When you’re doing home improvement or construction, the right screw size can make or break a project. But if you’re not familiar with the many variations in screw heads, shanks and points, it can be challenging to choose the perfect fastener for the job. This guide is intended to cover all of the intricacies and specs you’ll see on a screw chart, so you can be confident that you’re selecting the best self-tapping screws for your needs.
Screws are generally categorized by their head type, head size, diameter (or gauge) and threads per inch (TPI). The number of threads on the screw’s shank determines how quickly it works through different materials, with coarser threads working more effectively in softwoods than finer ones in metals or hardwoods. Screws that are designed for general construction or woodworking are usually labeled with a one- or two-digit number, and are also often marked with a fraction.
Choosing the correct length for your screws is another important factor in their effectiveness. For wood construction, the general rule is to select a length that will allow the tip of the screw to penetrate the bottom piece of lumber by at least half its thickness. This will prevent the screw from pulling out as it is tightened, and will also ensure that the screw will be fully threaded into the next piece of lumber above it, allowing for maximum holding power.
Screw heads are also available in a wide variety of shapes and styles, depending on the purpose for which they’re intended. For instance, gutter screws feature a hex-shaped head and #10 shanks, while wood screws are typically round heads with a hex drive for easy hand tightening. Screws that are designed for heavier applications, like roofing, feature a hex washer to provide extra strength and stability.
Finally, the point of a screw determines whether or not it’s a self-piercing or non-self-piercing screw. Self-piercing screws have sharp points that are designed to self-pierce and work through light sheet metal and wood. These types of screws don’t need to be pre-drilled before use, but they should be fully inserted into the material to avoid snapping off.
In addition to these basic specifications, screws are sometimes further classified based on their coating or material. For example, exterior wood screws can be coated with a zinc finish to resist rusting when exposed to the elements. Screws that are designed for use in pressure-treated wood can be coated with Alkaline Copper Quaternary, or ACQ, to ensure they stand up to the chemicals used to treat the wood. Similarly, stainless steel screws offer increased corrosion resistance over common iron-based alternatives. #10 screw diameter
