Why treated lumber is harder on metal
FEMA's Technical Bulletin 8 explains that chromated copper arsenate (CCA) was removed from most building-use lumber in 2004, and several of its replacements are “more chemically reactive and therefore more corrosive.” Two things raise the rate: the chemical used and how much of it stays in the wood (its retention). Ground-contact lumber, often used for posts, carries more preservative and can corrode hardware faster than lumber rated for above-ground use. Moisture speeds all of it up.
The tag stapled to the end of treated boards names the preservative and its use rating. If you still have tags, receipts or the builder's plans, keep them. They tell a contractor which hardware the lumber and connector makers allow.
What the code expects
The International Residential Code, which Pennsylvania's building code is based on, requires corrosion-resistant fasteners in preservative-treated wood (section R317.3.1, as summarized by FEMA). Connectors must follow the lumber or connector maker's instructions. Where they give no direction, the minimum is G185 hot-dip galvanized steel, about twice the zinc of standard G90 hardware. Type 304 or 316 stainless steel exceeds that. Mechanically galvanized coatings aren't allowed on driven nails because they can chip off as the nail goes in.
Mixing metals causes its own problem. FEMA warns that dissimilar metals in contact corrode faster, and that stainless connectors need stainless nails. So adding stainless screws to an old galvanized hanger isn't automatically an upgrade. The hanger and its fasteners should be one matched system from the manufacturer's literature.
Philadelphia and Bucks County are far from salt spray, so FEMA's guidance for buildings well inland treats code-required galvanizing as acceptable, with thicker galvanizing or stainless as the better choice. The Building America deck guidance notes that decay and fastener corrosion often contribute to deck failures, which mostly happen at the ledger and railings.
What rust actually tells you
FEMA says the spots that corrode fastest are partly sheltered: under a deck, where hardware stays damp and isn't rinsed by rain. That's exactly where joist hangers and ledger bolts sit. From a stable spot, without crawling under a sagging deck, look for:
- Red rust on hangers, post bases or screw heads. A dull gray coating is the zinc doing its job; FEMA says more than thin rusty edges means the zinc is used up and the steel is now thinning.
- Empty nail holes in hangers. NADRA's deck evaluation checklist asks whether every connector hole is filled with the proper fastener, and notes that slotted and Phillips-head deck screws have never been appropriate for connectors.
- Dark, soft or crumbling wood around fasteners. NADRA's homeowner checklist says a corroded fastener can cause deterioration in surrounding wood.
- Ledger boards held only with nails, which NADRA says is not acceptable.
If the deck sways, sags or has pulled away from the house, stay off it until someone qualified has looked. Our guide on when joists or posts need a professional inspection covers those signs, and loose stairs or railings deserve the same caution. A soft spot right at the wall can be a ledger leak; see why a porch floor feels soft near the house.
Who to call and what to ask
A deck contractor who does structural repairs can assess most hardware corrosion. If many connections are rusted, the ledger is involved or the deck moves, ask for a structural engineer. FEMA recommends that replacement connectors go into undisturbed wood rather than old fastener holes, so a proper repair may mean different hanger sizes, sistered framing or new lumber.
- Which hardware and coating are you specifying, and does the connector maker list it for this lumber treatment?
- Will the fasteners be the type the hanger maker specifies, not deck screws?
- Is any wood around the corroded hardware damaged, and how will it be fixed?
- Does this repair need a permit? Deck permit rules differ in Philadelphia and each Bucks County township, and replacing structural hardware is often more than cosmetic work.