Q: Can you provide information on the various screw fasteners for attaching panels to cold-formed steel framed partitions?
A: The successful installation requires the correct fastener for a specific condition. In this case it is selecting the proper screw fastener when attaching gypsum or cement panels to either wood or cold-formed steel framing. Using the incorrect fastener can lead to inadequate holding power that does not meet performance requirements. These performance requirements include not only structural, as in wall limiting heights, but also assembly fire resistance.
The process of fastener selection starts with understanding the various parts of the screw and how they can differ. The screw designed for attaching panels comes in two forms: bugle head and wafer head. The bugle head facilitates gypsum panel installation by providing sufficient holding capacity while allowing the top of the head to be positioned just below the plane of the panel surface. This allows for a smooth monolithic finish. The wafer type has a larger diameter head, and in some cases has “reamer nibs” on the backside of the head to remove panel material. These were designed for cement board installation where countersinking the fastener is not allowed.
There are two basic thread configurations for fasteners, according to the ASTM standards, that are applicable to the gypsum industry. They are categorized as fine (Type S) or coarse (Type W, Type G, Type A). Each type of fastener will have its own thread geometry. The fine thread is designed for cold-formed steel framing. The Type W is designed with a coarse thread for applying gypsum panels to wood framing. The Type G has a course, high thread for attaching gypsum to gypsum. Type A is for attaching metal plaster bases to wood or steel. There is a high-low thread not directly covered in the gypsum industry ASTM standards. This is a fastener with two thread diameters, one large and one small.
Equally important are the two fastener points (or tips) available, each designed for a specific purpose. The first is called a self-piercing point. The point pierces the steel after which the threads engage the steel. The second is called self-drilling. It bores through the steel before the threads start engagement.
The self-piercing point is defined in ASTM C1002, Standard Specification for Steel Self-Piercing Tapping Screws for Application of Gypsum Panel Products or Metal Plaster Bases to Wood Studs or Steel Studs. This type of point to be used “in fastening gypsum panel products and metal plaster bases to cold-formed steel studs less than 0.033 in. (0.84 mm) in thickness and wood members, and for fastening gypsum panel products to gypsum board.”
The self-drilling screw is defined in ASTM C954, Standard Specification for Steel Drill Screws for the Application of Gypsum Panel Products or Metal Plaster Bases to Steel Studs from 0.033 in. (0.84 mm) to 0.112 in. (2.84 mm) in Thickness. The screw has the “ability to drill its own hole, form or cut mating threads.” Using the wrong point of screw will have one of the following results:
The self-piercing screw will simply not pierce steel that is too thick. The fastener will be damaged.
Self-drilling screws have the tendency for removing excessive material during the drilling process when used in too thin a steel. This will impede the ability of the fastener to thread properly.
Occasionally, the self-drilling screw may appear to be properly installed. The only indication of failure is the screw will continue to spin in the thin steel.
The thickness of the steel framing then dictates which fastener point to use. For steel less than or equal to 33 mil, the self-piercing point should be used. For thicker steel, the self-drilling is the proper point. When using 33 mil steel, the contractor does have a choice of which point to use. The second determining factor is the panel type that to be installed. Cement board manufacturers specify that the steel be no less than 0.0312 inch. This is also a requirement of Tile Council of North America. In this application the head should be the wafer head with reamer nibs.
Standard gypsum panels can be successfully installed on 15 mm steel framing. Steel thickness is a function of the structural demands of a given design including wall heights, fixture loads or architect’s preference. Certain high-performance gypsum panel applications have special requirements as indicated by the following Gypsum Association policy statement:
“The Gypsum Association recommends 30 mil (minimum 0.0296" base metal thickness) steel studs for use with abuse-resistant and impact-resistant (AR/IR) gypsum panels. However, if you choose to install AR/IR panels on 20 EQ studs, with measured minimum thicknesses of 0.019”, then sharp point fine thread or sharp point high-low thread screws shall be used and the screw gun speed reduced to 2500 RPM. The use of drill point screws on 20 EQ studs is not recommended. If experiencing problems with screw spinout on 20 EQ studs, changing screws and/or reducing the speed of the screw gun may help alleviate the issue.”
The contractor must be cognizant of which type of panels are being proposed for installation on what thickness steel. If there are any questions relating to the type of fastener or choice of points, the contractor should follow manufacturer’s or association-written specifications.
Robert Grupe is AWCI’s director of technical services. Send your questions to firstname.lastname@example.org, or call him directly at (703) 538.1611.