When it comes to the ideal size screw to use for a 2×4, there’s a lot to consider. 2x4s – like any other lumber type – can be attached in several different ways such as face-to-face, toenailing, or attaching to end grain.
The right length screw will largely depend on the specific application in question. As a general rule though, you’ll want to use the longest screw possible without punching through the second board.
Let’s take a detailed look at what size screw you should use for a 2×4 – including several common scenarios.
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What Size Screw to Use When Joining 2x4s?
When joining two 2x4s together, the right size screw will vary depending on the application.
Face-to-Face
When joining two 2x4s together face-to-face, you’ll typically want to use 2-1/2” screws.
While you might be tempted to use 3” screws, this will lead to the screw punching through the other end. Despite their name, 2x4s only measure 1-1/2” thick – so two of them placed face-to-face will measure 3” thick. This means that a 3” inch screw driven through two 2xs will pop out the other end.
End-Grain
When joining 2x4s with one board being an end-grain, you’ll want to use 3-1/2” screws.
End-grain doesn’t hold screws nearly as well as cross-grain, so you need significantly longer screws to achieve the same level of fastening. As a rule of thumb, whenever you’re attaching to end-grain you’ll want 1/3 of the screw length in the top and board and 2/3 in the bottom board.
Mounting to Wall Stud
If you’re mounting your 2×4 to a wall stud, then you’ll want to go with at least a 3-1/2” to 4” long screw.
Typical residential drywall measures 1/2″ thick, so a 3-1/2” screw driven through the 2×4 and drywall will embed 1-1/2″ inches into the wall stud. Another good option for this type of application is using lag screws – as these are specifically designed to withstand the shear forces that come with hanging heavy objects from studs.
Other Considerations
There are several other considerations to keep in mind when attaching 2x4s.
Pre-drilling
For most applications, it’s recommended to pre-drill holes before fastening your screws. This will make driving the fastener easier and will reduce the chances of the wood splitting. This is especially important when working with hardwoods like oak or maple as they’re more prone to splitting than softwoods.
Countersinking
Countersinking is another good idea – especially when working with hardwood lumber. Another useful application for this is when you’re fastening boards face-to-face and you want to get a bit more of the screw’s depth into the second board.
Lateral Pressure vs. Withdrawal Pressure
Wood screws require enough pressure in the sub-material to hold the two materials together. Two forces work against the screws and can cause them to fail.
Lateral pressure – also known as shear pressure or shear force – is the sideways sliding pressure of two materials in contact with each other. An easy way to picture this is the above example of mounting a heavy object to a wall stud. The weight of the object is pulling downward, trying to shear the fastener in half.
In instances with heavy lateral pressure, a thicker fastener – like a lag screw – will better resist these shear forces. Another option is adding additional fasteners to increase the overall strength.
Withdrawal pressure is the force pulling two connected materials away from each other. Think of a heavy lighting fixture mounted to a ceiling. To compensate for higher withdrawal pressure, you’ll want to use a longer fastener with more threads buried into the sub-material. Unlike lateral pressure, thicker fasteners won’t help to resist withdrawal pressure.
Type of Screws for 2x4s
The best type of screw to use for 2x4s will depend on the specific application. For indoor use, you can use less expensive zinc screws. Outdoor jobs will require screws with corrosion resistance like stainless steel or epoxy-coated deck screws.
As far as screw diameter is concerned, screws are sold with gauges between #2 and #16. For general construction, #8 is the most common, but if you want a screw with more strength go for a beefier #12-16 screw.
