Timber Flooring Cupping Over a Liquid Screed Floor

A moisture content check of the timber itself, compared against manufacturer tolerances, combined with an RH or moisture reading of the screed beneath, will usually indicate whether the screed is still releasing moisture into the floor build-up. Where a vapour barrier was correctly specified and installed, the screed's residual moisture should be less relevant, so checking the vapour barrier detail is part of the diagnosis.

It's also worth checking whether the cupping is uniform across the floor or localised, since localised cupping near an external wall or wet area points to a specific moisture ingress point rather than a general screed drying issue.

Cupping is usually easiest to see in raking light across the floor, where the slightly concave board profile catches a shadow along each edge. It's often first noticed underfoot as a faint ridge between boards rather than as an obvious visual defect, particularly on narrower board widths where the distortion is less pronounced.

In more advanced cases, gaps can open between boards as they cup and shrink at the same time, and finish (lacquer or oil) can crack along the raised edges as the wood fibres are put under uneven stress across the width of each board.

Beyond a screed that was simply laid too wet or tested inadequately before covering, cupping can stem from timber that wasn't acclimatised to site humidity before fixing, from a vapour barrier that was punctured, poorly lapped, or omitted at a junction, or from an adhesive-fix system that allowed moisture-laden vapour to pass through unevenly. Board species and cut also matter, since quarter-sawn timber is generally more resistant to cupping than flat-sawn boards of the same species.

Under-floor heating adds a further variable: heat drawn upward through the screed changes the moisture gradient across the board thickness, so a system commissioned too quickly, before the timber has had time to equalise, can trigger cupping even where the screed itself was within its moisture tolerance at the point of laying.

If the screed is confirmed to be too wet, the timber floor generally has to be lifted so the screed can dry or be force-dried and re-tested. Re-laying without addressing the moisture source risks repeat cupping or, once the moisture eventually leaves, boards crowning or developing gaps instead.

Where the vapour barrier detail was the failure point rather than the screed's actual moisture level, correcting that detail before the timber is put back may be sufficient without further drying time.

Preventing recurrence starts with a documented moisture test of the screed against the timber flooring manufacturer's specific threshold, not a generic figure, since requirements vary between solid and engineered products and between fixing methods. Specifying and correctly detailing a vapour barrier at every junction, including around pipe penetrations and at perimeters, closes off one of the most commonly overlooked gaps.

Because timber flooring installers, the screeder, and sometimes an under-floor heating specialist all have a hand in the final moisture balance, agreeing responsibility for testing and sign-off before the timber goes down avoids ambiguity if cupping does appear later.