Lag Bolts

A lag bolt and a lag screw are the same fastener. A lag screw and a lag bolt are the exact same fastener. “Lag bolt” is the older term, still common at lumberyards. Technically, the screw name is more correct. Bolts pass through material and are secured with a nut. Lag screws thread directly into the material and stop there. One veteran contractor said it plainly: there’s no such thing as a lag bolt because a bolt always has a nut.

Either way, you know it when you see it. A lag screw is one of the most sturdy fasteners you’ll find outside of through-bolted assemblies. It has a thick shaft, deep coarse threads, and a head designed for serious torque. Most have hexagonal heads so they can be tightened with a wrench or socket rather than a driver bit. That coarse thread is the whole point. Because their threads are designed to bite deeply into wood fibers, lag screws provide significantly higher pull-out resistance than standard wood screws.

This guide covers the common sizes, how to pick diameter and length, which finish survives Montana weather, and — the part most people get wrong — when a lag is actually the right fastener versus when code wants something else.

Sizing: Diameter, Length, and the Pilot Hole You Can’t Skip

Two numbers define a lag: diameter and length. Diameter is the thickness of the bolt’s shaft, measured in fractions of an inch. Larger diameters mean stronger holding power. Length is measured from under the head to the tip — longer bolts allow for deeper penetration and stronger joints. Common diameters range from 1/4 inch to 1 inch, and lengths can vary from 1 inch to more than 12 inches.

The 3/8” and 1/2” diameters handle most deck, fence, and heavy-hardware work — those are the two sizes worth knowing first. The 1/2” diameter alone spans a wide range of lengths, from short 2” lags up to 12” for deep timber connections.

Get the length right by penetration, not by feel. At least half the bolt length should penetrate into the main supporting material, and penetration should equal about 7 times the bolt’s diameter for maximum strength. For light tasks, reach for shorter sizes; use shorter bolts (1-1/2” – 3”) for light-duty woodworking and furniture, and longer bolts (4” – 8”) for decks, beams, and structural applications.

Now the part nobody should skip. Lags are not self-drilling. Lag screws have a sharp gimlet point and coarse threads that bite into wood. Because of their diameter, a pilot hole is required. Skip it and you’ll regret it — skip it and you split the wood or snap the lag. Pre-drill every time, even on the small ones. Always pre-drill, even on a 1/4” lag. Skip the pilot and you’ll snap the head off before the lag sets.

Size the pilot to the wood. Too small means splitting and over-torque; too large means low holding power. Softwood compresses, so it takes a smaller bit; hardwood needs more room. In softwood the pilot hole can be slightly smaller since the fibers compress more easily; in hardwood a larger pilot hole is needed to prevent splitting or excessive resistance.

Lag Diameter	Pilot (Softwood)	Pilot (Hardwood)	Hex Head
1/4”	3/16”	13/64”	7/16”
5/16”	13/64”	15/64”	1/2”
3/8”	9/32”	5/16”	9/16”
1/2”	3/8”	13/32”	3/4”
5/8”	15/32”	1/2”	15/16”
3/4”	19/32”	5/8”	1-1/8”

This serves as a practical reference whether you’re a professional contractor or a weekend DIYer.

Two more placement rules keep the wood from splitting. For maximum strength, embed threads into side grain (not end grain) and keep the screw centered in the member’s middle third. And mind your edges — for lag bolts in tension on a narrow face of a 2x, you need to maintain an edge distance of 1.5 times the diameter and an end distance of 4 times the diameter. So for a 1/2-inch lag, that’s a minimum edge distance of 3/4 inch. The concern with larger diameter bolts is the risk of splitting the wood before the load can shift to other bolts.

Finish: Pick the Coating for the Conditions

This is where lags fail early in Montana — wrong coating in wet or treated lumber. The rule is simple. Zinc-plated steel is affordable and widely available, best for indoor use where moisture is minimal. Hot-dip galvanized steel has a thick, protective zinc coating that makes it excellent for outdoor use, especially decks and fences. Stainless steel (304/316) is the gold standard for marine or coastal applications, extremely resistant to rust.

For anything touching pressure-treated lumber, galvanized or stainless is non-negotiable. Modern PT chemistry eats plain steel. Modern pressure-treated lumber (ACQ, MCA, CA-C copper chemistry) corrodes plain steel and electroplated zinc fast. Code requires fasteners in PT contact to be hot-dip galvanized, stainless, silicon bronze, or copper. Zinc-plated lag bolts in PT typically fail in 2 to 5 years. The picks, straight: hot-dip galvanized for outdoor PT lumber, 304 stainless inland — and never zinc-plated in PT wood.

That two-to-five-year failure window matters here. Pressure-treated decks and fences see freeze-thaw and spring snowmelt at the ground line. A zinc-plated lag in treated wood is a warranty call waiting to happen.

When a Lag Is the Right Call — and When It Isn’t

Here’s the highest-stakes decision a buyer makes. Over the last fifteen years, structural screws have taken over a lot of work lags used to do. 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.

Use a lag when the job is a heavy bolted-style connection into one piece of wood, and the bracket is built for it. 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 bracket has a 5/16” or 3/8” hole, the lag fits, and the load is well within range.

A few specifics worth knowing:

• TV mounts and heavy hardware to studs. 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.
• Hammock and swing eye lags. Use 1/2” minimum diameter for adult hammock loads, 5/16” minimum for kids’ swing hardware. A 1/2” eye lag needs 4” of length minimum. And it has to hit structure — the eye lag must hit a structural piece (joist or beam), not just decking or paneling.
• Fence, gate, and shed brackets. 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.

Now the no list. Structural framing has code-listed alternatives that beat a lag. 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.

Never put a lag in a metal connector. 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.

And a lag alone won’t hold in concrete. Not directly. For masonry, use anchors designed for concrete/brick (wedge, sleeve, or lag shields). You can thread a lag into an appropriate lag shield anchor in masonry.

When you do drive one, finish it by hand. Lag screws are tightened with a wrench or socket, not a screwdriver, to ensure proper torque. Using washers under the head distributes the load and prevents the head from sinking too far into the wood. Drive with an impact driver or wrench, then finish by hand to avoid over-torque and head snap.

Frequently Asked Questions

Are lag bolts and lag screws the same thing? Yes. Functionally, lag bolts and lag screws work the same way. They don’t require a nut; instead, they are driven directly into the material to create their own secure hold. The two names are habit, not engineering — “lag bolt” is the older lumberyard term.

Do I really need to pre-drill? Every time. Lag bolts do not self-tap. They require a pilot hole. The gimlet point helps start, but a pilot is necessary for proper performance. The pilot prevents splitting and keeps you from snapping the head off before the lag seats.

What coating should I use outside or in treated lumber? Hot-dipped galvanized or stainless (18-8, or 316 for coastal) to resist corrosion. Plain zinc-plated belongs indoors only. In pressure-treated wood, zinc-plated lags typically corrode through in two to five years.

What size lag for a typical TV mount? 5/16” x 3” or 3/8” x 3” works. 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.

Can a lag bolt be structural? It can, with the right detailing. Lag screws are used in structural applications when sized and detailed per code/engineering — species, spacing, edge distances, embedment. Always verify with local code officials or a design professional. For deck ledgers, beam-to-post, and joist hangers, though, code-listed structural screws are usually the better and faster choice.

How long does the lag need to be? Select a length that allows at least half its length to penetrate into the second material for a secure grip. For full strength, aim for thread penetration around seven times the bolt diameter into the main member.