Lag Bolts vs. Structural Screws: Which One Is Best?

What Are Lag Screws?

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Also called lag bolts, these old workhorses are easily identifiable by their large, hexagonal heads. They’re typically larger than most wood screws and fasteners because they’re designed to have lots of holding power, and they’re commonly used for connecting lumber to other heavy materials, like masonry.

• Lag bolts require you to predrill two holes: one for the threads and a larger clearance hole for the shaft.
• Structurally rated lag bolts are available, but most home centers carry generic versions of varying quality.
• All home centers carry a large assortment of lag bolts in different sizes.
• Lag bolt screws cost a third of the price of structural screws.

What Are Structural Screws?

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Sometimes called construction screws, these are fastening solutions made from hardened steel and heat-treated for maximum strength that is longer and skinnier than lag bolts. They’re especially popular in the trades because of how quickly you can drive these screws, shaving time off the building process.

• Structural screws are so thin and sharp that you can skip any predrilling and get right to driving.
• Structural screws meet stringent engineering standards. Hardened, high-quality, heat-treated steel means virtually no chance of shear-offs.
• Home centers carry the most commonly used structural screw styles and sizes, but usually only one brand.
• Structural screws aren’t cheap: they generally cost several times more than an equivalent lag screw.
• You’ll need a Torx or Spider drive bit for structural screws.

The labor difference between structural screws and lag screws is so huge that by the time you finish drilling pilot holes and ratcheting in just a few lags, you could have finished the whole job with structural screws and be sipping a cold one.

Structural fasteners are made from stronger steel and are heat-treated for maximum strength. So even though they look less “beefy,” they’re actually one-to-one replacements for their fatter cousins. One brand has a modified drill bit embedded into the screw point, so the screw literally removes wood as it spins. A few brands have rippled screw threads near the tip that saw a path for the remaining screw threads.

Structural screws also feature Torx (six contact points) or Spider (eight contact points) drive heads in addition to traditional hex-head styles. The additional contact points spread the driving torque and prevent the kind of “cam-out” you get with Phillips or square-head drive screws. The straight-sided Torx and Spider bits also hold the screw in place so you can “aim” and drill with one hand. You can’t do that with a Phillips head or a hex-head lag.

What Are Structural Screws Used For?

Among the most common applications are decking, framing, ledger boards, etc. Like lag bolts, they’re made to handle lots of force and hold heavy loads.

Downsides of Structural Screws

There are only two downsides to structural screws: cost and availability. GRK brand screws are the most expensive and are only available from professional lumber yards. Spax and FastenMaster brands are sold by home centers, but not all stores carry the complete line.

Which Is Better, Structural Screws or Lag Bolts?

The editors here at Family Handyman use a lot of structural screws on our projects. While they are more expensive than lag bolts, their ease of use and time saved give them an edge. You’ll have to make sure to choose the right kind depending on the application, e.g. attaching ledger boards to concrete or masonry.

If you’re building or upgrading a deck and your local building inspector isn’t familiar with structural screws, call the “800” number of the screw manufacturer, whose engineers will satisfy the inspector’s concerns by providing all the testing and approval data to support their use in place of lags.