You walked in for lag bolts. The guy behind the counter handed you a box of lag screws. Same product, two names, and that's where the confusion starts. In the last 15 years, structural wood screws like Simpson SDWS and SDWH have quietly replaced lag bolts in most code-rated outdoor framing. Sometimes a lag is still the right call. Often it isn't.
We've spent 28 years in outdoor timber construction and stock every Simpson product below, so we'll tell you straight which one fits your project.
TL;DR When to use a lag screw (and what to use instead)
| Your Project | What to Use | What Contractors Use |
|---|---|---|
| TV mounts, hammocks, swings, fence brackets, shed hardware | Lag screw | Hex lag, 1/4" to 3/8" |
| Deck ledger, beam-to-post, pergola, doubled joists | Structural screw | Simpson SDWS Timber |
| Joist hangers and metal connectors (code requirement) | Connector-rated screw | Simpson SDS Heavy-Duty |
| Heavy timber, pile, dock, marine | Structural hex screw | Simpson SDWH Timber-Hex HDG |
| Concrete or masonry | Anchor, not lag alone | Sleeve anchor or Tapcon |
- Lag screw vs lag bolt: Same product, different names. "Lag bolt" is the older term still used at most lumberyards.
- Pre-drilling: Required, every time. Clearance hole sized to the smooth shank through the top board, smaller pilot through the receiving member. Skip it and you split the wood or snap the lag.
- Most common deck ledger size: 1/2" x 4-1/2" HDG lag per IRC R507.9.1.3. Simpson SDWS Timber is the code-listed alternative that installs in half the time with no pre-drill.
- Material picks: Hot-dip galvanized (ASTM A153 Class C) for outdoor PT lumber. 304 stainless inland. 316 stainless coastal or marine. Never zinc-plated in PT wood.
- Simpson's own claim: Two SDWH Timber-Hex HDG screws replace one 5/8" or 3/4" lag-bolt assembly in pile and framing work, per IAPMO-UES ER-192.
- The single biggest mistake: Lag bolts in joist hangers (code violation), or in a deck ledger when Simpson SDWS does the job in half the time at equal load.
Lag Screw vs Lag Bolt: Same Thing, Two Names
A lag screw and a lag bolt are the exact same fastener. "Lag bolt" is the older term, still common at lumberyards.
"Lag screw" is technically more accurate, since the fastener threads directly into wood with no nut. As one veteran contractor put it bluntly: there's no such thing as a lag bolt because a bolt always has a nut. It's either a lag screw that threads into wood, or a machine bolt that threads through it.
The shape is the same either way. Hex head on top (square on older or restoration pieces), a smooth machined shank below, and a sharp wood thread to the tip with a tapered point.
The shank passes through the top board cleanly so the threads can pull the joint tight. Lag screws are wood fasteners and aren't designed to tap into metal. ANSI/ASME B18.2.1 is the dimensional standard either name refers to.
When to Use a Lag Screw: The Quick Answer
Use a lag screw when you need a heavy bolted-style connection in a single piece of wood and the hardware is sized for it. TV mounts, gym equipment, hammock and swing hooks, fence post brackets, and shed bracketry all fit. The match is mechanical: the bracket has a 5/16" or 3/8" hole, the lag fits, and the load is well within range.
Don't use a lag screw when there's a code-listed structural alternative. Deck ledger to band rim, beam-to-post, doubled joists, pergola post-and-beam, and heavy timber pile work all have structural screws that are faster, code-rated, and engineered for the job. Simpson SDWS Timber, GRK RSS, and FastenMaster TimberLOK are the common alternates.
Don't use a lag screw in a joist hanger or any pre-fabricated metal connector. Hanger manufacturers test their hardware with specific connector screws (Simpson SDS) or hanger nails. A lag in a hanger nail hole is one of the most common mistakes code officials see in the field, and it fails inspection every time.
Don't use a lag screw in concrete on its own. If you're going into concrete or masonry, you need an anchor sized to your fastener, or a concrete screw like a Tapcon.
When Lag Screws Are the Right Call: Application by Application
Here's where the lag still wins. Each subsection tells you yes or no, plus the size, material, pre-drill, and the brand pros use.
TV Mounts and Heavy Hardware to Studs
LAG: YES, use lag screws for TV mounts and heavy hardware to studs. TV mount manufacturers ship lag bolts in their kits for a reason. The engineers who design wall mounts say it plainly: cabinet screws and small hardware screws can shear under the way an articulating arm pulls and twists, and lags hold the depth into the stud that keeps the bracket from pulling out.
For a typical residential mount, 5/16" x 3" or 3/8" x 3" works. The 3" length clears 1/2" of drywall plus the mount plate and gets at least 1-1/2" into the stud. A standard 3/8" lag in a 2x4 stud holds around 1,200 lbf static load when properly pre-drilled, more than enough for any flat panel up through 75 inches. Commercial AV installers who hang dozens of TVs a year have settled on 5/16" as the practical default, stepping up to 3/8" for heavier articulating arms.
Material: zinc-plated for indoor dry rooms, 304 stainless for humid spaces. Always pre-drill, even on a 1/4" lag. Skip the pilot and you'll snap the head off before the lag sets. As one experienced electrician put it, the expensive flat screen not falling off the wall is worth the two minutes to drill the pilot holes.
Hammock, Swing, and Eye Lag Mounts
LAG: YES, use lag screws for hammocks, swings and eye lag mounts. specifically eye lags for hammocks and swings. Use 1/2" minimum diameter for adult hammock loads, 5/16" minimum for kids' swing hardware. A 200-pound rider generates 300 to 500 lbf of pull when shifting weight, and play set swinging generates cyclic loads up to 3 to 5 times body weight. The code (NDS Section 12.1.4.6) calls for 8 thread diameters of bite to develop full design value, so a 1/2" eye lag needs 4" of length minimum.
Material: hot-dip galvanized for outdoor exposed beams, 304 stainless for indoor humid spaces. Pre-drill at half the lag diameter. The eye lag must hit a structural piece (joist or beam), not just decking or paneling.
Fence Brackets, Gate Hardware, and Sheds
LAG: YES, use lag screws for bracket-to-post. 3/8" x 3" or 5/16" x 3" works for most fence post brackets, rail brackets, and shed kit hardware. Hot-dip galvanized minimum for any PT lumber contact. Step up to 304 stainless for cedar or coastal exposure.
Modern pressure-treated lumber (ACQ, MCA, CA-C copper chemistry) corrodes plain steel and electroplated zinc fast. IRC R507.9.1.3 requires fasteners in PT contact to be HDG, stainless, silicon bronze, or copper. Zinc-plated lag bolts in PT typically fail in 2 to 5 years.
Heavy gates with daily motion are a different story. Veteran installers prefer through-bolts with backing plates for any gate swinging hundreds of times a week, since lag bolts work loose over time when a gate cycles that often.
Decorative and Restoration Work
LAG: YES, use lag screws specifically square-head lag bolts for period-correct barns, post-and-beam restorations, and decorative timber gates. Black oxide, oil-rubbed bronze, or natural finishes match the visual style. For structural-plus-decorative use (barn door tracks, pergola braces), specify HDG or stainless underneath the finish.
When Structural Screws Replaced Lag Bolts
Are lag screws as strong as bolts? For the fastener itself, often yes. For the assembly, no. A 1/2" through-bolt with washer and nut beats a 1/2" lag of equal length on pull-through and shear. And in modern framing, both have been outpaced by code-listed structural screws that install in a fraction of the time.
Read the case directly from Simpson, on the SDWH Timber-Hex HDG product page we sell:
"Speed up your next pile job by replacing 3/4" and 5/8" HDG bolt/washer/nut assemblies (two screws for one bolt in many conditions) with the Strong-Drive SDWH Timber-Hex HDG screw."
That's the manufacturer telling builders to stop using lag bolts on heavy timber. The shift happened because structural screws install without pre-drilling, often carry equal or higher rated load per fastener, and have a code listing under IAPMO-UES ER-192 for IBC and IRC structural applications.
Working contractors have been clear about the change for years. One veteran builder: "I haven't used a lag screw in four years." Another, a structural engineer-adjacent contractor: "Lags should never be used by anyone, ever again." That's not unanimous, but the trade has moved.
Deck Ledger Boards
LAG: NO. Use a structural screw. IRC R507.9.1.3 still permits 1/2" HDG lags with the two-step pre-drill, and it's still code-compliant. But Simpson SDWS Timber, FastenMaster LedgerLOK, and GRK RSS all get approved as code-listed alternates through their evaluation reports, with no pre-drilling at the same code-rated spacing.
Best structural screw to use: Simpson SDWS Timber, 0.220" diameter, 4" or longer, depending on ledger and band joist thickness. From $33.00 per box. For coastal or marine ledgers within 3,000 feet of saltwater, switch to the Simpson SDWS Timber 316 stainless at 0.276" diameter.
Some older suburbs still ask for 1/2" lags by default if the inspector hasn't seen the structural screw paperwork. Working contractors handle this by calling Simpson's or FastenMaster's tech line, where the engineers provide ICC-ES or IAPMO data directly to the inspector.
Beam-to-Post and Pergola Connections
LAG: NO. Use a structural screw. For beam-on-top construction (the code-compliant method, since IRC prohibits sandwiching beams on either side of a post unless notched), Simpson SDWS or GRK RSS, 0.220" to 0.276" diameter, 5" to 8" length beats a lag on every axis. No pre-drilling, equal or higher shear capacity, and a 4-post pergola goes from a half-day with lags to a couple hours with structural screws.
A manufacturer comparison test makes the time math concrete: 91 GRK RSS structural screws driven in the same 15 minutes it takes to drive 49 lag screws. Nearly double the work per battery charge.
Heavy Timber, Pile Work, and Marine Construction
LAG: NO. Use Simpson SDWH Timber-Hex HDG. Two SDWH27G screws (0.276" shank, hot-dip galvanized to ASTM A153 Class C) replace one 5/8" or 3/4" HDG bolt-washer-nut assembly in many conditions. The integrated 0.93" washer face means no separate washer to lose overboard.
Per IAPMO-UES ER-192, two SDWH27G screws in Southern Pine total 574 lb/in withdrawal capacity, exceeding a 5/8" lag (517 lb/in) and approaching a 3/4" lag (592 lb/in). On allowable shear, two of them more than double a single 5/8" or 3/4" lag. From $65.00 per box, in 4" through 12" lengths. For direct saltwater contact, the SDWH Timber-Hex 316 SS variant.
Doubled Joists and Built-Up Beams
LAG: NO. Use a structural screw. FastenMaster TimberLOK, Simpson SDWS, or GRK RSS in 5" to 6" length to sandwich a doubled 2x10 or built-up 2x12. Pull-through resistance is higher than a lag at equal diameter, and you're not stopping every fastener to drill twice.
Side-by-Side: Lag vs Structural Screw
| Application | Lag Screw | Structural Screw | Best Brands |
|---|---|---|---|
| Deck ledger to band rim | Code-compliant but old-school | Better (no pre-drill, code-listed) | Simpson SDWS Timber |
| Beam-to-post (beam on top) | Acceptable | Better (faster, equal shear) | Simpson SDWS or GRK RSS |
| Pergola structural | Acceptable | Better (faster on dozens of joints) | Simpson SDWS, SPAX PowerLag |
| Heavy timber pile or dock | Acceptable | Better (two screws replace one lag bolt) | Simpson SDWH Timber-Hex HDG |
| Doubled joists or built-up beams | Acceptable | Better (no pre-drill) | Simpson SDWS, FastenMaster TimberLOK |
When You Need Something Else Entirely
Sometimes the right answer isn't a lag at all. Three cases catch most of the mistakes.
Joist Hangers and Metal Connectors
LAG: NO. Code violation. This is the most common mistake code officials see in framing inspections. Joist hangers are tested with hanger nails or with manufacturer-specific connector screws like the Simpson SDS. Random lag bolts in hanger holes change the tested capacity and fail inspection every time.
Best connector screw to use: Simpson SDS Heavy-Duty Connector Screw, 1/4" diameter, 1-1/2" through 6" lengths. From $30.00 (HDG) or $48.00 (316 stainless). The packaging on every Simpson connector lists which fasteners are approved for that piece of hardware, and the connector screws are typically stocked right next to the hangers.
The reason a lag fails here isn't strength. It's the shear pattern. Hanger nails and SDS screws bend in a specific way under repeated load, and the hanger is designed around that. A lag in a hanger nail hole puts the load in the wrong spot, and the hanger no longer holds its rated capacity.
Concrete and Masonry
LAG: NO on its own. A lag bolt in concrete is an application, not a single product. The options:
- Plastic or lead expansion anchor + lag screw for light to moderate loads
- Sleeve anchor or wedge anchor for heavier loads (different fastener entirely)
- Concrete screw like a Tapcon for direct-into-concrete fastening
If the bracket you're mounting needs to attach to a foundation or wall, ask your supplier for a sleeve anchor sized to your bracket hole, or the right concrete screw for the load.
Carriage Bolt Applications
LAG: NO when you have access to both sides of the lumber. A carriage bolt (with nut and washer on the back) is often stronger and easier than a lag of equal diameter. For dock cleats, ladder brackets, and any cyclic load that might pull a lag, through-bolt with a backing plate is the durable answer.
Lag Screw Sizes: How to Pick the Right Diameter and Length
Lag screws come in standard diameters from 1/4" to 3/4", with 5/8" and 3/4" reserved for heavy timber. Common lengths run 1-1/2" to 12". ANSI/ASME B18.2.1 is the dimensional standard. Sizing is rarely about an abstract chart, it's about the application:
| Application | Common Size |
|---|---|
| TV mount or heavy hardware to studs | 1/4" x 2-1/2" to 3/8" x 3" |
| Indoor hammock to ceiling joist | 5/16" x 2-1/2" or 3/8" x 3" eye lag |
| Outdoor swing set to 6x6 beam | 1/2" x 4" eye lag minimum |
| Fence post bracket or shed hardware | 5/16" x 3" or 3/8" x 3" |
| Deck ledger (if still using lag) | 1/2" x 4-1/2" HDG per IRC R507.9.1.3 |
| Heavy timber pile or dock | 5/8" x 6" or switch to Simpson SDWH27G |
How Long Should a Lag Screw Be?
At least 5 thread diameters of penetration into the main member for general work. For a 3/8" lag, that's 1-7/8" of thread minimum. For a 1/2" lag, 2-1/2" of thread. NDS Section 12.1.4.6 calls for 8 thread diameters to develop full design value.
Total length adds the top piece thickness. A 3/8" lag through a 1-1/2" bracket needs at least 3-3/8" total, so a 3-1/2" or 4" lag is the practical pick. For a deck ledger, the lag tip must extend fully beyond the inside face of the band joist (1-1/2" ledger + 1/2" sheathing + 1-1/2" band = 4" minimum, practically 4-1/2").
A common mistake is matching the lag length to the top board only. Pros size for the top board plus at least 1-1/2" to 2" of penetration into the receiving member.
Wood Density Changes Everything
A 3/8" lag in pressure-treated Southern Yellow Pine (specific gravity 0.55) holds about 352 lb per inch of thread engagement. The same lag in SPF framing (G = 0.42) holds about 234 lb/in, around 33% less. In Western Red Cedar (G = 0.36), it's 185 lb/in, roughly half what it holds in Southern Pine.
For cedar fences and gates, oversize the lag or use structural screws explicitly rated for low-density species. Simpson SDWS22DB tabulates Western Cedar allowable loads; most other fasteners don't. In hardwoods like oak (G = 0.67), pre-drilling becomes brutal: undersized pilot holes snap lag heads off mid-install.
For the full sizing chart by gauge and length across all applications, see our complete wood screw size chart.
Lag Screw Materials: Galvanized, Stainless, and What to Avoid
Three categories cover almost every job.
Hot-Dip Galvanized (HDG) per ASTM A153
The standard outdoor lag for pressure-treated lumber. ASTM A153 Class C is the heavier coating (2.0 oz of zinc per square foot) for fasteners 3/8" and larger. Class D is the lighter version (1.25 oz/ft²) for smaller fasteners. For ACQ-treated lumber, Class C is preferred where geometry allows.
True HDG looks dull and slightly rough, sometimes almost crusty gray. Shiny means thin (electroplated), and thin means short service life in PT. Service life of HDG Class C in ACQ lumber is 25 to 40 years in typical exposure.
Stainless Steel 304 vs 316
Two grades that matter:
304 stainless (also called 18-8) for inland outdoor work. Cedar fences, redwood projects, freshwater dock framing, and any PT application where you're avoiding the galvanic stain HDG can leave on tannin-rich woods. 304 also avoids the black iron staining that zinc-plated and bare steel cause in cedar and redwood.
316 stainless anywhere within sight of the ocean. The difference is 2 to 3% molybdenum, which prevents the chloride-induced pitting that destroys 304 in saltwater. Coastal builders are clear about this: any hardware that will see saltwater needs 316 minimum. Coastal, marine, dockside, swimming pool, and road-salt locations all call for 316.
Never Use Zinc-Plated in Pressure-Treated Wood
Zinc-plated (electroplated) lag bolts in ACQ pressure-treated lumber typically fail in 2 to 5 years. The thin electroplated zinc layer gets consumed by dissolved copper ions, acidic moisture in ACQ, and galvanic corrosion. IRC R507.9.1.3 effectively prohibits zinc-plated fasteners in outdoor PT contact.
Plain steel and black oxide finishes are interior-only, fine for dry workshop builds and decorative restoration. Outside, they rust on contact with moisture.
Mixing metals causes its own trouble. Stainless hardware on aluminum brackets near saltwater can corrode the aluminum within weeks. Isolate dissimilar metals with a non-conductive washer, or match the metals throughout. For a deeper dive on outdoor corrosion, see our guide on how to prevent rust on screws outdoors.
How to Install Lag Screws: The Pre-Drilling Rule
Should you pre-drill for lag screws? Yes, every time, no exceptions. This is the number one install mistake, and it's the engineering reason structural screws got invented.
Skip the pilot and the unthreaded shank wedges wood fibers apart, splitting the board. Once split, the lag is holding nothing. Lag bolts are also softer than people expect, sometimes described on contractor forums as "soft like nails and not hard like screws." They snap under impact-driver torque if the pilot is too small or skipped. NDS Section 12.1.4 requires pre-drilling for any lag 3/8" diameter or larger, no matter which direction the load runs.
The Two-Step Pre-Drill
Pros use two bits, two sizes, two distinct holes:
- Clearance hole through the top piece. Same diameter as the lag's smooth shank. The shank passes through cleanly so the threads can pull the joint tight. For a 1/2" lag, that's a 1/2" bit.
- Lead (pilot) hole through the bottom piece. Smaller than the shank, sized for the thread root. For a 1/2" lag in Southern Pine, that's a 5/16" bit.
The biggest DIY mistake is using one bit at the thread-root size all the way through. The shank binds in the top piece, the threads don't pull tight, and the board cracks.
Pilot Hole Chart
For Southern Yellow Pine, Douglas Fir, and other softwood framing (G = 0.42 to 0.55):
| Lag Diameter | Clearance Hole (top) | Pilot Hole (softwood) | Pilot Hole (hardwood) |
|---|---|---|---|
| 1/4" | 1/4" | 11/64" | 3/16" |
| 5/16" | 5/16" | 7/32" | 1/4" |
| 3/8" | 3/8" | 1/4" | 5/16" |
| 1/2" | 1/2" | 5/16" | 3/8" |
| 5/8" | 5/8" | 3/8" | 7/16" |
| 3/4" | 3/4" | 7/16" | 1/2" |
The 1/2" lag with 5/16" pilot is the exact IRC R507.9.1.3 deck ledger spec. For hardwoods (oak, maple, hickory, ipe), bump the pilot up one increment to prevent splitting and snapped lag heads.
Driving and Washers
Yes, always use a washer with a hex-head lag. Standard for a 1/2" lag is a 1-3/8" diameter washer with a 9/16" hole. Without it, the hex head sinks into the wood and crushes the surface, loosening the connection over time. Exception: Simpson SDWH Timber-Hex screws have a 0.93" integral washer face built into the head, so no separate washer is needed.
Don't use a cordless drill on lag bolts 3/8" and up. The motor doesn't have the torque to set the lag without snapping the head. Use a ratchet or a high-torque impact wrench. Veteran home theater installers say it straight: "Just remember not to use a drill to get those lag bolts in, made that mistake and snapped a couple, do it by hand."
What Screws Don't Require Pre-Drilling?
Most structural screws (Simpson SDWS, GRK RSS, FastenMaster TimberLOK, SPAX PowerLag) drive direct under code listings IAPMO-UES ER-192 and ICC-ES ESR-2442. The SawTooth, 4CUT, and Type-17 self-drilling tips cut through wood fibers like a small drill bit instead of wedging them apart. That's the mechanical reason no pre-drill is required. As one industry video put it, you can "zip those right in" with a standard drill instead of pre-drilling and ratcheting like you would with a lag.
Exception: dense hardwoods (oak, maple, ipe) may still split if you skip the pilot. Screw brands recommend a small pilot (5/32" for SDWS, 7/32" for SDWH27G) if you see the wood starting to split.
What Lag Screws Should You Buy?
After 28 years of outdoor framing, we reach for Simpson Strong-Tie structural screws on the connections that carry real load: ledgers, beams, pile work, and joist hangers, where a lag fails inspection or works loose. We pair them with Eagle Claw 304 and 316 stainless on the deck boards above, so the whole assembly outlasts the wood. Shop the full line below.
Picking the Right Lag Screw for the Job
If you're mounting a TV, hanging a hammock, or attaching brackets to studs, a lag bolt is the right fastener. Pre-drill, pick the right size, and match the material to the environment.
If you're building a deck ledger, a pergola, a pile or dock structure, or any wood-to-wood structural connection in modern outdoor framing, the structural screw wins. Simpson SDWS Timber for ledgers and beams, Simpson SDWH Timber-Hex HDG for heavy timber, and 316 stainless variants for coastal work.
If you're attaching to a joist hanger or any metal connector, neither lag nor general structural screw passes inspection. Use the Simpson SDS Heavy-Duty Connector Screw.
We sell every product mentioned above. Browse our timber construction collection or construction screws collection, or get in touch and we'll match the fastener to the job.
