Why does my deck bounce or feel spongy when I walk on it?
Short answer
Bounce and sponginess are two different problems. A deck that bounces evenly across a span usually has joists that are undersized, over-spanned or missing blocking, which is uncomfortable but not always dangerous. A soft spot in one place, where a board sinks under your foot, is rot or a failed connection beneath that board, and it is a warning sign. Probe the framing under any soft spot before deciding whether it is a nuisance or a hazard.
Bounce versus soft spots
Walk the deck slowly and pay attention to what moves. If the whole area between two beams flexes under you like a diving board, and springs back, that is bounce. The structure is intact but not stiff enough. If one board or a cluster of boards sinks and feels mushy, or you hear a crunch, that is a soft spot. Something under your foot has lost strength.
Bounce calls for stiffening. Soft spots call for an inspection of the framing before anyone decides anything, because the cause is almost always rot or a failed hanger, and rot does not stay in one place.
What causes bounce
Joists sized or spaced for their span carry the load without noticeable deflection. Joists that are too small for the distance between supports, or spaced too far apart for the decking on top, flex. Composite decking in particular needs closer joist spacing than wood, and a composite deck laid over an old frame built for wood boards is a common source of bounce. Missing blocking or bridging between joists lets each one flex on its own instead of sharing the load with its neighbors.
Bounce also shows up where a beam is spanning too far between posts, or where a post has settled and left the beam partly unsupported. Sight along the beam from the end. A visible dip tells you where to look.
- Joists too small or too far apart for the span
- Composite decking on a frame spaced for wood
- No blocking between joists
- Beam spanning too far between posts
- A post that has settled or lost its footing
What causes soft spots
Rot in the decking itself, usually where water sits: at the ends of boards, under planters, below a downspout, at a low corner. The board softens and sinks under your weight while the joist beneath is still sound. This is the least serious version and the board is simply replaced.
Rot in the joist under the board. The board is fine but it is sitting on a joist whose top edge has turned soft where fasteners let water in. Several boards in a line will feel soft over the same joist. This needs the joist repaired or sistered, and it means the other joists should be probed.
A failed connection. A joist hanger whose nails have rusted away, or a joist end that has rotted out of its hanger, lets that joist drop under load and spring back. This feels like a soft spot but behaves like bounce in one narrow strip. It is a structural repair.
How to find out which one you have
Get under the deck if there is access, or lift a board where it is softest. Probe the joist top, the joist ends and the beam. Tap with a hammer and listen for a dull sound. Look at hangers and fasteners for rust. On a low deck without access, pulling two or three boards at the worst spot is a quick and reversible way to see the frame.
Levi does this as part of a repair scoping visit in Carlsbad and the surrounding North County communities. The goal is to find out whether you have a decking problem, a joist problem or a frame problem, because the three cost very different amounts of effort and the right fix for one is wrong for the others.
Fixing bounce
Adding solid blocking between joists at mid-span, tightening or replacing hangers, and adding an intermediate beam and posts under a long span all stiffen a deck considerably without rebuilding it. Where joists are badly undersized for their span, sistering a second joist alongside each one doubles their stiffness. If you are resurfacing a wood deck with composite, this is the moment to add joists or blocking so the new boards are properly supported.
Related questions
Is a bouncy deck dangerous?
Usually not by itself if the framing is sound and properly connected. It is uncomfortable, it loosens fasteners over time, and it can hide a soft spot. If the bounce is new or getting worse, something has changed underneath and it should be checked.
Can I just screw down the soft board?
If the board is the only thing that is soft, replace it rather than screwing it down. If the joist under it is soft, a screw into rotted wood holds nothing. Pull the board and probe before deciding.
Why is the soft spot always near the house or at a corner?
Water collects there. Near the house, the ledger area traps runoff from the wall. At corners, boards end and expose end grain, and the deck often slopes toward a low corner. Those are the first places rot starts on almost every deck.