One of the most important questions to answer before joining two boards is what kind of fasteners to use. The second most important question is what size of fasteners to use.
In this article, I’ll walk you through the process of choosing the right screw size for a wide range of situations. And, while there is no one-size-fits-all formula, there are some general rules to follow and factors to consider.
Before jumping in, though, a quick answer in case you are looking for an answer to the most common situation. 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.
However, even if that’s the case, I recommend reading through the whole article before you commit to that size.
A Rule of Thumb for Choosing the Right Screw Size
Further down, I’ll get into a more systematic approach to choosing the right screw size for your application. Before that, though, here’s a quick rule of thumb you can refer to.
It says that the anchor length of a fastener should be twice that of the workpiece you are fastening.
Considering that rule and the fact that a finished 2×4 is 1.5 x 3.5 inches, you would need a 4.5 inches long screw to attach it to another piece of wood. Of that, 1.5 inches would go through the 2×4 being attached with the remaining 3 inches would anchor the board into the main piece.
Keep in mind, though, that it’s just a rule of thumb.
After all, if you followed it literally, your screw would be sticking out of your workpiece anytime you tried to join two pieces of the same thickness.
How to Choose the Right Screw Size
The more structural of a joint you are doing, the more important it becomes to pick the right screw. After all, while a picture frame falling off the wall would cause relatively minor damage, a shed you built in your garden falling apart could have more serious consequences.
What Determines Screw Size
There are two things that determine the size of a screw:
- Length: It’s typically denoted in inches and consists of the length of the screw’s shank (the unthreaded part) and of the thread.
- Gauge: It’s typically denoted in a simple number (4 gauge, 5 gauge, etc.) and it determines the thickness of the screw’s thickness.
The longer and the thicker a screw is, the stronger it holds two pieces together. However, you have to be careful not to have a screw that is so long that it sticks out of your workpiece. Or so thick that it splits your board.
Factors to Consider When Picking Wood Screw Size
There are several things that you need to consider when choosing the right wood screw size for your application. There are some criteria that are more important than others, and so I sorted them roughly in the order of importance.
Size of workpieces: Obviously, you cannot have a screw that is longer than the sum of the widths of the two pieces you are joining. If you want to change the ratio of the length of the screw in the two pieces, you can use counter-sink holes. In other words, you can place the screw deeper in your top piece.
Structural importance: You should consider whether your application is cosmetic or structural. If it’s structural, make sure to use stronger screws, or even consider using nails. Also, if it’s a larger job, make sure you are following your local regulations.
Type of load: If your joint will need to be able to withstand a lot of withdrawal load (the two boards pulling apart from each other horizontally), make sure to use longer screws than you normally would. If it needs to be able to withstand lateral load (twisting), make sure to use thicker screws or nails.
Grain orientation: If you are joining two boards face-to-face, you will not need screws as long as when joining screws face-to-end grain.
Type of wood: Hardwoods (walnut, etc.) are more prone to splitting than softwoods (pine, etc.). As such, if you are screwing through a hardwood piece, you will want to either go with a thinner screw or drill a pilot hole through the hardwood board.
What Screw Size to Use for a 2×4
The perfect screw to fasten your 2×4 with will depend on a variety of factors many of which I listed above. However, considering that 2×4 is one of the most common board sizes, I’ll give you some specific advice about what screws to use in selected situations.
Joining 2x4s Face-to-Face
If you are joining two 2x4s face-to-face, I recommend using either 2.5-inch or 3-inch screws.
However, keep in mind that the thickness of the joined piece will be 3 inches. As such, if you decide to use a 3-inch screw, make sure to put it in at a slight angle and not to sink its head too far below the workpiece’s surface.
In fact, unless you require the extra strength that the 0.5-inch of extra length would provide, just go with a 2.5-inch screw.
Joining 2x4s Face-to-End
If you are joining two 2x4s face-to-end, I recommend using 3.5-inch or 4-inch screws.
That’s because to achieve the same strength when joining face-to-end as when joining face-to-face, you need slightly longer screws.
Joining 2x4s Using Pocket Holes
If you are joining two 2x4s using pocket holes, you can use 2.5-inch screws just like you would when joining the boards face-to-face.
For drilling the pocket holes, I recommend using a Kreg jig.
Considering how many different combinations of boards, grain orientations, wood types, and other factors there are, it is hard to recommend a single screw size. Instead, you will have to decide what the best size for your application is.
When doing so, you should not forget to consider what kind of load your joint will need to bear as well as how important it will be structurally.
All of the above said, for many “generic” DIY woodworking projects using 2x4s, using 2.5-inch long gauge 8 or 9 wood screws will work fine in most cases. As for the type of drive, it does not really matter, but generally, I recommend Phillips over slotted.