Lag Bolts

A lag bolt is a big wood screw: thick shaft, coarse threads, a sharp point and a hex head you turn with a socket or wrench. Lag bolt, lag screw, coach screw, it’s all the same fastener. It holds by biting into the wood, with no nut on the back, which is why it’s the fastener for anything you can only get at from one side: a ledger board against a rim joist, a bracket on a post, a shelf cleat on a stud.

WBC carries hex lags from 1/4 inch up to 3/4 inch in diameter, and from 1 inch up to 18 inches long, plus black lags in the smaller sizes. Here’s how to pick one.

Which diameter for which job

Think of diameter as how much work the lag does, and length as how far it has to reach.

Diameter	WBC lengths	What it’s for
1/4 inch	1 to 5-1/2 in.	Light brackets, shelf cleats, hardware on studs; black in 1-1/2 and 3 in.
5/16 inch	1 to 5-1/2 in.	TV mounts, fence and shed brackets, cabinets on studs; black in 1-1/2, 3 and 5 in.
3/8 inch	1-1/2 to 10 in.	TV mounts, fence and gate brackets, workbenches, beam-to-post; black in 1-1/2, 3 and 3-1/2 in.
1/2 inch	2 to 16 in.	Deck ledgers, heavy framing, machinery feet on wood floors; black in 3 in.
5/8 inch	4 to 18 in.	Heavy timber, post-frame and log work
3/4 inch	5 to 16 in.	The heaviest timber connections

A few jobs most people come in for:

• Deck ledger. A 1/2 x 4-1/2 inch lag is the common deck-ledger size. Your deck plan or the inspector sets the size, spacing and pattern, so bring that with you. Outdoor ledgers on treated lumber need a hot-dip galvanized or stainless lag; more on that below.
• TV mount. A 5/16 x 3 or 3/8 x 3 inch lag into the studs handles a typical residential mount. If the mount came with lags, use those and follow its instructions.
• Fence posts, rail brackets and shed kit hardware. 5/16 x 3 or 3/8 x 3 inch works for most.
• A hammock or swing. That’s a job for suspension hardware rated for it, into a joist or beam, never just decking or paneling. A person shifting in a hammock or swinging pulls a lot harder than their weight.
• Timber frames, log work and pole barns. That’s where the long 1/2, 5/8 and 3/4 inch lags earn their keep, reaching through a thick member into the next one.

On anything structural, a deck ledger, a beam or a girder, the size and layout come from the plan or the code, not from a counter guess. What a lag holds depends on the wood species, its moisture, how deep the threads go, the spacing and edge distance, and which way the load pulls.

How long it needs to be

Lag length is measured from under the head to the tip. Add up what the lag passes through first (the board, the bracket, the drywall) and the rest is what’s left to bite into the wood that holds it. On a TV mount, a 3-inch lag through a bracket and 1/2-inch drywall puts well under 3 inches into the stud.

How much thread has to go into the holding wood is set by the connection: the mount’s instructions, the deck plan or the code. Short of that, more thread in solid wood holds better than less. Too long is a problem too: the tip can poke out the far side.

Indoor, outdoor and treated lumber

This is where most lag jobs go wrong, and it doesn’t show for a few years.

• Indoors and dry: zinc-plated lags are fine.
• Outdoors: use hot-dip galvanized or stainless. Montana decks and fences sit through snowmelt and freeze-thaw every spring, and a plain or zinc-plated lag rusts out.
• Pressure-treated lumber: never zinc-plated. Today’s copper-based treatments eat plain steel and electroplated zinc, and zinc-plated lags in treated wood can fail in a few years. Building code calls for hot-dip galvanized, stainless, silicon bronze or copper fasteners in treated lumber. For inland work, 304 stainless is plenty; 316 is for salt and coastal exposure.
• Black lags: black is for looks, the head matching dark brackets, barn-door track or furniture. Black by itself doesn’t tell you it’s made for the weather. Some black finishes, like black oxide, give only mild rust protection, so for an outdoor pergola brace or a bracket on treated lumber, check that the coating is rated for that use.

Check the finish on the bin label before you buy for an outdoor job.

Drilling and driving

A lag goes in with a pilot hole, not a hammer.

1. Clamp the pieces together, lined up.
2. Drill a clearance hole the size of the smooth shank through the top piece.
3. Drill a smaller pilot hole into the holding piece, at least as deep as the threads will go. Too small and the wood splits or the head snaps; too big and the threads don’t grip.
4. Clear the chips, add a washer if the connection calls for one to spread the load, and drive it with a socket, wrench or impact.
5. Finish by hand. Snug, not cranked. Overtightening strips the threads in the wood and crushes the fibers.

Pilot sizes by wood. Spruce studs count as softwood; Douglas fir falls between soft and medium:

Lag	Softwood pilot	Medium	Hardwood
1/4 inch	3/32	5/32	3/16
5/16 inch	9/64	3/16	13/64
3/8 inch	11/64	15/64	1/4
1/2 inch	15/64	5/16	11/32
5/8 inch	5/16	13/32	29/64
3/4 inch	13/32	1/2	9/16

Pilot guides vary from source to source, so if your hardware or plan gives a pilot size, use it. In hardwood, a little soap or wax on the threads helps. Put the threads into side grain, not end grain, and keep the lag in the middle of the member, away from the edges.

Concrete and brick

A lag won’t hold in concrete or brick by itself. Set a lag shield in the masonry first, then run the lag into the shield, matched to it.

The Pick

For a deck ledger, 1/2-inch lags in the length and finish your plan calls for, hot-dip galvanized or stainless on treated lumber. For a TV mount, 5/16 x 3 or 3/8 x 3 into the studs. For fence and shed brackets, the same 5/16 or 3/8 x 3. For timber and log work, the long 1/2, 5/8 or 3/4 inch lags. Where the head shows against dark hardware, the black lags.

Frequently Asked Questions

Are lag bolts and lag screws the same thing?

Yes. Lag bolt, lag screw and coach screw are all names for the same fastener: a heavy wood screw with a hex head and no nut.

What size lag bolt for a deck ledger?

1/2 x 4-1/2 inch is the common size. Your deck plan or local code sets the size and spacing, and on treated lumber the lag must be hot-dip galvanized or stainless.

What size lag bolts for a TV mount?

5/16 x 3 or 3/8 x 3 inch into the studs works for a typical mount. Use the hardware that came with the mount if it has some, and find the studs first.

Do I need to drill a pilot hole for a lag bolt?

Yes. Drill a clearance hole through the top piece and a smaller pilot into the holding piece. Skip it and you’ll split the wood or snap the head.

Can I use zinc-plated lag bolts outside?

Not on treated lumber, and not where they’ll stay wet. Use hot-dip galvanized or stainless outdoors.

What’s the difference between a lag bolt and a carriage bolt?

A lag bolt threads into the wood and works from one side. A carriage bolt passes all the way through both pieces and clamps them with a nut, so you need to reach both sides.