Deck collapses are not caused by rotten decking. They are caused by the deck separating from the house, almost always at the ledger — the board bolted to the building that carries roughly half the deck's total load. It is the most safety-critical connection in the whole structure and the one most likely to have been done badly, because it is quick to get wrong and completely hidden once the decking goes on.
The good news is that most of what matters can be checked from underneath with a flashlight and twenty minutes. Here is what to look for.
First, the ledger fasteners. Get under the deck and look at where the frame meets the house. You are looking for bolts or heavy structural screws in a staggered pattern — two rows, offset. What you do not want to see is nails. A ledger attached with nails is the classic pre-collapse condition: nails have very little resistance to being pulled straight out, which is precisely the load a ledger puts on them. If you can see nail heads and nothing else, stop using the deck and get it looked at.
Second, flashing. Above the ledger there should be a metal or membrane flashing that runs up behind the siding and out over the top edge of the ledger board, so water running down the wall is thrown clear instead of soaking into the joint. Without it, water sits in the gap between the ledger and the house and rots the band board — the framing member the ledger is bolted to. This is the quiet killer, because the bolts can be perfect while the wood they are anchored into turns soft. If there is no flashing, push a screwdriver firmly into the band board and the ledger at a few points. It should feel like driving into solid timber. If it sinks in, that connection is compromised regardless of what it is bolted with.
Third, the posts and footings. Each post should sit on a concrete footing that goes below the frost line onto undisturbed soil, and the post should be attached to it with a metal base rather than just resting on it or, worse, buried directly in dirt. A post set straight into the ground will rot at grade level — the point where it is alternately wet and dry — and it will rot from the inside, so it can look fine and be hollow. Push hard against each post. It should not move at all.
Fourth, the railing. This one is easy and it is where most injuries actually happen. Grab the railing at the middle of each span and push outward, hard — harder than feels polite. It should feel completely solid. Railings are required to resist a substantial lateral load precisely because people lean on them in groups, and a railing fixed only through the decking into the rim, with no through-bolts or blocking, will not. Also check the gaps: no opening in a guardrail should let a four-inch sphere pass through, which is the standard for keeping a small child from fitting through it.
Fifth, the stairs. Look at how the stringers are attached at the top — hangers or a ledger, not toe-nailed — and check that the treads are even. Uneven rise between steps is the most common trip cause on a deck, and if there is more than about a three-eighths of an inch difference between the tallest and shortest riser in a flight, people will catch their foot on it.
If several of these check out badly, that does not automatically mean the deck has to come down. A frame with sound joists and beams but a bad ledger and undersized footings can often be re-anchored, re-flashed and re-footed for far less than a rebuild, and the decking can be reused if it is in decent condition. What you should not do is resurface a deck — new boards over the existing frame — without having someone look at the frame first. It is the most common way an unsafe deck gets made to look brand new.
Top Line Remodel LLC builds and rebuilds decks and porches across Apex, Cary, Fuquay-Varina, Knightdale, Benson and the wider Triangle. Call (919) 396-2777 for a free on-site estimate — we will check the ledger and the footings with you before we quote anything.