Sanding down cupped boards flat, or filling gaps with a wood filler or slivers of matching timber, is the fastest way to make a floor look right again — and if the underlying moisture issue hasn't been fixed, both approaches fail in a predictable way. Sanded cupped boards that are still absorbing moisture from below simply cup again, but now with less material left to sand a second time. Filled gaps push back open as the timber continues shrinking or swelling seasonally.
A repair that lasts fixes the moisture source first, then deals with the boards.
The correct sequence of work
Fixing the moisture source before relaying is the step that's most often skipped because it takes longer and isn't visible in the finished result. But relaying boards onto a subfloor that's still damp, or before new boards have acclimatised to the room, essentially guarantees a repeat of the same fault within a similar timeframe.
- Identify and correct the moisture source — clear blocked airbricks, address a damp subfloor, or adjust how underfloor heating is run
- Allow the subfloor void and, where necessary, the boards themselves to reach a stable, appropriate moisture content before any permanent work is done
- Acclimatise new or existing boards properly on site before fixing, rather than fitting them straight from delivery
- Relay or refit boards with a correct expansion gap left at the room's perimeter, to be covered later by skirting or beading
- Where underfloor heating is involved, commission it on a slow, staged ramp-up rather than switching straight to full temperature
- Sand and refinish only once the floor has settled at its new stable moisture level
Why sanding or filling alone doesn't hold
Sanding removes the visible ridge from cupped boards but doesn't stop the moisture differential between the top and underside of the board that caused the cupping in the first place. If that differential is still present, cupping recurs, and because sanding has already removed material, there's less margin left before boards become too thin to sand again.
Filled gaps behave similarly — timber that's still adjusting seasonally will continue to move by roughly the same amount each year, so a filler or timber sliver either gets crushed as the boards swell back together, or the gap simply reopens around it as they shrink again.
Interfaces with heating and ventilation
Underfloor heating installers and floor fitters need to coordinate on commissioning — a heating engineer ramping the system to temperature on their own schedule without reference to how quickly the specific timber floor can tolerate drying is a common cause of exactly this fault. Subfloor ventilation, where airbricks or vents have been blocked externally by landscaping, needs correcting as building fabric work, separate from the floor itself, before the floor is finished.
How you know the repair has worked
The floor should go through at least one full heating season without new cupping or gapping appearing beyond what's genuinely normal seasonal movement for solid timber. A well-installed floor with a correct expansion gap will still move slightly year to year — the test is whether that movement stays within a normal, stable range rather than progressively worsening.
Common questions
How long should new boards acclimatise before fixing?+
It depends on the timber and the room's conditions, but boards should be left in the room they'll be fitted in, out of their packaging, until their moisture content matches the environment they'll be living in — rushing this step is one of the most common causes of gapping.
Can cupped boards always be sanded flat?+
Only if there's enough material thickness left, and only once the moisture cause has been fixed — sanding boards that are still cupping from an unresolved moisture source is a wasted step that will need repeating.
Is engineered flooring less prone to this than solid boards?+
Generally yes — engineered boards are more dimensionally stable across seasonal humidity changes than solid timber, which is one reason they're often specified over underfloor heating, though the same principles around subfloor moisture and acclimatisation still apply.
Get your floor properly diagnosed and repaired
We fix the moisture cause first, then relay or refinish the boards, with a written quote covering the whole sequence.