Lag Bolt vs. Through Bolt
Both bolt heavy wood joints, but they hold differently: a lag bolt threads into the wood itself, while a through bolt passes through and clamps with a nut. Access decides — strength follows.
| Feature | Lag Bolt | Through Bolt |
|---|---|---|
| How it holds | Coarse threads grip the wood | Nut and washers clamp the stack |
| Needs far-side access | No | Yes |
| Relative strength | Strong — limited by thread bite in wood | Strongest — full clamp plus shear, hole wear irrelevant |
| Re-tightenable | Re-drives, but loosens the hole | Yes — re-tightens like new |
| Installation | Pilot hole plus wrench | Clearance hole plus two wrenches |
| Washers | Always — a flat washer stops the head crushing fibers | Always, both sides — spreads clamp and protects the wood |
| Outdoor / treated lumber | HDG or stainless — plain zinc dies in ACQ | HDG or stainless, same rule; isolate dissimilar metals |
| Code and screws | Structural screws now rival lags with no pilot hole | Still the inspection-proof choice for ledgers and beams |
| Best for | One-side access: posts, railings, ledgers into framing | Beams, ledgers, and every critical joint you can reach through |
Verdict
Shop This Comparison
- Lag bolt assortment (HDG) — the one-side-access side
- Hex bolt, nut and washer kit — the through-bolt side
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Also in the toolbox
- Deep socket set (SAE and metric) — seats lags and torques nuts
FAQ
Which is stronger, a lag bolt or a through bolt?
A through bolt, all else equal — it clamps the full stack with a nut instead of relying on thread grip in wood fibers. A lag bolt in a stripped or oversized pilot hole is dramatically weaker; a through bolt does not care about the hole.
Can I replace a through bolt with a lag bolt?
Only where the plans allow it. Through bolts are specified for critical joints (ledgers, beams) precisely because they cannot pull out. Downgrading to a lag where a through bolt is specified needs an engineer’s sign-off, not a guess.
Do lag bolts need washers?
Yes — always fit a flat washer under the head. Without one, the hex head crushes wood fibers as you torque it, losing clamp and inviting loosening as the lumber seasons. In softwood, step up to a large-diameter fender or square plate washer for even more bearing area.
What is the pull-out strength of a lag bolt?
It depends on diameter, thread length in the wood, and species — a 3/8 in lag with 3 in of thread in Douglas fir holds roughly 1,500–2,000 lb in withdrawal, while the same bolt in soft pine holds far less. Pilot-hole size is the hidden variable: too big and you lose half your strength, so drill the shank clearance plus a pilot at 65–75% of thread diameter.