Lag Screws vs Structural Screws and Bolts

Lag screws are the heavy hex-headed screws people reach for whenever a connection looks serious — a ledger board, a post base, a swing set, anything where an ordinary screw feels inadequate. They work, and for a lot of those jobs they are now the second-best option, because modern structural screws do the same work with less preparation.

The three heavy fasteners — lag screw, structural screw, through-bolt — hold in genuinely different ways, and picking between them is mostly about what you can reach.

The three fasteners and how each holds

Lag screws cut a coarse thread into solid timber. All their holding power comes from that thread's grip in the far piece, so the pilot hole matters enormously.

Structural screws are hardened, engineered fasteners with thinner shanks, aggressive threads and cutting tips. They drive without predrilling in most timber and are supplied with published load values.

Through-bolts pass all the way through and are secured with a washer and nut. They do not depend on threads gripping wood at all, which makes them the strongest and the most tolerant of timber that shrinks, splits or degrades over time.

Why a lag screw's pilot hole is not optional

This is the single most common error with lag screws, and it is the same two-diameter logic that applies to ordinary screws, only with much higher consequences.

A lag needs a clearance hole through the near piece at the shank diameter, so the shank passes freely, and a pilot hole in the far piece sized to the screw's root diameter, so the threads have material to bite without splitting it.

Get it wrong in either direction and the connection is compromised. Too small a pilot in dense timber and the lag either splits the piece or snaps while being driven — and a lag that shears off in a ledger is genuinely difficult to remove. Too large and the threads have nothing to hold. No clearance hole at all and the threads engage both pieces, so the lag cannot pull the joint tight, exactly as the pilot hole guide describes for smaller screws.

Two further points specific to lags. Always use a washer under the head — the hex head is small relative to the load and will otherwise crush into the timber. And drive with a socket and wrench, not an impact driver run flat out; lags are meant to be tightened to snug, not until something gives.

What structural screws changed

Engineered structural screws did to heavy timber connections what cordless drivers did to framing.

No predrilling in most applications, which removes the two-step pilot and most of the labour.

Published, tested load values, which is the substantive difference. A generic lag screw carries capacity from published timber design values; a structural screw is tested as a product and its ratings are specific to it.

Better shear behaviour than an ordinary hardened screw, because they are engineered for it — though the general caution in the nails vs screws guide still holds: only a screw actually rated for structural use belongs in a structural connection.

Coatings rated for exterior and treated timber, which matters because modern treatments are corrosive to plain steel.

Less splitting, which is the quiet advantage. A thinner shank displaces less fibre than a lag of equivalent capacity, so connections near the end of a member — where splitting is most likely and most damaging — survive better. That also means more fasteners fit into a given area without the piece coming apart, and spreading a load across several smaller fixings is generally better practice than concentrating it in one large one.

There is a trade-off worth naming. Structural screws achieve their capacity through hardening, and hardened steel is less ductile than the mild steel of a lag or a bolt. In a connection that will see repeated shock rather than steady load, that brittleness is a real consideration, and it is part of why bolts remain the specified answer in the heaviest connections.

The one thing they do not change: where a code, a connector manufacturer or an engineer specifies a particular fastener, that specification governs. Joist hangers and post bases are tested as systems with named fasteners, and substituting something that seems equivalent invalidates the rating.

Choosing between lag screws and the rest

Use a through-bolt whenever you can reach both sides and the connection is important. It is the strongest option, it can be re-tightened as timber shrinks, and it does not depend on wood threads holding for decades.

Use a structural screw where you can only reach one side, which is most real situations — ledgers, blocking, post connections, pergola posts and similar.

Use lag screws where the size or head style is specified, where you are matching existing hardware, or where a structural screw of the required dimension is not available.

Follow the specification for anything holding a deck to a house, supporting a structure, or carrying people. Ledger attachment in particular is a code-governed detail with prescribed fastener types, sizes and spacings, and it is one of the most common serious failures in residential construction. That is a case for the published requirement and, where there is any doubt, a professional — not for a rule of thumb from an article.

For everything below that threshold, the practical summary is short: bolt it if you can reach both sides, use a rated structural screw if you cannot, and if you are using lag screws, drill both holes properly and put a washer under the head.