Franklin Deck Collapse: ACQ Lumber & Joist Hanger Failure

By The Franklin Deck Building Team — Deck Building professionals serving Franklin, TN

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It was a Saturday afternoon in mid-July, the kind of humid Tennessee evening when everyone drifts outside. A group of guests moved together toward the outer corner of the deck — the shaded end that hangs over a slope on a wooded lot in an established Franklin neighborhood — and the corner dropped. Not catastrophically, not all at once, but several inches in a single, sickening lurch. No one was seriously hurt. But the homeowner's backyard was effectively closed until someone could tell them why a Franklin deck that had passed every visual check that spring had just tried to swallow a group of their friends.

They called us the next morning.

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What We Found When We Got on Site

The deck was a ranch-style home addition, well-shaded by a mature tree canopy that kept the framing cool and damp through most of the year. From the surface, the decking boards looked reasonable — some weathering, nothing alarming. The post bases looked intact. A homeowner walking across it in April would have had no reason to worry.

We pulled up enough decking to expose the joist framing within the first hour, and the problem was immediately apparent to anyone who knows what to look for: the joist hanger hardware had corroded from the inside out. The connectors were standard G90 galvanized joist hangers — the kind that was widely specified and installed throughout the mid-2000s — and they were in contact with ACQ (alkaline copper quaternary) pressure-treated lumber.

Here is the technical reality that too many decks built during that era are carrying around as a ticking liability: when CCA (chromated copper arsenate) lumber was phased out for residential use in 2004, the industry transitioned to ACQ and other copper-based preservative treatments. ACQ does a better job protecting the wood. But it carries a significantly higher copper content, and copper accelerates galvanic corrosion on standard zinc-coated (G90) hardware. The connectors don't fail visibly. They don't rust red and flake the way a fence nail does. Instead, the zinc coating degrades quietly at the wood-metal interface, the base metal follows, and the connector's load-transfer capacity erodes over years until — under the right load, in the right conditions — it simply gives way.

The shaded, damp microclimate under that tree canopy had accelerated the process. What might have taken fifteen years in a sunny, open-air installation had played out in under a decade. When we measured the remaining section thickness on the failed hangers, they had lost nearly all meaningful structural capacity. The connectors looked intact from above. They were not.

The correct specification for ACQ-treated framing lumber — and this has been in the Simpson Strong-Tie and USP/MiTek documentation since the mid-2000s — is hot-dip galvanized (HDG) connectors meeting ASTM A153, or stainless-steel hardware. G90 is not rated for contact with ACQ or CA (copper azole) treated lumber, full stop.

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How We Fixed It

The scope of work was clear: full demo and rebuild on an accelerated schedule. There was no salvageable path forward — not with the existing hardware compromised throughout the framing system, and not with a homeowner who needed a safe, inspectable structure before the summer was out.

Demo came first. We stripped the decking, pulled the joists, and removed the ledger connections to inspect the house rim joist for any related moisture intrusion or fastener corrosion at the attachment point. The rim joist was sound; we documented the condition and proceeded with framing layout.

The replacement framing was specified correctly from the first nail: Simpson Strong-Tie HDG joist hangers and post bases, every one of them rated and marked for use with ACQ and CA treated lumber per the manufacturer's current load tables. Framing lumber was allowed to acclimate on site before installation, and joist bays were spaced and detailed for drainage — a detail that matters on a shaded lot where standing moisture is a recurring condition rather than an exception.

We pulled the Williamson County AHJ framing inspection as a priority, coordinating directly with the inspection office to get the deck into the queue without the delays that can stretch a standard permit cycle. The framing inspection cleared, and the project finished within the agreed window — decking, fascia, and all finish-out completed before the homeowner's next scheduled outdoor event.

This is exactly the kind of job where preconstruction clarity matters. The homeowner understood from our first site visit what the root cause was, what the correct fix looked like, and what the inspection sequence would be. No surprises on scope, no change orders driven by undiscovered conditions — because we had exposed and documented the conditions before we quoted the work.

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What to Watch for on Your Own Deck

If your deck was built between roughly 2004 and 2012 — the transition window after CCA lumber was phased out but before HDG hardware became standard practice on most residential jobs — there is a real possibility that your joist hanger hardware is not rated for the lumber it's holding.

A few things worth knowing:

• Standard G90 galvanized connectors are not rated for ACQ or CA treated lumber. If your deck framing used pressure-treated lumber installed after 2004, G90 hardware is the wrong specification.
• Corrosion at the wood-metal interface is not visible from the deck surface. You will not see it during a casual inspection. The connector may look fine while carrying a fraction of its rated load.
• Look for 'ZMAX' or 'HDG' markings on exposed connector hardware as a baseline indicator that the correct product was used. If you can't find a marking, or if the hardware looks like bare silver-gray stamped metal, it warrants a closer look by a qualified contractor.
• Shaded, damp installations accelerate the timeline significantly. A deck under tree canopy, on a north-facing elevation, or in a low-drainage area is at higher risk than an open, sun-exposed installation.

A proper assessment means pulling enough decking to visually inspect the joist hangers and post bases — not just walking across the surface and calling it good. If a Franklin deck in your neighborhood was built during that era, it is worth a professional set of eyes before the next gathering season.

The failure mode we documented on this job is not rare. It is a known, documented materials-compatibility issue that played out on thousands of decks built during the CCA-to-ACQ transition. The good news is that it is entirely preventable — and entirely correctable — when it's caught before someone gets hurt.

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Names and details are illustrative; the problem and fix reflect real jobs we do.

If you have a Franklin deck that was built in the mid-2000s, or if you witnessed any movement, bounce, or settling in your deck framing this season, don't wait for a gathering to tell you what the hardware already knows. Call The Franklin Deck Building Team at (629) 245-4428 — we'll schedule a site assessment and give you a straight answer about what you're standing on.