What size screws do I need?
Screw size is something that should be taken into consideration after you have thought through most of the project. The project design, environment, materials, and proper size of the boards/panels have been decided, and now you want to make it all come alive.
Picking the right screw size is kind of like choosing the field for a match. You can iterate on many things, but the field has to remain more or less the same, irrespective of everything else. The two most important factors that lay this field are the screw length and screw diameter.
Screw Length and Screw Diameter
As per the general rule, the length of the screw should at least be half of the thickness of the bottom material. For instance, if you're drilling into a 2 x 4 wood panel, the length of the screw should be equal to or more than 3/4 inches. The deeper the screw, the better volume of wood fibers held between the threaded and unthreaded shank.
The other factor is the screw diameter or gauge. The gauge number must increase as the load increases. Size 6 (usually 3.5mm diameters) gauge can be great for residential and DIY projects but for heavier projects, you may need more than #10 or #12-grade screws. Their minimum screw head diameter is 4.8mm.
Now that we have the broad size decided in questions let's tune our choices better to fit the project and its life better. The following project and tool characteristics must be kept in mind.
Environment
This is perhaps the biggest question. For indoor applications, you can use zinc screws or brass screws for visual appeal. However, for outdoor projects, the right screws must have corrosion resistance and tensile strength, such as stainless steel screws or any other screw material that can withstand moisture and high sheer forces due to wind, etc.
Screw Head Types
Yes, screw head types greatly impact the ability of the screw to be fastened tightly. The better the surface area available for the screw drills to transfer the torque, the more secure the screw along the shank and thread. Flat head and phillips head screws are the basic ones with single or four touch points. They may not create a proper countersunk hole with a while tightening with a hand or a simple drill and may compromise stability. For screws with coarse threads, it's important to keep this in mind. If you're using steel wood screws, stainless steel wood screws, or oval head screws, it is recommended to have Torx or a star drive to maximize the transfer of drill torque.
Thread Pitch
Fine threads along the entire length provide more threads per inch as compared to coarse threads. The small distance between the threads allows evenly distributed forces and offers better resistance to vibration and shear. However, in some large-scale projects where speeds are of the essence, you may use course threads since they turn better. Also, keep in mind to check the thread pitch. A 60-degree pitch typically aids self-locking especially when using wood screws to join wall studs.
Choosing the appropriate screw size for your project is absolutely critical. Therefore, it's imperative to consult experts if you're unsure. It's crucial to consider all the relevant factors to ensure that you select the right screw size. Using screws that are too small or too large can lead to weak connections, instability, or other serious problems. If you're unsure, it's better to err on the side of caution and opt for slightly longer screws, which can be trimmed or filed down if necessary, rather than using screws that are too short.
