Accoya eats ordinary steel. Which fasteners and hardware to use with acetylated wood

Accoya is wood treated with acetic anhydride, a process that makes it highly resistant to rot and keeps it from swelling and shrinking. The manufacturer says that, like most durable woods, it holds a little acid, and it strongly recommends corrosion-resistant or non-ferrous fasteners such as good stainless steel. A hardware guide that Accsys, the maker, published with UK supplier Coastal Group compares the acid level to oak and western cedar and names moisture as the trigger for corrosion.

On US jobs the rule is firmer. ICC-ES evaluation report ESR-2825, which recognizes Accoya as preservative-treated wood under the IBC and IRC, limits fasteners to Type 304 and 316 stainless steel. Where the wood would touch damp concrete, CMU or cement-based siding, the report calls for stainless or aluminum flashing or another noncorrosive barrier, and it requires Accoya's instructions to be on the jobsite during installation. Suppliers sell those grades as A2/304 and A4/316, and the Accsys guide names A4/316 as the most durable choice for exterior butt hinges.

The harder case is hardware that isn't made in stainless, such as galvanized sash hinge pins or multipoint lock bodies. The guide's answer is a barrier: coat the metal or the wood, or keep the two from touching. Lock housings and other cutouts get end-grain sealer, and the edges of doors and sashes, including the strip hidden behind the lock, get the same full paint coat as the outer face. Spraying locks and their mechanisms with PTFE or lithium grease adds protection. Coastal says Winkhaus locks tested to EN 1670 corrosion class 5 are validated for Accoya, and the guide lists MACO Tricoat-Evo window hardware as suitable.

A coating only works while it is intact. The guide warns that if paint or plating chips, the metal underneath is likely to corrode unless it is stainless. Coastal's summary of Accoya practice allows zinc-coated screws only in dry interior spots and calls for stainless or brass glazing pins on both sides of exterior doors and windows.