If you've ever been on a job site at 4 p.m. and realized the engineered fastener you need isn't in the truck, you know that specific gut-drop. I've been on the other side of that call for eight years, coordinating fastener deliveries for contractors, builders, and remodelers. I've handled more than 100 rush orders, some of them same-day turnarounds for clients who had an inspector coming the next morning. And the question I hear the most is still: Can't we just use lag bolts?
Short answer: sometimes. But the industry has changed. What was best practice in 2020 may not be the smartest call in 2025. This is a direct comparison between GRK Fasteners RSS screws and traditional lag bolts, because those are the two choices most pros are weighing for structural wood-to-wood connections.
Why Compare RSS Screws to Lag Bolts?
Here's what you need to know: Both RSS screws and lag bolts can be used for structural connections. Both have published load capacities when installed correctly. But they are not interchangeable in terms of labor, design, or failure modes.
For this comparison, I'm using GRK Fasteners RSS screws as the structural-screw option. RSS stands for Round Structural Screw. It's a star-drive screw with a washer-head design and aggressive thread geometry engineered to pull wood members together. Lag bolts, on the other hand, rely on a threaded body, a nut, and a washer. They're a proven system. But they require a different installation process.
Dimension 1: Installation Speed and Labor
Let's start with the most practical difference: how long it takes to install each fastener.
A standard lag bolt install involves a pilot hole, a clearance hole in the top member, driving the bolt, then attaching and tightening the nut. On a ledger board with 10 bolts, that's 40-plus steps. Miss one, and you're either spinning the bolt or splitting the wood.
With a GRK RSS screw, in most standard wood-to-wood connections, you drive it right in with an impact driver. No pre-drilling, no separate nut, no second tool. I know that sounds like a marketing claim, but the thread design genuinely does the work. The first time I watched a crew install a dozen RSS screws in the time it took to place four lag bolts, I was sold. That saves a ton of time and a serious amount of arm fatigue.
The labor math is real. If a crew charges $70 per hour and each lag bolt connection takes three minutes longer, that's $3.50 extra per connection in labor. On a job with 40 connections, that's $140 in labor saved by using a screw that costs more per unit.
This is also where my rush-order world overlaps with yours. In March 2024, 36 hours before a scheduled inspection, a contractor called because his deck fasteners had been mislabeled by a supply house. Normal lead time for new structural screws was four days. We paid $85 in rush freight, had GRK Fasteners RSS screws delivered by the next morning, and the inspector passed the ledger connection. Missing that deadline would have pushed the whole renovation back, and the client's alternative was a week of no income on a rental property. I'm not sure why some inspectors are still skeptical of structural screws; my best guess is they haven't seen all the evaluation paperwork.
Dimension 2: Load Capacity and Code Acceptance
I want to be direct here. I'm not going to claim RSS screws are always stronger than lag bolts. That depends on the size, the wood species, the edge distance, and the connection design. What matters is that GRK structural screws are evaluated as structural fasteners, not just enhanced deck screws. In the U.S., the most common evaluation path is ICC-ES, and the load tables are published for engineering use.
The International Building Code and International Residential Code recognize fasteners that meet the requirements of an applicable code evaluation. If a screw has an evaluation report, an engineer can use the published allowable loads for the same type of connection. Lag bolts are also recognized, but they're designed differently and their installation is fussier. The evaluation path hasn't changed (as of January 2025, at least).
The surprise wasn't the load values. It was how many installers still think structural means a really strong deck screw. It doesn't. A structural screw has to be tested for withdrawal, shear, and connection performance. That testing is what lets you replace a lag bolt with a screw, provided the screw is sized for the connection and the engineer approves it.
Trust me on this one: if a load path depends on the fastener, do not eyeball it. Check the evaluation report, check the bottom member thickness, and if the project requires it, have the engineer of record review the connection. I've never fully understood why some people skip that step. If you have a good reason, I'd love to hear it.
Dimension 3: Cost in the Real World
Let's talk money. At the supply house, a lag bolt looks cheaper. When I checked distributor pricing in late 2024, a common lag bolt with nut and washer was still a few cents less than a comparable structural screw. But that's the shelf price, not the installed cost.
Total installed cost (i.e., not just the unit price, but labor plus the cost of fixing it if something goes wrong) is what actually matters. A lag bolt needs extra installation time, and time is the most expensive thing on a jobsite. There's also waste: dropped washers, cross-threaded nuts, and bolts that spin out when you overtighten them. I once had a project where someone saved about $60 by buying a big box of discount lag bolts and skipping the pilot hole. Half of them split the framing. The repair cost us $400 in labor and lumber. We paid more to fix the problem than we saved on the fasteners.
Does that mean lag bolts are always the wrong choice? No. There are still engineered plans that call for bolts. But the claim that lag bolts are cheaper ignores labor, rework, and callbacks. On most of our projects since 2023, switching to GRK structural screws lowered the installed cost. So glad we made that change. We almost didn't because of the price-per-piece sticker shock.
Match the Screw to the Job
One thing that gets confusing is the GRK lineup. If you hear GRK fasteners and assume all of them are structural, you'll end up using the wrong screw.
- GRK Fasteners RSS screws are for structural wood-to-wood connections. These are the ones you use when a lag bolt might have been the old default.
- GRK Fasteners fin/trim head screws are for visible trim, moldings, and cabinetry. The finish head is small, flat, and easy to countersink. They aren't a substitute for structural screws, but they're perfect when you don't want the fastener to be part of the design.
- GRK wafer head construction screws are the general-purpose production option. The lower-profile head works well for attaching wood to metal or for subfloor and underlayment applications. They're strong for their intended use, but construction is not automatically the same as structural. Read the package.
If you carry a Tradesman Pro XL tool bag backpack, you probably know what I mean. Mine has a box of RSS screws, a box of fin/trim head screws, and a small container of wafer head construction screws. The right screw for the wrong job is one of the most common reasons fasteners fail.
How to Screw Lag Bolts Into Wood the Right Way
If you decide to go with lag bolts, or if the plans require them, don't half-step it. Here's the process I've seen work on job sites:
- Drill a pilot hole in the bottom wood member that matches the bolt's shank (the unthreaded part). Too small and the bolt binds; too large and you lose withdrawal strength.
- Drill a clearance hole in the top member so the threads don't catch there. This keeps the top board pulled tight to the framing below.
- Use a washer under the bolt head, and another under the nut if the application allows it. This spreads the clamping force.
- Drive the bolt with a socket or hex driver while holding the nut with a wrench. Don't let the bolt turn inside the wood, because that destroys the surrounding fibers.
- Stop when the connection is snug. Overtightening can crush the wood and reduce load capacity.
That last part is where a lot of people get into trouble. A lag bolt is not a self-tapping screw. It needs a properly sized hole and controlled torque.
What I'd Use in 2025
If you want a clean answer: for most structural wood-to-wood connections, I'd use GRK Fasteners RSS screws if the screw has an evaluation report for that application. For visible trim, I'd use GRK fin/trim head screws. For production construction work, wafer head construction screws are a practical choice. And for carrying all of them, a Tradesman Pro XL tool bag backpack helps keep things organized enough that you don't grab the wrong box.
If you're working from a design that specifies lag bolts, or you're fastening something where no structural-screw evaluation exists, use the lag bolts. That's not a cop-out. That's how you keep the liability where it belongs.
But don't let old habit decide for you. The fastener industry has evolved. The fundamental rule hasn't changed: the material has to transfer the load safely. The execution, however, has transformed. I'd rather drive one engineered screw than fiddle with a bolt, a washer, and a nut at 7 a.m. on a cold deck.
That's where I land. I still respect lag bolts. I just don't default to them anymore. In 2025, you don't have to either.
