The short answer first
The clearest signs of a failing ledger are a gap opening where the deck meets the house, soft or spongy wood at that joint, rust streaks and corroded fasteners, missing or loose flashing, and any new bounce near the house wall. If several signs appear together, keep people off the deck and get a professional evaluation.
What the ledger board actually does
The ledger fastens the deck frame to the house. Joists run from it out to the beams and posts, so it carries one end of the deck and transfers that load into the house framing.
The NADRA checklist hosted by the Town of Bar Harbor puts this plainly: the ledger board is where a deck attaches to the house, and it is a common source of deck failure. A board that carries load and sits at a wet joint deserves close attention.
Design guidance treats it with the same seriousness. DCA 6 is the prescriptive guide from the American Wood Council based on the IRC for single level residential wood decks, so this is a planned structural joint.
A gap opening at the house wall
Start by looking along the line where the deck meets the house. The joint should look closed and even. A dark shadow line that was not there before, a gap you can fit a fingertip into, or boards pulling away from the siding suggest the ledger is moving or losing strength.
Check from below where you can do so safely. Look for daylight at the joint or joists that no longer sit tight against it. Movement here rarely fixes itself. Decayed wood lets fasteners slip a little more each season, so a small gap tends to grow.
A gap also lets in more water, which speeds up the decay that caused it. That loop is why an opening here is a sign to act on soon.
Soft wood and what a careful probe can show
Decay often starts inside the ledger, where water sits against the house and drying is slow. The surface can look acceptable while the wood behind it softens. Damp patches that stay dark long after rain are an early clue.
The NADRA checklist advises checking damp areas with an ice pick or screwdriver, and notes that easy penetration of 1/4 to 1/2 inch, or soft and spongy wood, may mean decay is present. Firm wood resists the point. Wood that lets the tool sink in with light hand pressure is telling you its fibres have broken down.
Keep this check gentle and limited to inspection. Compare damp spots with a dry, sound area. Do not pry, lever or remove boards at the joint. If the probe sinks in near fasteners or across a wide stretch, stop there. A deck repair provider can judge how far the damage runs.
Rust streaks, corroded fasteners and nails only
Fasteners hold the ledger to the house, so their condition is a direct clue to the strength of the joint. Look for rust streaks running down from bolt or screw heads, heads that look thin or crumbly, and fasteners that sit loose in enlarged holes.
The NADRA checklist warns that corroded fasteners can cause deterioration in the surrounding wood. Wet wood corrodes metal, and corroding metal helps break down the wood around it, so the hole weakens faster than either problem alone.
The same checklist notes that a ledger should not be fastened with only nails. Nails can work loose as wood swells, shrinks and decays, and they do not clamp the joint the way a proper structural fastening does. If you see only nail heads along the ledger, treat that as a serious weakness.
Flashing that is missing, loose or out of place
Flashing is the thin barrier that directs water away at the house to deck joint. Done right, it sends rain and melt water out over the ledger face instead of letting it run behind the board, as the NADRA checklist describes.
Look for flashing that is absent, cracked, bent back, loose at its edges or tucked so water can run behind it. Stains below the joint and damp wood at the top of the ledger often go with flashing faults, and this wood dries slowly against the house.
The AWC explains that wood regularly exposed to rain needs a wet service adjustment because water affects moisture content and strength, and that wood protected from moisture reaches an equilibrium moisture content below 19 percent. A ledger kept wet by failed flashing lives at the hard end of that range.
Movement, bounce and stains inside the house
Some ledger problems show up as feelings and stains rather than visible rot. A deck that bounces near the house or feels less solid than last year may be moving at the ledger. Test this with one person walking slowly, and stop if movement is clear.
Go inside and look at the wall just below the deck line. Water stains, bubbling paint or damp patches there can mean water is getting in at the deck joint, with the ledger area as the likely path.
A stain that grows after each storm, or movement worse this spring than last, points to an active problem. That pattern may shape whether deck replacement or a focused structural repair is the right scope.
When to keep people off and get help
Keep people off if you find several signs together: a growing gap at the wall, soft ledger wood, badly corroded or loose fasteners, a ledger fixed with nails only, or clear movement near the house. Do not hold a gathering on a deck in that state.
The NADRA checklist itself advises seeking a professional, such as a deck builder or an ASHI inspector, for an evaluation. Judging how far decay runs into the house framing takes experience.
This post covers inspection only, because a failing ledger is not a weekend fix. Spot the signs early, keep the deck out of use when they are serious and hand clear observations to a professional. See our areas we cover page.
Frequently asked questions
What does a deck ledger board do?
The ledger board fastens the deck frame to the house and carries part of the deck load into the house framing. It sits at the joint where deck and house meet, so water control there matters a great deal. If the ledger rots, pulls loose or was fixed weakly, the whole deck edge at the house is at risk.
How can I tell if ledger wood is decaying?
Look for dark, damp or cracked wood near the house joint, then use the careful probe described in the NADRA checklist. An ice pick or screwdriver pushed into damp wood should meet firm resistance. Easy penetration of a quarter to half an inch, or soft and spongy wood, may mean decay is present.
Why is flashing at the ledger so important?
Flashing directs water away from the joint between house and deck, where the ledger sits. If flashing is missing, loose, bent or placed so water runs behind it, rain and melt water can reach the ledger and the house framing again and again. That repeated wetting is what gives decay and fastener corrosion time to grow.
When should I keep people off the deck?
Keep people off if the deck moves at the house, a gap is opening at the wall, the ledger feels soft, fasteners look badly corroded or the deck bounces more than it used to. Do not host a group on it. Treat those signs as a reason to get a professional evaluation promptly rather than waiting for another season.
Sources
Common questions
What does a deck ledger board do?
The ledger board fastens the deck frame to the house and carries part of the deck load into the house framing. It sits at the joint where deck and house meet, so water control there matters a great deal. If the ledger rots, pulls loose or was fixed weakly, the whole deck edge at the house is at risk.
How can I tell if ledger wood is decaying?
Look for dark, damp or cracked wood near the house joint, then use the careful probe described in the NADRA checklist. An ice pick or screwdriver pushed into damp wood should meet firm resistance. Easy penetration of a quarter to half an inch, or soft and spongy wood, may mean decay is present.
Why is flashing at the ledger so important?
Flashing directs water away from the joint between house and deck, where the ledger sits. If flashing is missing, loose, bent or placed so water runs behind it, rain and melt water can reach the ledger and the house framing again and again. That repeated wetting is what gives decay and fastener corrosion time to grow.
When should I keep people off the deck?
Keep people off if the deck moves at the house, a gap is opening at the wall, the ledger feels soft, fasteners look badly corroded or the deck bounces more than it used to. Do not host a group on it. Treat those signs as a reason to get a professional evaluation promptly rather than waiting for another season.