Why does my decking bounce when I walk on it?
Telling deflection from failure, then the four ways to stiffen a deck that is already down — noggins, sistering, an extra bearer and bracing.
- Difficulty
- intermediate
- Time
- 4 hr
- Read
- 5 min
- Safety
- warning
Short answer
Get underneath and find out whether the frame is intact. If it is, the deck is simply under-stiffened and adding solid noggins between the joists at mid-span is the single most effective fix, followed by a new bearer under the middle of the span. If a joist is soft, a post has settled or the ledger is loose, that is a failure and it gets repaired before anything is stiffened.
A deck that flexes underfoot is telling you the joists are spanning further than their depth comfortably allows, or that something has stopped supporting them. Both feel identical when you walk on it. The distinction is made underneath with a torch and a screwdriver, and it decides whether you are doing an afternoon's stiffening or a structural repair.
Safety
What you'll need
- Torch and a screwdriver for probing
- Timber to match the joist section, treated
- Joist hangers or structural screws
- Circular saw or handsaw
- Spirit level
- End-grain preservative
- Adjustable prop or acrow (optional)
Step by step
- Get underneath, or lift boards until you can see the frame.Everything below depends on seeing the structure. On a low deck this means lifting a run of boards; do it where the bounce is worst and where a board is already loose.
- Probe every joist and bearer with a screwdriver, especially at the ends.Joist ends sitting in hangers, on a wall plate or on a bearer are where water collects. Soft timber anywhere means you have a failure, not a stiffness problem, and the repair comes first.
- Check the posts and pads.Push hard against each post. A post that moves, or a pad that has sunk into the ground, drops the bearer above it and the deck bounces over that whole area. Look for a gap between post top and bearer.
- Check the ledger if the deck is fixed to the house.Look at the fixings and at the gap between ledger and wall. A ledger pulling away is a serious defect and takes priority over everything else here.
- Measure the joist span and section.The clear span between supports, and the joist depth. As a working guide for domestic decking, 150 x 47mm treated softwood is comfortable to about 2.4m of clear span; beyond that it deflects. If your span is significantly longer than the section supports, no amount of noggins fully fixes it.
- Fit solid noggins at mid-span.Cut blocks of the same section as the joists and fit them tight between every pair of joists in a line across the deck, staggered slightly so you can fix through the joist face into each end. They stop the joists twisting and share load sideways, and on most bouncy decks this alone transforms the feel.
- Sister the worst joists.Bolt or screw a second joist of the same depth alongside an existing one for its full length, bearing on the same supports at both ends. Doubling a joist roughly doubles its stiffness. Do this to alternate joists across the springy area if the whole area moves.
- Add a bearer and posts under mid-span if you can.The most effective structural fix by a wide margin: halving the span quarters the deflection. A new bearer across the underside of the joists, carried on posts on proper pads or concrete footings, turns one long span into two short ones.
- Brace the posts on a raised deck.Diagonal bracing between posts in both directions stops the whole frame swaying, which reads as bounce even when the joists are stiff enough. On anything raised, this is not optional.
- Treat everything you cut and put the boards back properly.End-grain preservative on every cut, two fixings per board per joist, and the 5 to 6mm board gap maintained.
Tips
- Noggins are cheap, quick and startlingly effective. Do them first and reassess before deciding on the bigger interventions.
- Cut noggins slightly tight and tap them in. A loose noggin does nothing at all.
- If you are lifting boards anyway, number them on the underside — decking boards weather to slightly different shades and putting them back in order is invisible where a shuffle is not.
- Use joist hangers where a new bearer or joist meets existing timber. Screwing into end grain is a fraction of the strength.
- Check the fixings while you are under there. Bright zinc screws in treated timber corrode away and a joist can be effectively unfixed while looking perfectly sound.
Common mistakes
- Adding more boards or a thicker board to cure bounce — The bounce is in the joists, not the deck surface. A thicker board makes the deck heavier and no stiffer between supports.
- Propping the deck on bricks or blocks stood on soil — It feels better for a season, then the block settles or tips and you are back where you started with a trip hazard underneath. New support means a pad or a footing.
- Stiffening a frame that has a rotten member in it — You are transferring load onto timber that is already failing. Repair first, stiffen second.
If it doesn't work
Bounce in one small area only
Cause: A single joist has failed, a hanger has pulled, or a pad under one post has sunk. — Fix: Local repair — replace or sister the joist, refix the hanger properly, or re-bed the pad. Stiffening the whole deck for a local failure is wasted work.
The whole deck moves together, side to side
Cause: The post frame is unbraced and racking. The joists may be perfectly adequate. — Fix: Diagonal bracing between posts in both directions, and check the post bases are actually restrained rather than just standing on pads.
Bounce plus a soft, spongy feel underfoot on a board
Cause: The board itself has rotted, independent of the frame. — Fix: Replace the board and check the joist beneath it — a rotten board usually means a wet joist under it.
Bounce that has appeared recently on an older deck
Cause: Something has changed: a post has settled, a fixing has corroded through, or rot has reached a bearing point. — Fix: Treat it as a failure and inspect underneath before use. Decks rarely become less stiff on their own.
Questions people ask
How much movement is normal?
A domestic deck should feel like a solid floor. A very slight liveliness on a long span is common and not dangerous, but if glasses on a table move when someone walks past, or the deck drums, it is under-stiffened and worth fixing.
Can I fix this without taking any boards up?
On a raised deck with access underneath, yes — noggins, sistering and a new bearer can all be done from below. On a low ground-level deck you will have to lift boards, and it is usually easier to lift a whole run than to work through a small hole.
Is a bouncy deck dangerous?
Not in itself, if the frame is sound and the deck is close to the ground. It becomes a real concern on a raised deck, where the same movement may be a post, a ledger or a rotten bearing rather than deflection — which is why the inspection comes before the diagnosis.