Walk to the middle of the deck and shift your weight heel to toe without picking a foot up. If the whole field moves with you and settles back level, that is deflection — the frame bending under load and coming back, which is what wood does. If one spot goes down under your foot while the boards either side of it stay where they are, that is not bounce at all. That is something under there that has stopped carrying.
Both arrive on the phone wearing the same word. One is a frame doing what its span table always permitted. Another is wood that quit being wood where you cannot see it. A third is steel that has quietly let go. You can separate them standing on the deck, and which one you have decides who to call.
Bounce, sway and a soft spot are three findings, not one
Get somebody to help. One of you walks the deck and bounces gently — at mid-span, near the house, then over each corner. The other stands in the yard at ground level, eye running along the ledger line and the top of the beam. Three motions, pointing three directions.
Up and down, over the whole deck. Everything moves together and returns to level the instant you step off. Glasses ring on the table wherever the table is. That is deflection, and it is a frame question.
Side to side, or a rock that is not vertical at all. Watch the gap between the deck boards and the siding open and close. Watch the top of a post move against its base. Put a flat hand on a guard post while your helper walks past it. Sway is about bracing, hardware and footings, not joist depth.
One board, one bay, one footprint. Everything else stays put, and whatever is wrong is directly under that spot. Do this barefoot if the weather allows — feet read the difference between a springy deck and a dead board faster than eyes do.
An area that goes down and stays down is not deflecting. It is settling, and something under it has lost height or lost strength. Sight along the boards at a low angle from a corner, in raking light early or late in the day: a sag shows up immediately from there and is invisible standing over it.
One board, one bay, or the whole deck
Kneel and press hard with the heel of your hand, first directly over a joist, then halfway between two. A tired deck board flexes in the middle and firms up over its bearing points. A bad joist gives even where the board is fully supported, because the support is the problem.
If it is one board, take it up and look. How boards come off and go back — face screws, hidden clips, which joist the seam lands on — is covered in laying deck boards, and it matters here because a clipped board usually means lifting its neighbor too. Nothing under a deck gets diagnosed from the top of it.
Cause one: a frame that was always going to be springy
Joist depth, spacing and clear span decide how much a deck moves, and the span tables account for strength first and springiness second. A deck framed right at its maximum is fully compliant and can still feel like a diving board. Missing blocking makes it worse — joists free to roll at mid-span share load poorly, so each one takes your weight closer to alone.
How you tell it is this and not rot: the bounce is even across the deck, worst at mid-span, the same in every bay, the wood probes sound, the hardware is clean, and the deck has felt like this since it was new. That last one is the giveaway. Frames do not become over-spanned. They start that way.
Get under it with a tape and write down four things: clear distance from ledger to beam, joist depth, spacing, and whether there is any blocking. Add the grade stamp if you can read one. Those are the inputs anybody honest starts from, and joists, beams and the hardware that holds a deck up sets out what each does to the answer.
The real repair is shortening the span: another line of support, which means a beam, posts and footings below the frost line. Central Indiana practice is commonly in the range of 30 to 36 inches, and the depth that governs your deck is the one your local building department gives you. Sistering new lumber alongside the joists can work, but only where a full-depth member gets in and bears at both ends, which the ledger and beam geometry often will not allow.
Blocking is the cheap thing to try and it is worth trying. Be honest about what it does: it takes some liveliness out and helps joists share load. It will not turn an over-spanned frame into a floor.
Cause two: rot where water sits
Rot is not about rain. Wood outdoors gets wet and dries constantly and is fine. Decay needs wood that stays wet, so it starts where water is held rather than shed — joist end grain at the ledger, the ledger behind the boards, post bases at grade, stringer bottoms, and under anything that has sat in one place for years.
What an awl tells you
Use an awl, or the slimmest screwdriver you own. Sound wood stops the point almost at once. Lever it sideways: sound wood lifts a long splinter that stays hinged at one end, while decayed wood breaks off short and brash, in crumbs rather than fibers. Wood that is properly gone swallows the shank under hand pressure and brings wet punk back out with it.
Then rap along the joist with a hammer handle. Solid wood knocks sharply; a rotten section answers dull and flat, and the dull stretch tells you how far it runs.
Color lies. Gray is ultraviolet weathering. Black streaking below a fastener head is corrosion, a different problem on the same deck. Neither proves decay. The awl decides. After a run of dry days, look for an area still darker and cooler to the touch than the rest — that patch is holding water, and the reason is underneath it.
Where to probe first
- Joist ends at the ledger. Water past the flashing runs straight into the end grain of every joist at once. Probe every end, and the hanger seat if you can reach it.
- The ledger, and the rim joist behind it. Neither is visible without taking a board up or a band of siding off. It is the joint with no second load path, which is why the ledger board has a post of its own.
- Post bases at grade. A post standing in soil, or in a collar of concrete that holds water against it, rots from the bottom out. A post that lifts and drops seasonally instead is a footing that never reached depth, and frost depth and things that move in spring covers that.
- Stringers and the landing. The bottom of a stringer sits closest to the ground and gets the least air, and stairs take the hardest weight on the deck.
- Anything that never moves. A grill mat, a planter, an outdoor rug. Lift it.
One soft deck board over sound framing is a board. A soft joist end at the ledger is the deck's whole connection to the house, and by then the rim joist inside the wall is a fair question rather than a paranoid one. If there is a finished room behind that rim — a finished basement is the common case here — the drywall is part of the scope too. What may be growing on the inside face of wet framing is a question for an independent testing professional and the state health department's guidance. We do not test, diagnose or remediate; what has to happen once somebody else has tested is its own subject.
Sometimes the honest answer is that it has to be opened up to know. A joist can probe sound where you can reach and be gone where it meets the ledger, behind the hanger, in the last hand's width nobody sees. Pulling two boards at the house end settles it. Guessing costs more.
Cause three: fasteners and connectors that have let go
This one announces itself with noise. Deflection is close to silent. Movement that comes with a knock or a metallic tick is two parts working against each other, and the repair is at that joint rather than in the lumber. From underneath, with a light, look for:
- Hanger holes left empty, or filled with deck screws. A deck screw holds a board down. It is brittle in shear and its numbers have nothing to do with the hanger's.
- Nail heads standing proud of the steel, a flange pulled off the ledger face, or rust streaks under fastener heads — plain steel against copper-based preservative.
- Through-bolts you can move with a wrench. Treated lumber arrives wet, gives up moisture for months, and shrinks around everything driven through it. Bolt holes wear oval where a beam is bolted to the post faces.
- A missing post cap letting the beam roll, or a guard post notched and lagged to the rim joist, which feels fine until somebody leans on it.
Have your helper bounce while you hold a hand on a joist end at the hanger. You can feel a joist lifting off a seat it is supposed to bear on. Water in a loosened bolt hole freezes, works the hole larger and thaws — the same freeze-thaw cycle that takes mortar and slabs apart everywhere else outside here.
Which of these is a Saturday, and which is not
A Saturday, with the right hardware and a willingness to lie on your back: replacing a soft deck board where the joist beneath it probes sound; filling empty hanger holes with the connector nails or structural screws that hanger's manufacturer names, in a corrosion rating matching the connector; snugging through-bolts after a hard winter; clearing leaves and dirt off the tops of joists and away from post bases so wet wood can dry.
Not a Saturday, and we will say so on the phone rather than after:
- Anything needing a new footing. Depth, size and bearing get settled before the concrete, and structural exterior work generally goes in front of a building department.
- Anything at the ledger. It carries about what the beam carries, through fasteners into somebody's floor framing, with nothing behind it.
- A soft joist end, post base or stringer. Sistering only works into sound wood at both ends, and finding sound wood is half the job.
- Guards and stair rails, which take their load at the top of the rail and lever hard on the base of the post.
- Anything carrying a hot tub or a roof. Engineering questions, and doubling a couple of joists is a guess said out loud.
When it belongs to another trade
Some of what rots a deck is not carpentry, and new lumber under the same water only resets the clock. A downspout discharging onto the deck or at the ledger, or a roof plane dying into the wall above without a kick-out, is a gutter or roofing contractor's fix and it happens first — where the roof water goes makes the same argument about basements. If posts sit on a patio slab that is tipping, or a corner has dropped and kept dropping across seasons, the question is what the soil is doing. That is a foundation contractor's work. We do not guess at soil, and neither should anybody quoting you.
The short version
Bounce that is even across the deck, worst at mid-span, there since it was new and probing sound is a span-and-blocking problem — a lumber, post and footing job. A spot that gives under one foot and does not come back is decay or a connection that let go, and an awl tells you which. Sway is bracing and hardware. None of it gets diagnosed from the top.
The decks in our projects gallery look like ordinary decks from the yard, which is the honest problem with this part of the trade: what decides how one feels is the part nobody photographs. If yours moves in a way you do not like, ask for an estimate and we will get under it and probe before anybody writes a number.
