Carriage bolts
A carriage bolt has a smooth rounded dome head with no drive slot and a square neck directly below the head. The square neck embeds into wood as you tighten the nut, preventing the bolt from spinning. Used where a smooth, finished head appearance is required on the visible side.
Specifications
| Property | Description |
|---|---|
| Head style | Dome (button head) — no wrench flats, no drive recess |
| Neck style | Square shoulder below head — embeds in wood to prevent rotation |
| Thread type | UNC coarse thread (standard) or metric |
| Requires nut? | Yes, always — nut and washer on the threaded end |
Common sizes
| Diameter × thread | Common lengths | Use case |
|---|---|---|
| 1/4"-20 | 1" – 4" | Light furniture, fixtures |
| 5/16"-18 | 1-1/2" – 6" | Deck railings, outdoor furniture |
| 3/8"-16 | 2" – 8" | Deck ledger connections, heavy furniture |
| 1/2"-13 | 3" – 12" | Structural framing, heavy timber |
Installation
- Drill a through-hole that is the bolt's nominal diameter (e.g. 3/8" hole for 3/8" bolt).
- Insert the bolt from the "show" side — the head will be visible here.
- Tap the head with a hammer to force the square neck into the wood.
- Place a washer and nut on the threaded end and tighten. The square neck prevents the bolt from spinning.
Material and finish
- Zinc-plated — indoor or protected outdoor use
- Hot-dip galvanized — deck construction, exterior connections, pressure-treated lumber contact
- Stainless steel — dock, marine, or harsh outdoor environments
When used with ACQ pressure-treated lumber, use hot-dip galvanized or stainless steel only. Zinc-plated fasteners will corrode rapidly in contact with ACQ chemicals.
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