sides of the sheet, measured in hundredths of an ounce per square foot of sheet surface (oz/ft²):
- G60: 0.60 oz/ft² total zinc coating (both sides combined)
- G90: 0.90 oz/ft² total zinc coating (both sides combined)
G90 has 50% more zinc than G60. More zinc means a thicker protective barrier against moisture, oxygen, and chlorides — the primary drivers of steel corrosion.
How Galvanizing Protects Steel Deck
Hot-dip galvanizing bonds a layer of zinc to the steel surface. Zinc acts as both a physical barrier (blocking moisture from reaching the steel) and a sacrificial anode (corroding preferentially to protect the steel at any exposed edges or scratches). The thicker the zinc coating, the longer the steel beneath it is protected in a given environment.
G60 vs G90: Side-by-Side
| Property | G60 | G90 |
|---|---|---|
| Zinc coating (total, both sides) | 0.60 oz/ft² | 0.90 oz/ft² |
| Corrosion resistance | Standard | Enhanced — 50% more zinc |
| Typical applications | Interior/dry environments, roof deck | Composite floor deck, humid/coastal areas |
| Concrete contact applications | Acceptable in most cases | Preferred — standard for floor deck |
| Coastal/high-humidity areas | Consult structural engineer | Recommended |
When G60 Is Sufficient
G60 is specified for interior applications in dry, conditioned environments. The most common use case is 1.5" B roof deck in a standard commercial building where the deck will be covered by insulation and a roofing membrane immediately after installation. In controlled interior environments with no direct moisture exposure, G60 provides adequate service life for the design life of the structure.
When G90 Is Required or Preferred
G90 is the standard coating for composite and non-composite floor deck. The reasoning: floor deck is exposed to wet concrete during the pour, creating an alkaline environment that can accelerate zinc consumption. G90's additional coating weight extends the protection through this vulnerable construction phase and through the service life of the floor system.
G90 is also the appropriate choice for:
- Roof deck in coastal or high-humidity regions (Gulf Coast, Florida, Pacific Northwest)
- Any application where the deck will experience extended exposure before enclosure
- Parking structures where de-icing salts or moisture intrusion are expected
- Industrial facilities with process moisture or chemical vapor exposure
- Projects where the owner's specification explicitly requires G90
What Your Project Specification Will Say
Structural drawings and project specs typically call out the galvanizing requirement in the steel deck specification section. Look for language like "galvanized per ASTM A653, G90 minimum" or a direct reference to the SDI standard. If the spec is silent on coating weight, confirm with the structural engineer of record before ordering — specifying G60 where G90 is intended is a field issue you want to avoid.
What We Supply
At Cutting Edge Decking, we supply:
- Roof Deck (1.5" B) — G60 or G90 available
- Composite Deck (1.5", 2", 3") — G90 standard
- Non-Composite Deck (1.5", 2", 3") — G60 or G90 available
All orders are Grade 50 steel as standard. If your project calls for a specific coating weight or you need to confirm what is appropriate for your application, reach out to our team with your project details.