The approved list
IBC 2021 2304.10.6.1 says fasteners, including nuts and washers, in contact with preservative-treated wood shall be of:
- hot-dipped zinc-coated galvanized steel
- stainless steel
- silicon bronze
- copper
Electro-galvanized steel is not on the list. Treating it as acceptable is a listed common error. The word that matters is hot-dipped.
Coating weights and the mechanical zinc option
The same section states that coating weights for zinc-coated fasteners shall be in accordance with ASTM A153. Hot-dipped nails for treated sill plates therefore follow ASTM A153.
Mechanically deposited zinc per ASTM B695 Class 55 minimum is allowed only for fasteners other than nails, staples, timber rivets, wood screws and lag screws. So:
- A bolt with its nut and washer may use ASTM B695 Class 55.
- Nails, staples, timber rivets, wood screws and lag screws may not. They need hot-dip galvanizing per ASTM A153, stainless steel, silicon bronze or copper (staples: stainless only).
Other standards that do not govern fastener coatings: ASTM A653 G185 is the minimum for exterior connectors, not fasteners, and AWPA M4 governs the treated wood itself.
Staples are stainless only
Staples in preservative-treated wood must be stainless steel. Hot-dipped galvanized steel and silicon bronze are fine for other fasteners but not for staples, and the ASTM B695 Class 55 option expressly excludes staples. Missing the stainless-only rule is a listed common error.
When plain carbon steel is allowed
The only carbon-steel exception is for SBX/DOT or zinc borate preservative-treated wood used in interior, dry conditions. There is no exception for:
- painted carbon steel in exterior exposure
- wood treated to AWPA U1 for ground contact
- staples in any treated wood
If the wood is not borate-treated and dry indoors, carbon steel is out.