I probably should have done more research on screws before working on my first outdoor furniture project. I had no idea how overwhelming it could be to choose a screw. I had no idea there were so many options! The hardware store has a bazillion types of screws! I was relieved to notice they were categorized by material: drywall screws, interior, exterior, multipurpose, etc. Perfect. I think I want exterior for outdoor furniture… or maybe multipurpose. But I quickly saw that within those categories, there’s a bazillion more sub-categories. And then, what length or size do I want? What is self-tapping and do I need that? Does thread count matter? In this article we’ll break all that down so you will be able to choose the best screw for your woodworking project.
the tale of the square headed screw
There are many drive styles (60, actually) but for wood working you really just want to focus on two of them. You might be asking “What’s a drive style?”. This is the pattern recessed in the head of the screw.
One you may be familiar with but, you don’t want to use for building wood furniture (more on that later) is the Phillips screw. The one you do want to know about is a square drive is called a Robertson screw, named after it’s inventor. And it has a really cool history. Check this out: Robertson, a Canadian, invented this screw in 1908. Even before the more popular Phillips head was invented (that was developed in the 1930s). But it wasn’t until the 1970s that the Robertson screw migrated to the US. Apparently, what happened was Henry Ford wanted exclusive rights to this square headed screw and Robertson said ‘No way, eh!’. And so it gained popularity in Canada but was virtually unheard of in the US.
But the American woodworking and furniture builders of the 1970s came to realize the benefits of the Robertson and popularity of this screw spread amongst that community. Frustration with the popular Phillips screw was mounting and helped lead to a major shift to the Robertson screw. If you’ve ever worked with a Phillips head, you know that the darn thing keeps getting stripped. Meaning that your bit slips and the shape of the drive gets damaged and before you know it the screw is impossible to drive in or reverse out. I think they were actually designed to strip out so that in assembly line building, the screw wasn’t forced too far into it’s material.
What makes the Robertson screw so great?
The Robertson screw is perfect when you need a lot of torque. Now, if you’re new to woodworking, you’re probably thinking, “when do I need a lot of torque?”. Okay, let’s look at an example. Let’s say I’m using a 3″ long screw to connect some pretty solid 2×4 boards. I’ve pre-drilled holes but it might still take a lot of force to get that long of a screw all the way in and flush. You need torque to drive it home.
How does a square head give you more torque? Well, the square shape allows a better “hand in glove” engagement between the drill bit and the square screw head. It’s a more solid connection that is less likely to slip. I recently was introduced to this screw head when I bought my pocket hole jig that came with a square drill bit. OMG, it is all that it claims to be! I was driving in screws like butter!
The other benefit of this drive style is that it alleviates the need to apply a lot of pressure to “push” the screw in. Because of that ‘hand in glove’ connection, your screw will sink in without need to apply the pressure you would with other screws. For me this makes it my all time favorite.
The other drive type you should get to know
Prior to getting to know and falling head over heels for the Robertson, I really liked the Torx drive type. This screw was developed in the late 1960s and is very common amongst woodworkers. It has a unique star shaped head with six points.
Like the Robertson, this screw is also not likely to slip or strip out. That’s because of that six-point connection to create a good grip between bit and screw. It’s way more common and far easier to find on hardware shelves in the US. But I do find that you need to use force or apply much pressure to sink a longer screw with the Torx head.
Selecting the right length, thread count, and head shape
Your most important decision is choosing the best lengths of screw for your project. Most woodworkers would tell you that your screw should be long enough to reach halfway through the thickness of the bottom board. So if your top board is a 1×4 (or 3/4″ actual thickness) and your bottom board is a 2×4 (or 1-1/2″ actual thickness), then you want your screw to be at least 1-1/2″ long. Think through all your connections to get the right mix of screws.
Width of the screw is designated by a number, #0 to #20. I don’t really pay much attention to these. When you are shopping for screws for wood projects and suitable for outdoor use, the most common size is #8. If you were to purchase a #6, 8, or 10 – you’ll be just fine. And you’ll be hard pressed to find anything much skinnier or fatter in the exterior wood screw section anyway.
Finally, the last thing to consider is the shape of the screw head. There are many types of head styles, but unless you are trying to highlight the hardware versus conceal it, you’ll likely choose a flat head screw. This is a tapered, flat head that sinks into the wood and sits flush with the face of the board you’re sinking it into.
Entering the world of DIY can certainly have its challenges. Who would have thought that deciding which screws to buy would be one of them?! Fortunately, getting over this hurdle is easy. Once you narrow the choices down based on preference and project needs, you’re off and building! What’s your favorite in the hardware aisle? Drop a comment and let us know!