Two different things get reported under this heading, and they're worth telling apart from the start. Cupping is when the edges of individual boards rise slightly higher than the centre, so the floor feels ridged underfoot. Gapping is the opposite in a sense — boards shrink and pull apart, leaving visible gaps wide enough in some cases to see the joists or subfloor below.
Both are timber responding to moisture, just in different directions. Cupping usually means the board is absorbing moisture faster on its underside than its top; gapping usually means the timber is drying out and shrinking, often because it was too wet when it was laid or the underfloor environment has changed.
Cupping versus gapping — different moisture directions
Cupped boards are swelling more on the underside, which pushes the edges up relative to the centre — that pattern points to moisture reaching the boards from below, from the subfloor or void, rather than from the room above. Gapping is shrinkage, which happens either as boards that were laid too wet finally dry out to their equilibrium size, or as an already-settled floor dries further due to a change in heating or ventilation.
The likely causes
- Boards laid before the building had properly dried out from construction or a previous leak
- Underfloor moisture with inadequate ventilation, common where airbricks have been blocked or covered externally
- Underfloor heating commissioned and ramped up to temperature too quickly, drying the timber faster than it can adjust
- No expansion gap left at the perimeter of the room, which stops the floor accommodating normal seasonal movement
Checking which is behind it
Note whether the movement is seasonal — worse in winter or summer and improving in between — or whether it's been steadily one-directional since the boards were laid. Seasonal movement that reverses points to normal timber behaviour reacting to humidity changes, often made worse by a missing expansion gap. One-directional gapping since installation suggests the boards were simply too wet when fixed and have been drying out and shrinking ever since.
Check external airbricks for the subfloor void aren't blocked by paving, planting or paint — this is a very common and easily overlooked cause of persistent underfloor moisture leading to cupping. If underfloor heating is present, ask how it was commissioned; a fast initial ramp-up is a frequently overlooked cause of both cupping during the ramp and shrinkage-gapping once the floor has dried out unevenly.
A quick fix versus a repair that lasts
Sanding a cupped board flat or slipping a sliver of timber into a gap changes how the floor looks on the day it's done, but neither of those actions touches the moisture imbalance that caused the movement. That's the difference between a quick fix and a lasting repair: a quick fix treats the board, a lasting repair treats whatever is making the board absorb or lose moisture unevenly in the first place, whether that's a blocked airbrick, a leak, or heating that's been ramped up too fast.
A useful test when comparing options is to ask what happens next winter. If the answer relies on the same moisture source not causing the same movement again without anyone having changed it, it's a quick fix rather than a repair, however neat the finish looks on the day.
When to involve a structural engineer
Cupping and gapping caused by moisture are almost always a job for a carpenter or floor fitter working alongside whoever deals with the ventilation or heating issue behind it — structural engineering input isn't usually needed. That changes if the boards are moving because the joists themselves have sagged, twisted or shown signs of decay, rather than the boards simply responding to humidity. Persistent bounce or springiness underfoot that doesn't correspond to the board pattern itself, or visible dipping across a wider area of floor, points to the structure below rather than the boards, and that's worth having assessed properly before any board-level repair is carried out.
Common questions
Will cupped boards flatten out on their own?+
Sometimes, once the moisture source causing them to swell is addressed and the timber dries back to a normal level — but if they've cupped significantly or for a long period, some permanent set can remain even after the moisture issue is fixed.
Are wide gaps between boards a sign of a serious problem?+
Not usually on their own — they're common where the timber wasn't fully seasoned before fixing. But it's worth checking they're not widening further each year, which would suggest an ongoing moisture or ventilation issue rather than one-off initial shrinkage.
Is it the underfloor heating causing this, or the boards themselves?+
Both can contribute — solid timber boards are more prone to movement with underfloor heating than engineered boards, and heating ramped up too quickly speeds up the moisture loss that causes gapping regardless of board type.
Get your floor moisture issue diagnosed
We check subfloor ventilation, moisture levels and how the floor was installed before recommending any repair, with a written assessment.