Waterproof sheet protection over a floor that still has moisture in it creates exactly the conditions for surface problems: trapped vapour, discolouration, soft patches and sometimes bond failure in coatings. This is one of the few cases where protection actively causes the damage it was bought to prevent.
Which floors need to breathe
Cementitious screeds and concrete in early cure need to lose moisture. Some timber systems need to equalise rather than be sealed. Certain stone and some coatings are sensitive to vapour being held at the surface. In each case the flooring contractor will state a period before impermeable coverings may be applied – get that in writing rather than assuming.
What actually happens
Trapped moisture does not always show immediately. The classic pattern is protection laid early, removed at handover, and a patchy dullness or staining that will not clean off. More seriously, trapped vapour under a resin coating can cause blistering or debonding, which means cutting out and replacing rather than cleaning.
Give it a way out
Where protection must go down early, provide ventilation. Leaving gaps at the perimeter, lifting panels periodically to air the floor, and avoiding sealed joints all help. Lifting every few days in the early period is tedious but far cheaper than a floor repair – and it doubles as the inspection walk recommended anyway.
Spilled water is a separate problem
Apart from trapped vapour, liquid gets under protection from above – wet trades, cleaning, leaks. A lipped or overlapping joint keeps most of it out, and a sacrificial top layer catches the rest. Where standing water does get underneath, the panels need lifting and drying rather than leaving it to evaporate through the joint.
Timing against the programme
The classic conflict: the floor needs another week and the programme needs access tomorrow. Where protection is essential for other trades to work, consider a temporary ventilated route – sacrificial runners along the traffic path only – rather than covering the whole floor.
Checking afterwards
Where there is doubt about whether a screed is dry enough, relative humidity testing gives an answer rather than a guess, and most flooring contractors can carry it out. The cost of a test is trivial against a delayed handover or a failed coating. We make the same argument about testing generally in testing without a laboratory.
Reading the numbers rather than guessing
Relative humidity testing in a screed gives a defensible answer about readiness, and the method and thresholds are standard enough that most flooring contractors can carry it out. The alternative – judging by eye or by calendar – fails often enough that a test costing a few hundred units protects a material installation costing tens of thousands.
Record readings with dates and locations. Where a reading is borderline, protecting only the traffic routes buys another week for the rest of the floor rather than risking the whole area, and that compromise is usually available even when the programme says otherwise.
Where escaping vapour comes from below
Groundbearing slabs without an effective damp proof membrane push moisture upward continuously, and no amount of ventilation above will remove it. Where protection is going onto such a slab for a long period, lifting periodically to air the surface is not optional – it is the only control available. Sequencing wet trades away from finished floors is the other half, described in keeping wet trades off finished floors.
What we can do
We supply PP sheet and protection panels with joint details that can be laid vented or sealed, and we will tell you plainly when a floor should be left uncovered. Send the floor build-up and the flooring contractor’s guidance and we will advise rather than sell.