Lag screws are widely used to attach large pieces of wood together in load-bearing applications, such as decks and barns. Are they the same as lag bolts, though? And where do structural screws, wood screws, and carriage bolts fit into the picture?
What is a Lag Screw?
Lag screws, often incorrectly called lag bolts, are tough screws used to attach pieces of wood together in load-bearing applications. They’re incredibly tough, and only a part of the shaft is threaded. This partial threading allows you to adjust the screw to suit the application. They don’t use a nut and washer, as is the case with carriage bolts, and they have a sharp tip, allowing them to drive deeper into the wood when needed.
Lag screws require a pilot hole since they’re not self-tapping or self-drilling. This is a two-step pilot hole: step one prepares for installing the shaft’s threaded portion, while step 2 makes room for the unthreaded portion. As mentioned in a previous article, hexagonal heads are used when the application requires a high torque value.
Lag Bolt vs. Lag Screw
What are lag bolts? The terms “lag bolt” and “lag screw” are often used interchangeably, and they usually mean the same thing. However, the a screw and bolt are not the same. Technically, “bolt” refers to a fastener with a machine thread and accepts a nut, which should be fastened to secure the bolt.
Screws are fastened by rotating the head, allowing the thread to penetrate in the material it’s installed in. They typically require you to drill two holes: one pilot hole for the thread and larger hole for the top portion of the shaft, which is not threaded and thus require larger diameter hole than the threaded portion.
The lag bolts vs lag screws confusion arises from the fact that generally, bolts are driven by gripping on to the external profile (usually hexagonal) with a wrench, whereas the screws usually have a socket drive on the head.
Lags are fasteners installed by turning the head to thread into the wood, and they don’t use a nut. This means that the proper term is “lag screw,” which could have either a hex or square head. The term “lag bolt” is technically incorrect.
Lag Screws vs. Structural Screws
Structural screws are new to the market and are fast replacing traditional lag screws. These structural screws tend to be stronger than their lag counterparts and can make longer-lasting connections.
Structural screws don’t require a pilot hole, which saves a lot of time during construction and minimizes the number of tools and drill bits you lug along on site. Fastening structural screws is also much faster than fastening lag screws since you can use a cordless drill for this. Lag screws typically require a ratchet or wrench, which is far slower to work with.
While they look flimsy, structural screws are pretty strong – as strong as the meatier-looking lag screws. They’re manufactured from stronger steel than lag screws and heat-treated for maximum strength.
Some structural screw brands are self-drilling, with rippled saw thread near the tip. These cut a path for the remaining screw threads, simplifying installation. There’s also a brand on the market boasting a modified drill bit at the tip. This removes wood shavings as it spins, simplifying and shortening installation times.
Since they’re new to the market, structural screws are not widely available yet. They’re also far more expensive than their lag screw counterparts. To decide which one you want, you’ll have to weigh the time saved during installation and the increased cost of material for each project before deciding which option to take.
Lag screws and bolts, by contrast, are the traditional option when bolting together structural joints. They’re rugged, widely available, relatively inexpensive, and they’ll last. They require a lot of work compared to structural screws, though. Lag screws require two pilot holes: one for the thread and one for the upper portion of the shaft. When opting for bolts, you’ll have to tighten a nut as well, which adds more time to your already lengthy site schedule.
Lag Screw vs. Wood Screw
Self-tapping woodscrews are hailed as the latest alternative for lag screws in load-bearing wooden construction.
Installing lag screws is time-consuming since they require a two-step pilot hole before installation. Self-tapping wood screws, in contrast, don’t require pilot holes at all. These new screws are tipped with specially engineered drill points and optimized thread designs. They even have a reamer included in the design that prepares the wood for the screw shank (a reamer widens drill holes).
Wood screws usually has self-tapping threads that considerably cut down on installation time by eliminating the need for pilot holes. On-site research showed that it cuts as much as half on installation time, amounting to massive savings in labor costs.
Self-tapping wood screws are designed with ductile failure in mind, which is crucial in areas with high seismic activity. This enables them to bear loads comparable to lag screws with a much larger diameter.
So, while self-tapping wood screws are typically more expensive than traditional lag screws, they are stronger and faster to install. The labor cost savings alone would make this swop worthwhile.
Carriage Bolt vs. Lag Screw
Lag screws and carriage bolts are used in construction, and both are typically treated to be corrosion resistant. However, they’re not the same and generally can’t be used interchangeably.
Lag screws (often incorrectly referred to as lag bolts) have a tapered thread, round neck, and hexagonal head. They add structural strength and can be directly screwed into the material without using washers or nuts. They’re not as strong as carriage bolts, though, and could pierce through wood if installed incorrectly. Lag screws are typically used for deck and rafter construction, where they join structural wood together.
What is a Carriage Bolt?
By contrast, carriage bolts, also known as plow bolts, have a cylindrical, domed head and square neck. The square neck functions as a movement check, preventing the bolt from turning after installation, rendering it more secure. They’re used to fasten joints, requiring a pilot hole, washers, and nuts for installation.
Carriage bolts have stronger than lag screws and can have far greater weight-bearing capacity. Since they don’t have pointed tips, they’re unlikely to pierce through wood, even when improperly installed. They’re typically used in rail, swingset, and dock construction, increasing joint strengths for a more robust installation.