What size of screw should I use for 2×4?

If you are looking to join 2x4s, the best screw size to use in most cases is 2.5 inches long, gauge 8 or 9.

Don’t Use Screws That Are TOO LONG or TOO SHORT! (Screw Length/How To Choose The Right Screw Length)

How to Choose the Correct Size Wood Screws

No matter whether inserting screws into end grain or cross-grained boards, many experts and skilled do-it-yourselfers always opt to drill pilot holes with a special countersink pilot bit. This type of bit drills a small-diameter hole into the length of the screw shaft, with a wider counter-sink portion that will allow the head to be sunk flush with the surface of the wood. Featured Video

As a general guideline, you can use #8 diameter deck-type screws for most utility tasks. Use 1-1/4 inch screws for basic tasks where strength is less of an issue or when attaching two 3/4-inch-thick boards across the grain. Keep some 2 1/2-inch deck screws for screwing into end grain, plus some stronger 3-inch screws for attaching into end-grained boards, or when a little extra added power is needed. Finally, if there will be a lot of sheer strength needed, you can opt for some 1/4-inch diameter lag screws.

The number and length of screws that are used will depend on the grain orientation of the receiving board. As a general rule, screws needed for attaching two boards across the grain do not need to be as long as when screws are inserted into end grain. When screwing into the end grain, aim for at least two inches of thread in the board.

Steel or stainless steel wood screws have thicker bodies and are typically used for precision woodworking and indoor furniture. When you buy wood screws, they are identified both by their length and also by their gauge, which refers to the thickness of the screw shaft. Larger gauge numbers indicate screws with thicker shafts. While utility screws are typically #8 or #10 gauge, steel wood screws are available in a much wider selection of gauge diameters.

The correct size, length, and gauge of wood screws can be difficult to determine when building woodworking projects. Adding other factors such as the type of wood, screw material, lateral vs. sheer forces, and the presence of pilot holes only complicates the issue. Fortunately, there are some general guidelines you can follow that will help you purchase and use the correct type of wood screw for the job.

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