Metal buildings are widely used in various industries, from warehouses and factories to retail stores and offices. One of the essential components in constructing these buildings is the humble screw, and the length of the screws is an important consideration.
Length essential when choosing screws for a metal building
Screws literally hold the components of the building together and the wrong length screw can result in a bad connection. In this article, we will discuss how to select the proper length self-tapping or self-drilling screws commonly used in the metal building industry.
The important length to know, the one that really holds that connection together, is called the bearing length.
Before we delve into the specifics of selecting the proper screw length, it’s important to understand the basics. There are two main types of screws used in the metal building industry – self-tapping screws and self-drilling screws. Self-tapping screws have a sharp point and are used to tap a threaded hole into the material. Self-tapping screws are usually used in thin-gauge metal. Self-drilling screws, on the other hand, essentially have a drill bit at the tip that allows them to drill and tap thicker gauge metals without the need for pre-drilling.
The head type of the screw is chosen based on the application and the aesthetic requirements of the project. The most common head types are hex washer head, pancake head, pan head and flat head. Sometimes, such as with a hex washer head screw, the length is measured from under the head to the point of the screw. Other times, such as for a flat head screw that is meant to be countersunk, the length is measured from the top of the head to the point.
With several different head styles and two different point styles, it is important to note that the length of a screw used in identifying the screw is not the effective length that is going to hold a connection together. Sharp point, self-tapping screws start with a thread that is very small and tapers up to full size, while self-drilling screws have a drill point where the thread does not start right away. The important length to know, the one that really holds that connection together, is called the bearing length. The bearing length of a screw accounts for where the first full size thread on the screw starts. Manufacturers will typically provide the minimum bearing length for each screw length to help determine the correct length for your application.
The most important thing to consider when selecting the proper fastener length is the total length of all the materials being connected. If insulation is being used between metal panels and the structure, the length of the screw will need to be longer to ensure that it can penetrate the insulation and still provide a secure hold in the substrate. The screw should be long enough to penetrate all the material and protrude at least 3 threads beyond the structural substrate. This ensures that the screw provides sufficient holding power without compromising the strength of the materials.
If the materials are uneven thicknesses, the screw length should be chosen based on the thickness of the thickest materials. Also, if a washer is used, the thickness of that washer has to be accounted for. The total length of everything that is being connected by the screw is called the attachment length.
Finally, simply choose a fastener with a minimum bearing length that is longer than the attachment length. This ensures you have the correct length fastener, resulting in a good connection.
Read and follow all the manufacturer’s recommendations when selecting the proper screw length.
Josh Krohn is engineering services manager at Triangle Fastener Corp., Pittsburgh. To learn more, visit www.trianglefastener.com or call (800) 486-1832.