What Accounts for 90% of Deck Collapses?

Deck collapses make the news because they're dramatic and injurious — but they're almost never random. The failures follow a predictable pattern, and understanding that pattern is the first step toward either building a safe deck or evaluating whether the one you have is still sound.

In Bozeman, the risk is compounded by freeze-thaw cycles, heavy snowmelt, and a significant number of older decks built before current code requirements. I've seen firsthand what separates a structurally sound deck from one that's slowly working toward failure — and the answer is almost always in the connections, not the surface.

What Causes 90% of Deck Collapses?

Three structural failures account for the vast majority of deck collapses:

Ledger board failure. The ledger board is the horizontal framing member that anchors an attached deck to the house's rim joist. When this connection fails — from undersized lag bolts, missing flashing, or a rotted rim joist — the entire deck can pull away from the house. This is the single most common cause of catastrophic collapse, and it's particularly dangerous because the failure can be invisible from above until it happens.

Rot and corrosion in posts, joists, and fasteners. Bozeman's snowmelt and moisture from wet shoulder seasons, combined with freeze-thaw cycling, accelerates rot in wood that wasn't properly treated or hasn't been maintained. Galvanized fasteners rust out. Joist hangers corrode. The structural members weaken from the inside of the assembly, invisible until the load they can no longer carry arrives all at once.

Poor footing installation. Posts must anchor to concrete footings that extend below the frost line — 36 to 48 inches in Bozeman. Shallow footings heave during freeze-thaw cycles, compromising post alignment and creating dangerous load imbalances. Footings set on unstable or improperly compacted soil can settle unevenly for the same reason.

Why Ledger Failures Are So Dangerous

A deck can look completely solid from above — good decking boards, tight railings — while the ledger connection is quietly deteriorating. Water infiltrates behind the ledger, rots the rim joist, rusts the fasteners. The degradation happens slowly and invisibly.

The failure isn't slow. When 15 or 20 people are on a deck with a compromised ledger, the combined load can exceed the weakened structure's remaining capacity in an instant. That's the scenario that ends up in the news.

Proper ledger construction requires:

• Through-bolts at minimum 1/2-inch diameter, not lag screws or nails, spaced per code based on joist spacing and deck span
• Self-adhering flashing installed behind the ledger board to prevent water infiltration
• Inspection of the rim joist condition before attaching anything — a rim joist that's already rotted needs to be replaced, not buried behind a ledger
• Stainless steel or hot-dipped galvanized hardware that holds up in Bozeman's corrosive winter conditions

None of this is complicated, but it requires doing the work correctly and not skipping steps because they're hidden from view.

Local Context: Why Bozeman Decks Are Higher Risk

Bozeman's climate is genuinely harder on deck structures than most of the country. Freeze-thaw cycles run deep and repeat often. Snow loads are substantial. Temperature swings from subzero winters to hot, dry summers drive material expansion and contraction in ways that stress connections over time.

A lot of older decks in Bozeman were built before updated building codes, which means they may lack proper flashing, may have undersized fasteners, or may have used non-pressure-treated lumber in structural members that have been in ground contact for years. If you inherited a deck with your house and don't know its history, that's worth taking seriously.

The City of Bozeman enforces the 2021 International Residential Code (IRC), and deck structural connections, footings, and framing all require inspection at specific construction stages for permitted work. That inspection process exists because these failures are preventable when the work is done right.

What Is the 3/4/5 Rule for Decks?

The 3/4/5 rule is a simple framing technique that ensures deck corners are square — which matters for even load distribution and structural integrity. It comes from the Pythagorean theorem (3² + 4² = 5²):

• Measure 3 feet along one edge of the deck frame
• Measure 4 feet along the perpendicular edge
• Measure the diagonal between those two points

If that diagonal is exactly 5 feet, the corner is square. You can scale this up — 6/8/10 or 9/12/15 — for larger structures. Square corners ensure joists align properly, loads distribute as designed, and the structure performs the way it was engineered to.

Is a Deck Collapse Covered by Homeowners Insurance?

Coverage depends on the cause and your specific policy, but the general breakdown:

Usually covered: Collapses caused by sudden or accidental events — structural failure during normal use, storm damage, or unexpected material defects. If a properly maintained deck fails due to a hidden defect, most homeowners policies provide coverage.

Usually not covered: Damage caused by neglect or gradual deterioration. If an inspection would have revealed rot, corroded fasteners, or obvious structural weakness that went unaddressed, the insurer may deny the claim.

Liability: If someone is injured during a collapse, your homeowner's liability coverage should apply — but only if you can demonstrate the deck was properly maintained and inspected. Documentation of construction methods, materials, and maintenance history matters more than most homeowners realize until they need it.

How to Check Your Deck Right Now

You don't need to be a contractor to do a useful visual inspection. Look for:

• Soft or spongy wood around the ledger board attachment to the house
• Visible rust staining around fasteners or joist hangers
• Posts that have shifted, leaned, or show soft spots at the base where they meet footings or the ground
• Decking boards that flex excessively underfoot, which can indicate joist problems below
• Any separation between the deck structure and the house wall

If you find any of these, the deck needs a closer look before your next gathering. Annual inspection of ledger boards, post bases, and hardware is a reasonable standard for any deck in Montana.

Want to make sure your existing deck is safe — or build a new one right? Schedule a site visit.