The Gauge Controversy Explained
© 2008 Door & Access Systems
Publish Date: Winter 2008
Author: Karl Hermann
Page 56
Technical Tip
The Gauge Controversy Explained
By Karl A. Hermann
Product Development for Painted and Coated Products
Steel Dynamics – Flat Rolled Division
Can a 24-gauge door actually be a 26-gauge door? Does it matter whether your door is referred to as a 24-, 25-, or 26-gauge door? Does the gauge of a door include the thickness of the galvanizing and the paint?
In an attempt to provide some consistency within the garage door and rolling door industry, the DASMA Commercial & Residential Door Division Technical Committee recently issued an updated version of Technical Data Sheet 154 (TDS 154). This article reviews the highlights of TDS 154 and provides some insight into its use.
Gage vs. Gauge
In the garage door industry, “gauge number” is commonly used to refer to the thickness of flat-rolled steel. In this article, this thickness will be denoted as “gage.” The term “gauge,” though, as commonly used by steel industry insiders, refers to the decimal thickness.
The gage number, such as “24 gage,” is not the same as the actual decimal thickness of the door panel. For example, in TDS 154, a 24-gage door with a G40 galvanized coating weight has a minimum thickness of 0.0216".
Wiggle Room
However, in the standard manufacturer’s gage table, the allowed tolerance for 24-gage galvanized product is 0.0236" to 0.0316". In this table, 0.0216" falls into the 25-gage range. Other gage tables that have evolved over the years show other conflicting ranges.
Also, the tolerance ranges of these gage tables sometimes overlap. These range overlaps create some wiggle room for how a door product can be marketed.
Because of ambiguities like this, the American Society of Testing and Materials International (ASTM) has eliminated the standard manufacturer’s gage table as a specification reference. ASTM now relies solely on specific decimal thickness as a more accurate and less ambiguous way to specify product. These same ambiguities have also driven DASMA to create an industry standard reference known as TDS 154.
Steel Tolerances
Due to the complex processing steps required to produce flat-rolled galvanized steel, the thickness of the final product is slightly variable. Consequently, ASTM has determined allowable thickness tolerance specifications for the production of flat-rolled steel.
As mentioned earlier, a 24-gage garage door with G40 galvanizing must be at least 0.0216" thick. To ensure that the product is not produced below this minimum, steel manufacturers set their equipment to produce a thickness of, for example, 0.0231". Due to processing variability, the final product could measure within a full tolerance range of 0.0216" to 0.0296" thick. Presently, though, “half standard” tolerance is the norm (i.e., 0.0216" to 0.0256").
How 22 Gage Can Be 24 Gage
According to the DASMA Steel Gauge Chart, G40 steel that is 24-gage could be as thin as 0.0216", but with allowable full ASTM tolerance could be as thick as 0.0296". G40 steel that is 0.0296" thick is actually in the tolerance range for 22 gage.
How, then, should this door be marketed: as a 24-gage door, a 23-gage door, or a 22-gage door? Yes, 22 gage sounds better than 24 gage, but is 22 gage the most ethical designation?
Gage = Steel Only?
In my opinion, gage should be determined by the actual substrate thickness of the steel plus the galvanized coating. The paint applied to the galvanizing is not considered as part of the decimal thickness supplied by the steel mills.
In the garage door industry, the gage number used in advertising/marketing should refer only to the thickness of the unembossed galvanized steel skin. It is not appropriate to determine the gage by combining the thickness of the steel door pan and the steel backer sheet if the door is insulated.
A Level Playing Field
This issue is admittedly confusing. That’s partly why ASTM eliminated gage references and switched to using specific decimal thicknesses.
Through TDS 154, DASMA has attempted to create a level playing field for the garage door industry so that all participating members have a consistent guideline to use as a reference. Yet, even with these standardized tools, gage numbers can be intensely debated. I hope this article erases some of the confusion and shows how the industry is trying to clarify the issue.
Resources:
1. ASM Metals Reference Book, Third Edition, 1993, Table: Comparison of Standard Gages, p.566.
2. ASTM International, 2007, A 924/A 924M-07, Standard Specification for General Requirements for Steel Sheet, Metallic-Coated by the Hot-Dip Process, Table 2, “Thickness Tolerance for Hot-Dip Metallic-Coated Sheet.”
3. DASMA Commercial & Residential Door Division Technical Committee, Technical Data Sheet 154, 10/09/08 Rev 4/08