This is a relatively simple question with a complex answer. I am currently planning an AWCI technical document that will go more in depth to assist architects and engineers with properly specifying steel framing thickness, but the short answer is to avoid the use of the term “gauge” or “gage” to refer to steel framing thickness, and instead specify a decimal thickness or a minimum level of performance.
What is currently available in the market today as either “20 gauge” or “20 gauge equivalent” ranges from 0.017″ to 0.0329″ thick. Some old tables show 20 gauge material as being up to 0.0359” thick, and different industries have different standards for what they consider 20 gauge. Focusing on the framing industry, we will narrow this down to three categories: structural, nonstructural and “EQ,” or equivalent thicknesses.
Both the Steel Stud Manufacturers Association and Steel Framing Industry Association have a steel thickness table near the front of their respective catalogs. Both tables show two entries for 20 gauge: 30 mil for “20 – Drywall” and 33 mil for “20 – structural.” Mil thickness is part of the “product designator” developed by SSMA and AISI in the late 1990s to standardize framing nomenclature. These tables not only include the reference gauge number but also the minimum bare steel thickness, the “design thickness,” and the bend radius associated with each thickness. The “gauge” column is marked as “reference only” in both catalogs. These thickness numbers do not include EQ studs, which have been engineered to provide equivalent performance as 30 and 33 mil nonstructural products. The thickness numbers also do not include coatings of zinc, paint or other metallic and non-metallic compounds. These coatings typically add at least 1 mil to any field-measured thickness.
So, what do you do when you see a specification or wall type that requires or requests 20 gauge?
• In most cases, the architectural community has accepted EQ studs that provide equivalent performance to 30 or 33 mil studs. Bidding using these EQ studs is common industry practice, although it is up to the stud manufacturer to provide proof of this equivalence.
• Check for performance requirements in the drawings and specifications. Sometimes architects will call out maximum deflection or a minimum decimal thickness or both. A future article will discuss deflection limits, but most manufacturers’ tables for partitions provide tables for L/120, L/240 and L/360 deflection limits. Composite tables are also included in ASTM C754 for standard 30 and 33 mil studs.
• Often, a minimum thickness is required because of a UL design or other fire-resistance rating. UL designs have two tables of MSG (manufacturers’ standard gauge) listed in their product IQ BXUV GuideInfo. The UL tables for both load-bearing and non-load-bearing steel studs list 20 gauge as having a minimum bare metal thickness of 0.0329 inches. For non-load-bearing applications, most manufacturers have performed fire tests on their nonstructural EQ studs, which are listed by name in many UL designs, even though the thickness is less than 33 mils. If architectural drawings call for a specific UL design, check with your stud manufacturer or the UL design listing to ensure that the EQ stud you want to use is approved (listed) in this design.
For decades I have worked to get architects, engineers and contractors to stop using “gauge” to refer to steel thickness. I realized several years ago that this is never going to happen, so the best we can hope for is to provide a common understanding of what gauge means in decimal equivalence. For structural framing with thicknesses of 43 mil to 118 mil (18 gauge to 10 gauge), this is fairly straightforward: The industry has coalesced around a series of thicknesses that are standard across the industry. For nonstructural products, this is a different story: With the maturation of EQ studs, which have been available in our industry for almost 20 years, architects and specifiers have accepted that thinner steel can meet performance requirements with less material, providing both cost and environmental benefits. And as the industry continues to mature with more innovative products and systems for framing walls and ceilings, there will be even more options for contractors to choose from and architects to specify.
So, back to the initial question: How thick is 20 gauge? Answer 1: Anywhere between 0.0179” and 0.0329” minimum bare metal thickness. Field measured thickness will be slightly thicker because of coatings. Answer 2: Structural: 33 mils, nonstructural, 30 mils, EQ 20: 18 to 24 mils, depending on the manufacturer.
