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How to choose the correct screw length
Fasteners come in all shapes and sizes, and it’s a contractor’s responsibility to identify the optimal screw length for the specific job. The following guide will help you determine the correct length based on the type of connection and depth of the material
Screw length/nominal length. Begins under the head and extends to the tip of the screw.
Pilot. The section at the tip of a metal screw that cuts through the substrate.
Shank. Unthreaded section of the screw between the base of the head and the start of the threads.
Thread length. The length of the threads on the screw.
Fully threaded screw. A fastener with threads that extend to the base of the head; no shank.
Major diameter. Diameter of the fastener to the end of the thread.
Minor diameter. Diameter of the inner shaft of screw, excluding the threads.
Thread pitch. The distance between two threads along the length of the fastener.
Fully developed thread. A thread that extends to its full size from the shaft of the screw.
Figure 1: Image displays fastener key termsSFS data sheets include specifications on two characteristics that help determine appropriate fastener length. Load bearing length is used for metal to metal connections and minimum embedment is used for metal to wood connections. Details of how these terms apply to the proper selection of fastener length are explored in the following section.
How to choose optimal screw length
How a screw is labeled
Before making your selection, it is important to understand how manufacturers label fasteners. In the United States, fasteners are typically categorized with two numbers and a nominal length. For example, SFS offers a #9-15 x 1” Woodrip fastener. The “#9” denotes the major diameter of the screw. The second number, “ - 15”, denotes the number of threads per inch (the inverse of the thread pitch). The third number denotes the nominal length. Take note, the nominal length of the screw does not include the head.
Choosing screw length for metal to metal
The depth and material into which the screw will be embedded determine the proper nominal fastener length. For connections that are metal to metal, the maximum load bearing length is used. This is the thickest amount of material that should be fastened together by any particular screw.There needs to be a minimum of three threads beyond the lower material to ensure that the application is secured by the fully threaded portion of the fastener (Figure 2). For fasteners with an unthreaded shank, three threads from the head side and three threads from the tip side are excluded in the safe region (Figure 3).
Figure 2: Image displays the maximum load bearing length for a fully threaded fastener with a pilot. Note that the first few fully formed threads are not included as discussed above.
Figure 3: Image displays the minimum and maximum load bearing length for a metal to metal fastener with a pilot and a shank. Note that the first few threads on either end of the thread length are not included as discussed above.
Choosing screw length for metal to wood
For metal to wood applications, there is a minimum embedment required for the screw to perform to its listed characteristics. When the fastener is fully seated against the upper substrate (metal panel), there should be a minimum of 3 fully developed threads into the lower substrate (2x4 or stud). This is essentially the opposite of maximum load bearing length. Here, at least 3 threads are installed into the material, whereas for metal to metal (load bearing length), at least 3 threads must be installed through the material.
One potential application is a thin gauge metal panel into a 2x4. In this case, the head of the fastener will be against the panel, and the rest of the fastener will be fully embedded into the wood substrate. When a fully threaded screw with 14 threads per inch is used, the fastener must be embedded at least 3 fully formed threads into the wood material. This value is around 0.43” when considering the tapering of the threads at the tip and the fastener point. So, if a 26 gauge carbon steel panel and a fully threaded screw with 14 threads per inch are used, a 0.5” (0.43”+0.01875”) long fastener would be the best choice. For this specific application, any fastener over 1/2" will meet the minimum length to be fully embedded into the wood substrate. Figure 2 will help visualize minimum embedment.
Figure 4: Image displaying an example of a metal panel being attached at a rib. The minimum embedment for this metal to wood fastener is shown with the green box. Note this is the minimum depth at which the screw is required to be embedded in the wood substrate.
Risks of Improper Fastener Selection
If the screw used in a specific application is not the correct length, the connection will not perform to its listed specifications.
Fasteners that are too short and do not have 3 fully formed threads in the proper position in the substrate are more likely to strip out of the material if there is an external force applied. In contrast, if the selected fastener is longer than necessary, the materials beyond the substrate are at risk of being damaged by the screw. Adhering to the load bearing length and embedment guidelines will ensure that the fastener performs as best as possible in the substrate being used.
For further guidance on choosing screws, read Fine Thread vs. Coarse Thread.
Summary
There are many factors to consider when selecting the proper fastener for a specific application. It is important to determine the correct application thickness of the job, as it will serve as the guideline when choosing the appropriate screw length. For metal to metal applications, the minimum and maximum load bearing length must be considered. For metal to wood connections, the minimum embedment into the substrate must be followed.
