Structural Screws vs. Lag Screws

Are these new, wimpy-looking structural screws really as good as the old-fashioned, meatier lag screws?

High-strength structural screws are beginning to displace traditional lag screws as the fastener of choice for structural applications. Here's how they compare.

By the DIY experts of The Family Handyman Magazine

Features, performance and price

Structural screws (also called “construction” screws) are stronger than lags and make longer-lasting connections. You can just zip them in with any 18-volt drill (no pilot hole required). The labor difference 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.

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

Installing a 5/16-in. screw with a battery-powered drill and no pilot hole is quite a claim. So I tested 5-in.-long screws from all three manufacturers (GRK, Spax and FastenMaster) to see if they really delivered. The GRK screw went in faster than the other two, but they all lived up to their billing. They really did zip right in. 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.

Structural Screw

These relatively new fastening solutions make quick work of just about any connection that relies on beefy screws.

  • 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.

Structural screw
Lag Screw

These old workhorses are labor intensive but readily available.

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

Lag screw
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Required Tools for this Project

Have the necessary tools for this DIY project lined up before you start—you’ll save time and frustration.

  • Cordless drill

You'll need a Torx or Spider drive bit for structural screws