Screws come in a wide array of types and sizes, designed for use on a host of construction tasks from woodworking to metal roof installations. Using the right screw for a task ensures a strong hold, prevents damage to the material and reduces the number of tools needed to complete the job. Choosing the proper length of screw is also critical. Screws are sized according to three essential measurements: gauge, threads per inch (TPI) and length.
Gauge is the diameter of the threaded part of a screw, which is listed as the first number in a screw size. A screw labeled “#6″ has a diameter of 6 / 32”. Screws with larger diameters have smaller numbers and vice versa. Generally, screws have a threaded portion that extends about half the length of the shaft unless they’re flat head.
Threads are measured in a system called the Unified Thread Standard (UTS) or the Imperial System, which uses inches of measurement. This system defines different types of threads, such as coarse (UNC) and fine thread (UNF). The first number in a screw size indicates the diameter of the coarse thread; the second number is the number of thread peaks in one inch of a screw’s length.
The length of a screw determines how deep it will anchor into the material. The ideal screw should enter at least half the thickness of the attached material to ensure a strong and secure hold. Choosing a screw that’s too long can damage the opposite side of the material, while a screw that’s too short won’t anchor properly and may even slip out or break off during the installation process. 1/4 to mm