Screws are used in countless construction tasks and when the wrong size is selected it can split the wood or negatively impact the strength of the structure. When selecting a screw, there are three fundamental qualities that every tradesperson should keep in mind: gauge, length and material on which the screws can be used.
A common misconception is that a screw’s gauge and threads per inch are the same thing, when in fact the former refers to the diameter of the head and the latter the spacing between each thread. A screw with a gauge of #6 has 18 threads per inch and is 1″ long. When purchasing screws, the numbers on the box will be listed as gauge (head), threads per inch and shaft length in inches.
During the 1800s, Joseph Whitworth developed a standard that would later be adopted as the UNS system. However, during the same time, various metric thread standards were evolving that usually employed 60deg profiles. These systems often developed into national or quasi-national standards that had a lot of intra- and inter-company variation.
Choosing the correct screw for any job is vital, but especially in the case of wood screws, which require a more precise gauge than nails. The type of wood being worked also dictates the type of screw required, with softwoods requiring coarser threading while engineered woods like OSB call for more fine-tuned threads. Drywall screws, for example, are available in coarse- and fine-thread versions to avoid tearing and can have a curve on the junction between the head and shank to prevent drywall from ripping off the screw. screw size chart