David Powell explains why moisture is such a persistent threat, what it can do to a flooring system,
and why understanding it is vital to protecting your work and reputation
FLOORING contractors know moisture far better than they used to, and fewer jobs fail because of it as a result. But it hasn’t gone away as a risk. When it does cause a failure, it’s rarely a quick or simple fix.
This article sets out why moisture is such a persistent threat, what it can potentially do to a flooring system, and why understanding it properly is one of the most important things you can do to protect your work and your reputation.
Why moisture is different from other risks
Most risks on a flooring job are visible. A cracked screed. A contaminated surface. A substrate that isn’t flat. You can see these things, assess them, and decide how to deal with them. Moisture is different. It hides. A subfloor can look and feel bone dry on the surface while carrying relative humidity (RH) levels well above the thresholds set out in British Standards. RH, in this context, is the measure of moisture vapour present within the subfloor.
Any reading higher than 75% RH means the subfloor is not ready for most resilient and textile floor coverings. For timber, the threshold is lower still, at 65% RH.
The problem is you cannot see 76% RH. You cannot feel it or smell it. If you don’t test for it, you simply won’t know it’s there until after the floorcovering goes down and the problems start.
What moisture does to a flooring system
The mechanism of failure depends on what the moisture encounters. Lay a water-based adhesive onto a subfloor with excessive moisture and the floorcovering appears bonded at first, but over weeks or months the adhesive can re-emulsify and the bond breaks down. The result is a floor that lifts, bubbles, or becomes soft and spongy underfoot.
Some types of smoothing compounds are also vulnerable. A screed with high moisture content can cause a smoothing compound to delaminate from below, especially if moisture continues to move upwards after application. The compound may look perfectly sound on the day of installation and fail weeks later once the moisture has done its work.
Then there is the floor covering itself. Impermeable materials such as vinyl act as a barrier. Once laid, the vinyl prevents the moisture already trapped in the subfloor from escaping upwards. That moisture has nowhere to go, so it concentrates at the interface between the subfloor, the adhesive, and the underside of the floorcovering. That’s exactly where you don’t want it.
Where the moisture comes from matters
Not all moisture is the same, and understanding the source affects how you deal with it. On newbuilds, the moisture you’re most likely to encounter is residual construction moisture. When a screed is mixed and laid, a significant amount of water goes into it. That water takes time to evaporate.
Standard cementitious screeds dry at roughly 1mm per day for the first 50mm, then 0.5mm per day for the remaining thickness, and only under the right conditions: 20deg C and 65% ambient RH. On most UK sites, those conditions are rarely sustained.
Cold buildings, poor ventilation, wet trades still working nearby, and programme pressure all slow the drying process considerably.
These figures are widely used as a rule of thumb, but actual drying rates vary by screed type, thickness, site conditions and manufacturer guidance.
Nowadays, in addition to traditional sand and cement screeds, installers encounter a wide range of substrates including cementitious flow screeds, calcium sulphate screeds, concrete, power-floated concrete, and proprietary fast-drying screed systems.
On older buildings without a structural DPM, ground-bearing moisture is a different issue. Here the moisture is not temporary. It’s a permanent, ongoing source that requires a surface-applied dampproof membrane (DPM) to suppress it. Misidentifying one type as the other leads to either unnecessary cost or, more seriously, inadequate treatment.
The role of UK construction practices
There is a broader context worth understanding here. One of the reasons moisture is such a persistent problem on UK sites is the speed at which buildings are built and handed over. In much of Europe, construction allows each phase to cure and settle before the next begins.
In the UK, programme pressure means floor coverings are often specified before screeds have had the time they need to dry properly.
This isn’t a new observation, and it’s not one that sits at your door. But it does mean the subfloor you inherit is frequently not in the condition it should be. That reality makes testing even more important, not less. You cannot control what happens before you arrive on-site. You can control whether you lay flooring onto a subfloor that is ready.
Underfloor heating systems
In all instances, UFH systems must be run through a full drying/commissioning cycle of heating up and cooling down before any subfloor preparation or installation of decorative floorcoverings. This period of commissioning is usually around 21 days.
After a full commissioning cycle, the system must be turned off for a minimum of 48 hours. After this point, a moisture test must be carried out to determine the moisture content of the substrate.
Alternatively, a recommendation for utilising a moisture control system must be given.
Why testing isn’t optional
BS 8204 Part 1 is direct on this point. It places the responsibility for confirming that a subfloor has reached a suitable moisture level squarely with the installer, before installation begins. If floorcoverings are installed onto a subfloor that doesn’t meet the moisture requirements set out in the relevant British Standard, responsibility may rest with the installer unless evidence shows otherwise.
The right approach is to test, record your results, and act on them. If the subfloor is above threshold, you have two options: wait until it has dried to the required level or apply an appropriate surface DPM to suppress the residual moisture.
Which option is right depends on the source of the moisture and the specifics of the job. What is not an option is proceeding without testing and hoping for the best.
A non-invasive moisture meter gives you a quick indication of problem areas across a larger floor. For a definitive result, you need a hygrometer test, where a sealed, thermally insulated box is fixed to the subfloor and allowed to reach equilibrium before the RH reading is taken. This is the test method specified in BS 8203, BS 8201, and BS 5325, and the result it produces is the one that matters when questions of liability arise. Record the date, location and result of every test. That documentation is your evidence if anything goes wrong.
The installer who tests is the installer who wins
There’s a commercial argument here as well as a technical one. Contractors who test thoroughly and can explain what they’re doing and why are consistently better placed to win work, justify delays, and defend their installations when things go wrong elsewhere in the build. It signals professionalism in a way that’s immediately visible to clients who’ve been let down before.
Moisture will keep being a cause of flooring failure for as long as UK construction programmes stay compressed and site conditions stay unpredictable. Knowing that, testing properly, and acting on what you find is the clearest line between an installation that lasts and one that comes back to haunt you.
david.powell@bostik.com
www.bostik-profloor.co.uk
David Powell is technical consultant at Bostik UK
