Timber Screws vs Lag Screws: Which Should You Use?

Ask three deck builders about lag bolts vs timber screws and you'll get three answers. One says Simpson SDWS for everything structural. Another says lag bolts have worked for 50 years. A third says it depends on the job. We've spent 28 years building decks, fences, docks, and timber structures. We're going to settle this.

For most code-rated structural connections, timber screws have replaced lag bolts. Simpson SDWS, FastenMaster TimberLok, and GRK RSS install faster, hold equal or higher load, and carry code listings. Lag bolts still make sense for TV mounts, hammocks, decorative work, and very dense timber.

How Pros Choose: Timber Screw or Lag Bolt?

If you're not sure whether to use a timber screw or a lag bolt, here's how we choose:

• Deck ledger to band rim: timber screw (Simpson SDWS Timber)
• Beam to post: timber screw (Simpson SDWS or GRK RSS)
• Pergola structural joint: timber screw (Simpson SDWS)
• TV mount to studs: lag bolt (5/16" x 3" zinc-plated)
• Hammock or swing from a beam: eye lag (1/2" x 4" stainless)
• Fence post bracket: lag bolt (3/8" x 3" HDG)
• Joist hanger: neither, use Simpson SDS Connector Screw
• One-off decorative restoration: lag bolt (square head, period-correct)

What's the Difference Between a Timber Screw and a Lag Screw?

First, clear up the question that trips up most people. A lag screw and a lag bolt are the same fastener with two names. Hex head, partial wood thread, gimlet point, ASTM A307 Grade A spec. Trade shops, lumber yards, and the IRC use both names interchangeably. The hex head makes people think "bolt," but it threads into wood like a screw, so "lag screw" is the more accurate term. Either way, it's one product, not two.

A timber screw is a different product entirely. Bold full-length wood threads, large washer head or hex-washer head, sawtooth or zip-style self-drilling tip. The shank is thinner than a comparable lag, and that thinness throws people off. A 0.220" SDWS replaces a 1/2" lag in many connections because the strength comes from heat-treated steel and thread geometry, not raw diameter.

Here's the difference between a timber screw and a lag screw when it comes to their appearance: a lag has a raised hex head and a smooth shank near the head. A timber screw has a flat washer head with under-head nibs and threads running almost the full length.

What Is a Timber Screw and Why Did Pros Switch to Them?

A timber screw is a structural wood screw built for heavy framing and outdoor construction. Self-drilling tip, bold thread, low-profile or hex head, with real shear and pull-out numbers the manufacturer publishes and an inspector will accept.

Simpson Strong-Tie started the category about twenty years ago with the SDS Strong-Drive screw, the first wood screw that could replace bolts without pre-drilling. FastenMaster, GRK, and SPAX followed. By the mid-2010s, every major deck builder we know was ordering structural screws in bulk.

We talked to a Massachusetts builder who switched after a single 80-foot retaining wall. He'd grabbed LedgerLok samples at a trade show, used them in place of spikes, and committed to never going back. Once you do one job with timber screws, you stop reaching for lags. That's the pattern we hear over and over. When framers who've already switched get asked why, speed is the answer that comes up first, not strength and not price. Price actually comes up last.

The engineering reason is simple. A timber screw has a Type 17 or sawtooth point that starts like a drill bit, plus an upper minor thread that bores out the clearance hole as the screw drives. The screw does both pre-drill operations in real time. That's why the lag bolt's drill steps disappear.

Can You Drive Timber Screws with a Cordless Drill?

After almost 30 years of building decks and timber structures, we've personally ruined a cordless drill trying to drive timber screws. The lesson: you need a half-inch impact driver, not a cordless drill driver. A regular drill stalls out and burns up trying to spin that much screw through dense wood. The impact driver hammers the screw home in short bursts so it doesn't strip out.

For more on structural screws for timber framing, see our post on the right wood screw fixings for timber structures.

What Is a Lag Screw and When Do People Still Use Them?

A lag screw is the traditional hex-head wood-thread fastener. Coarse partial thread, gimlet point, smooth shank near the head, ASTM A307 Grade A, made in diameters from 1/4" up to 3/4" and lengths up to 12" or so.

Lag screws need pre-drilling. Always. Two pilots: a clearance hole through the side member at shank diameter, and a smaller lead hole into the main member at roughly 60 to 75% of shank diameter for softwood. They also need a flat washer under the head. The IRC requires a 1 3/8" washer for a 1/2" lag. Skip the washer and the head sinks under torque, which kills the connection's design value.

Where lag bolts still make sense: TV mounts, hammock and swing eye-bolts, fence post brackets, decorative or period-correct work, and very dense timber. In every one of those cases the hardware or the wood is deciding, not the fastener strength. We get into the specifics below.

How Do Timber Screws and Lag Bolts Actually Compare?

The install-speed gap is what most pros notice first. FastenMaster's own demo timed a LedgerLok at 3 seconds and a 1/2" lag at 26 seconds. Real-world contractor pace is closer to 15 to 25 seconds for the structural screw versus 60 to 90 seconds for the lag. Still a 3x to 6x advantage.

We know a deck contractor in the Pacific Northwest who pulled 40 lag bolts from a three-year-old ledger. Half were corroding, two snapped on extraction. He replaced the whole assembly with 5/16" GRK RSS structural screws in one afternoon instead of two days, saving 11.5 labor hours and roughly $920.

The cost column has flipped in the last five years. At Home Depot pack pricing, the Simpson SDWS Timber 0.220" x 4" 50-pack runs about $0.67 per screw. A retail Everbilt 3/8" x 4" HDG lag at the same store runs $1.93 per bolt. The "structural screws cost 3x more" line only holds against bulk-supply lag bolts at $0.50 to $1.00 each.

Are Timber Screws as Strong as Lag Bolts?

In most lateral and withdrawal applications, yes. The published per-fastener shear values for a 1/4" Simpson SDWS Timber Screw match or beat a comparable lag in standard softwood. Same for FastenMaster TimberLok, which exceeds the shear values of a 3/8" galvanized lag screw per the manufacturer's published spec.

The reason is metallurgy, not mass. A hardware-store lag is soft, unhardened low-carbon steel that relies on diameter alone. A good structural screw is heat-treated, hard on the outside so it cuts through dense wood, but not so brittle it snaps when the deck moves under a load. The thread also runs nearly full length and grabs more wood per inch. That's why a thinner screw matches a fatter lag in a softwood pull.

Simpson puts a real number on the trade-off. On the SDWH Timber-Hex HDG product page, they tell pile builders they can replace 3/4" and 5/8" HDG bolt-and-washer-and-nut assemblies with two SDWH Timber-Hex screws "in many conditions." That's Simpson's exact wording, qualifier and all.

And they didn't just print that on the box. They ran the connection more than 50 different ways before they'd stand behind it, and they publish a table that tells you exactly how many screws equal two half-inch bolts so you and the inspector aren't guessing.

The moment most veteran builders come around is when they actually test one. A career shop owner we've talked to held a 1/4" structural screw next to a 1/2" stainless lag and said the new screw looked too thin to trust.

He committed only after he saw the load-test numbers. It's the same story every time. The screw looks too thin, the doubt is fair, and then the test numbers settle it. The honest exception is heavy vibration-cycled work, where some fabricators still want a lag with more give. It's a minority view, but it's a fair one.

When Should You Use Timber Screws on a Deck or Heavy Framing?

These five connections are the bulk of the structural work on a deck or frame, and timber screws own all of them. Each one gets a recommended Simpson product and size.

Best For Deck & Framing Work

Simpson SDWS Exterior Timber Screw

We reach for these on nearly every structural connection we frame, from deck ledgers to doubled joists. One worker drives them from one side with a T-40 impact bit, no pilot and no separate washer, and the double-barrier coating holds up against today's PT lumber without weeping rust into the wood. We've watched an inspector wave through a ledger switch once he saw the IRC R507.9.1.3 listing right on the box.

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Deck Ledger Boards

For residential deck ledger to band rim on PT lumber, use a Simpson SDWS Timber Screw at 0.220" x 5" or 6". The SDWS is listed under IRC R507.9.1.3 for deck ledger fastening, with its own published spacing table, and needs no pre-drilling in standard SPF or PT framing.

Coastal jobs step up to 316 stainless (Simpson SDWS Timber SS Heavy-Duty). Type 316 contains 2 to 3% molybdenum, which resists chloride pitting in salt air where 304 and HDG zinc both fail in a few years.

Simpson's Deck Connection and Fastening Guide (F-DECKCODE25) carries the spacing tables. We know a New England deck builder who keeps the engineering bulletin from the box in his truck. When an inspector raises an eyebrow on a switch, the box itself is the proof. For the full code-required fastener picture, see our post on what fasteners are required for deck construction.

Beam-to-Post Connections

For a 4x6 or 6x6 beam down to a post, use a Simpson SDWS Timber Screw at 0.220" x 5" or 6", or step up to the SDWH Timber-Hex HDG for heavy timber. GRK RSS is a strong alternative. The SawTooth point drives into 6x6 dry timber without splitting, where a 3/8" lag without a pilot would split the post or shear off the head. We've seen both on rebuild jobs.

Doubled Joists and Built-Up Beams

For sistering joists or building up a 2-ply or 3-ply beam, use a Simpson SDWS Timber Screw at 0.220" x 4" for 2-ply or 0.220" x 6" for 3-ply. One worker drives from one side, no back-side access, no through-bolt, no separate washer. That's why most production framers stopped through-bolting built-up beams about ten years ago.

Pergola Post-to-Beam Structural

For pergola structural joints, especially through-beam sandwich construction, use a Simpson SDWS Timber Screw at 0.220" x 6" minimum, or 0.276" x 8" for heavy-timber sandwich. Pergola joints take cyclic wind load. Timber screws hold thread engagement under cycling, where lag-and-washer assemblies loosen as the wood compresses around the washer.

Heavy Timber Framing (Pile and Stringer)

For dock, boardwalk, or pier connections in saltwater splash zone, use a Simpson SDWH Timber-Hex SS Heavy-Duty (Type 316 stainless), or SDWH Timber-Hex HDG (ASTM A153 Class C) for inland heavy timber. Sizes run 0.276" x 8" up to 12" by pile diameter. A coastal Florida dock owner we know had coated screws rusting and 12 hours before hurricane landfall.

He switched to 316 stainless structural screws, drove them fast with an impact driver and no pre-drill, and credits the speed for finishing before the storm.

So When Do People Still Use Lag Bolts?

This is the 20% case. Four situations where the lag is still the right call.

TV Mounts and Hardware to Studs

For a wall-mount TV bracket into studs, use a 5/16" x 3" zinc-plated lag for moderate loads or 3/8" x 3" for heavy articulating mounts. The reason isn't strength, it's hardware geometry. Mount slots are counterbored around a hex head, and a flatter timber screw head doesn't seat in the recess. A commercial mounting outfit we know hangs over 100 TVs a year and uses lag screws into wood studs every install. The job type didn't need a switch. Never skip the pilot, even into a stud, or you split the framing or shear the head off.

Hammock and Swing Set Mounts

For a hammock or porch swing into a beam, use a 1/2" x 4" eye lag in 304 or 316 stainless minimum, heavier for swing sets. The eye is the point, and no timber screw has a load-rated eye built in. One caution from a veteran builder: use an eye lag with a hex shoulder and coarse wood threads, not a thin eye screw. Hammock failures are almost always shock failures, not steady-state failures.

Fence Post Brackets to Wood Structures

For fence post brackets, gate hinges, or rail brackets to PT lumber, use a 3/8" x 3" or 4" HDG lag for inland, or 304/316 stainless for coastal. The bracket holes are slotted for a lag head, so the bracket controls the choice. ACQ pressure-treated lumber corrodes electroplated zinc lags within 2 to 5 years, so specify HDG or stainless. Our post on how to prevent rust on screws outdoors covers the chemistry.

Decorative Work and Very Dense Timber

Two different reasons land here. For barn restoration, log home repair, or any exposed-beam work where the fastener shows, use a lag with a black oxide, brass, or square-head finish. The exposed hex head is the design feature, and a flat T-40 head reads as 2010s on a heritage build.

The second reason is the one most articles skip. The structural screw's whole advantage is skipping the pilot hole. In old-growth oak, tropical hardwood, frozen timber, or dense LVL, even a premium structural screw needs a pilot to avoid snapping the head. Once you're pre-drilling anyway, the structural screw's labor edge disappears and the cheap, massive lag becomes the more economical and reliable pick. A pragmatic crew keeps both in the truck for exactly this reason.

Which Timber Screw Brand Should You Buy?

We stock the Simpson Strong-Tie line because that's what builders ask for. Here's the honest brand picture, including the ones we don't carry.

Best For Heavy Timber & Piles

Simpson SDWH Timber-Hex HDG Screw

We drive these when the timber gets serious, like a 6x6 beam down to a post or a stringer into a dock pile. The integrated hex-washer head seats clean and the screw bores into dry heavy timber without the splitting you get from a 3/8 lag with no pilot. On a coastal job we'll step up to the 316 stainless version, but the HDG handles inland heavy framing all day.

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Simpson Strong-Tie. The SDWS Exterior Timber Screw is the everyday structural pick: 0.220" shank, double-barrier coating rated equivalent to HDG for PT lumber, T-40 drive, listed under IAPMO-UES ER-192 and IRC R507.9.1.3. Eagle Claw stocks it starting at $33.00 per box. The SDWS Timber SS Heavy-Duty is the 316 stainless coastal version, 0.276" shank.

The SDWH Timber-Hex HDG and SDWH Timber-Hex SS are the heavy-timber pile screws with an integrated hex-washer head. The SDS Connector Screw is the joist-hanger fastener, not a lag replacement, covered in the mistakes section below.

FastenMaster. We don't stock it, but TimberLok and LedgerLok are common at big-box, with clear application-specific names that take the guesswork out at the lumberyard.

GRK. The RSS uses a Zip-Tip and W-Cut threads that drive with noticeably lower torque, which spares battery life and wrist fatigue over a shift. A framer we know switched to GRK partly because he reuses the same RSS screws for temporary bracing, backing them out and driving them again, and has broken cheap lag heads more often than RSS heads.

SPAX. The PowerLag is a lag-style structural screw with a 4CUT point, ICC-ES ESR-1782 listed. The brand choice often comes down to which line your local supplier stocks at a good price.

What Size and Material Timber Screw Do You Need?

For length, the rule is the same for both fasteners. Total length equals the side member thickness plus any sheathing or spacer plus thread engagement into the main member, with the tip past the inside face where required. For a 1/2" lag the NDS rule is 4 thread diameters minimum, 8 for full design value, so a 1/2" lag through a 1.5" PT ledger and 1/2" sheathing into a band joist needs 6.5" minimum.

Material by exposure:

• Zinc-plated: indoor or covered dry exterior only
• HDG (ASTM A153 Class C): outdoor PT lumber, inland framing, ACQ-rated
• 304 stainless: clean inland exterior, finish work
• 316 stainless: coastal, marine, chemical exposure, splash zone

Today's pressure-treated lumber eats galvanized fasteners about twice as fast as the older stuff did, and the IRC requires HDG, stainless, silicon bronze, or copper in PT. Electroplated zinc anywhere near treated lumber fails in 2 to 5 years and black-stains cedar and redwood.

For exact pilot sizes, head-to-head sizing, and the full diameter-by-length tables, see our complete wood screw size chart. We go deeper on grades in stainless steel vs coated deck screws and galvanized screws vs stainless steel.

Common Mistakes People Make with Lag Bolts and Timber Screws

Lag bolts in joist hangers. The hanger's published capacity assumes specific listed fasteners, usually 10d nails or Simpson SDS Connector Screws. A tapered lag shank doesn't fill the stamped hole, voids the listing, and is the fastest way to fail a framing inspection.

Timber screws on TV mount hardware. The flatter head doesn't seat in the mount's counterbored slots. Use the lag bolts the mount was designed around.

Skipping the pilot on a lag. In dry softwood the lag splits the wood; in hardwood the head shears off before full embedment. This is the single most common install failure in the research.

Over-driving with an impact driver. Run the impact past the point where the head seats and you either snap the soft lag head clean off or spin out and pulverize the wood threads into sawdust, leaving zero clamping force. Back off the trigger when the head meets the surface.

Galvanized timber screws on a saltwater dock. HDG fails in 2 to 3 years in marine spray. Use 316 stainless in any splash zone. The rebuild labor costs more than 316 fasteners did at install.

Deck screws instead of timber screws on a ledger. Deck screws aren't structurally rated and snap in shear. The ledger pulling from the house is the cause of most deck-collapse forensics. Our deck screws vs construction screws post covers the distinction.

Things to Keep in Mind: Glenn Mathewson, a code educator we follow, makes this point bluntly. A structural screw is not a drop-in replacement for a 1/2" lag at the same spacing. The wood crushing under the fastener is the limit, not the screw. When you switch, use the manufacturer's spacing table, not the IRC lag-bolt table.