Decking that feels bouncy or uneven: what's wrong underneath
The Abimeister team · Updated · 4 min read
If a terrace has started to feel springy, or one corner dips when you step on it, the boards themselves are rarely the problem. What's underneath them has usually shifted, weakened, or was undersized from the start. This guide explains what typically causes the movement, what you can check yourself before calling anyone, which signs mean the structure is unsafe to use, and how a handyman goes about inspecting and repairing a terrace that's already built and already sagging.
What usually causes the bounce
Most bounce comes from the joists — the horizontal timbers under the boards — being spaced too far apart, undersized for the span, or partly rotted where they sit on the ground or against a support post. Estonian terraces built with 45×95 mm joists on wide spacing can feel springy even when new; add a few years of moisture and the flex gets worse.
Support posts are the other common culprit. If a terrace was built with too few posts, or posts sitting directly on soil rather than a proper footing, the ground beneath them settles unevenly over time, especially after a wet autumn or a hard frost. One post sinking a centimetre or two is enough to make an entire section feel loose.
Rot is often invisible from above. Joists in contact with damp soil, or boxed in without ventilation, can soften from underneath while the top surface still looks sound. This is especially common on terraces built low to the ground, with little airflow beneath the deck.
What to check yourself before calling for help
Before booking anyone, a few checks from underneath the terrace will tell you a lot. If there's a gap you can crawl into, take a torch and a screwdriver and have a look.
Walk the deck slowly and note exactly where it moves — one board, one joist bay, or the whole structure. This narrows down whether it's a local repair or something more widespread.
- Press a screwdriver into any joist that looks grey or soft — if it sinks in easily, that timber is rotten
- Check whether support posts are still sitting flush on their footings, or whether there's now a visible gap
- Look for standing water or damp soil directly under the low points of the deck
- Check joist spacing where you can see it — anything wider than about 40–45 cm between joists is worth flagging
- Note whether the movement is worse near the house, at the outer edge, or around a single post
When it's a safety risk and when it can wait
A terrace that sags visibly under a single person's weight, or where a joist has clearly split or a post has come away from its footing, is a safety risk and shouldn't be used until it's checked. This matters most on raised terraces or ones with steps, where a sudden give could cause a fall.
A gentle, even springiness across the whole deck is uncomfortable but not usually dangerous in the short term — it's a sign the structure is under-built for comfort rather than failing outright. It's still worth having looked at before it gets worse, particularly before winter, when frozen, waterlogged ground can shift posts further.
If children, elderly relatives, or garden furniture with legs that could catch in a gap use the terrace regularly, treat any noticeable movement as something to deal with promptly rather than something to monitor.
How the inspection and repair usually go
A handyman inspection starts from underneath wherever access allows, checking each joist and post by hand and by eye rather than guessing from the top. This usually takes well under an hour and tells us whether the fix is local or structural.
Common repairs include sistering a new joist alongside a weakened one, adding extra support posts to reduce the span, replacing a rotted post on a new footing, or re-levelling posts that have settled. These are jobs that fit within our odd jobs rate, from 39 € an hour, when the scope is a section of decking rather than the whole build.
Where the joist layout is undersized throughout, several posts have failed, or rot has spread through most of the substructure, a partial or full rebuild of the frame is usually the more sensible option rather than patching piece by piece. For that scale of work we give a fixed quote after seeing photos or visiting, and our decking and terrace jobs start from 90 € depending on size and condition.
What's not worth attempting yourself
Replacing a single visibly rotten board is a reasonable DIY job. Working on the structure underneath is a different matter — a joist or post that's cut, packed, or levelled incorrectly can shift the load onto neighbouring supports and create a new weak point you can't see from above.
Anything involving footings set in concrete, or a terrace attached to the house at a height where a fall would cause real injury, is worth having assessed properly rather than shored up temporarily. If the terrace sits over or against a load-bearing wall of the house, that boundary is one we don't work on ourselves, and a structural engineer should be involved.
If you're unsure whether what you're looking at is surface wear or structural damage, a short call-out to have it checked is far cheaper than discovering the answer when someone puts a foot through the deck.
Bounce or sag in a terrace almost always comes from the joists or posts underneath, and it's worth checking from below before deciding whether it's a quick repair or a rebuild.