The floor did not fail. The slab did.
Bubbling, lifting edges and adhesive that never grabbed are almost never a vinyl problem. They are a moisture problem that was never measured, and by the time they appear the material is already down.
Why moisture destroys an adhesive bond.
Understanding what is actually happening makes the testing requirement obvious rather than bureaucratic.
A concrete slab holds water long after it is walkable. As that moisture migrates upwards and meets an impermeable covering, it cannot evaporate. It collects at the interface between adhesive and slab, where two things happen: the adhesive re-emulsifies and loses grip, and the alkaline moisture attacks the adhesive chemically. Resilient sheet vinyl is effectively a vapour barrier, so laying it over a damp slab traps the problem under the very material that reveals it.
The result is familiar. Bubbles that move under foot. Edges and seams lifting first because they are the weakest line. Adhesive still wet when a failed area is cut open months later. None of it is a manufacturing defect, and none of it is installation workmanship in the usual sense. The floor was laid onto a slab that was not ready.
Hot climates make this worse rather than better. A slab surface dries quickly in UAE conditions and reads dry to a surface meter while the body of the slab is still wet. Air conditioning then creates a vapour-pressure difference that actively drives moisture upward into a sealed building.
Surface readings are not evidence.
There is a real difference between a handheld meter and a test a manufacturer will accept.
A pin or capacitance meter held against the surface measures the top few millimetres. That is useful as a quick indicator and worthless as proof. The accepted method is in-situ relative humidity testing: probes are sealed into holes drilled to a specified depth in the slab, left to equilibrate, and then read. The result describes the moisture condition of the slab as a whole, which is what determines whether it will keep releasing moisture after covering.
| Method | What it measures | What it is good for |
|---|---|---|
| Handheld surface meter | Top few millimetres only | Quick screening; never acceptance |
| In-situ RH probe | Equilibrium humidity within the slab | Acceptance and manufacturer warranty |
| Hygrometer box on surface | Air above a sealed surface area | Indicative; slower, surface-weighted |
The number that matters is the threshold in the adhesive and covering manufacturer’s instructions, not a general rule of thumb, and it differs between products and between systems with and without a surface damp-proof membrane. Where readings exceed the threshold, the options are time, a liquid-applied moisture suppressant, or a different covering. All three have cost and programme consequences, which is exactly why the reading is needed early.
Probes need days, not an afternoon.
This is the practical reason moisture testing gets skipped, and the reason skipping it is expensive.
Equilibration takes time. A probe sealed into a slab needs a period measured in days before its reading is meaningful, and multiple positions are needed across an area because slabs are not uniform — edges, former wet areas, and anywhere a leak was repaired behave differently from the middle of a bay.
That has an obvious consequence for sequencing. If testing starts when the old floor comes up, the result arrives when the programme has already committed to laying. The test belongs earlier: where an existing covering can be lifted in a small area for access, testing can run while the rest of the programme is being mobilised, so the result is in hand before material is ordered.
- Test positions agreed across the area, not one reading taken centrally
- Probes installed early enough to equilibrate before the laying date
- Readings recorded against the manufacturer’s stated threshold, with the instruction referenced
- A written position on what happens if a reading fails, agreed before it does
An old slab is an unknown slab.
New construction at least has a known pour date. A refurbishment has history nobody wrote down.
On replacement work the slab may be twenty years old and bone dry, or it may have had a long-running leak under the previous covering that only becomes visible when the covering comes up. Neither can be judged by looking. The same lift also reveals what else needs correcting: bitumen residue from an older covering, several generations of adhesive, surface regularity that fails once the levelling effect of the old floor is removed.
This is why a quotation with no preparation allowance is a warning sign rather than a saving. Preparation is not an optional line; it is the part that decides whether the new floor lasts. A contractor who has priced it has looked. A contractor who has not will either return for a variation or lay over the problem.
Questions we are actually asked.
How long does subfloor moisture testing take?
In-situ relative humidity probes need several days to equilibrate before the reading is valid. That is why testing has to be planned into the programme ahead of the laying date rather than carried out once the old covering has been lifted.
Is a handheld damp meter good enough?
No, not for acceptance. A handheld meter reads the top few millimetres, which in a hot climate can be dry while the slab beneath is still wet. Manufacturers specify in-situ relative humidity testing because it describes the whole slab, which is what governs whether moisture will keep arriving after covering.
What if the reading is too high?
There are three routes: allow more drying time, apply a liquid surface damp-proof membrane suitable for the adhesive system, or change to a covering that tolerates the condition. Each has a cost and programme effect, which is the reason to test before ordering material rather than after.
Why do floors bubble months after installation?
Because moisture migrating from the slab has nowhere to go once an impermeable covering is laid. It collects at the adhesive line, weakens the bond and lifts the sheet, usually at seams and edges first. It appears months later because the moisture has to accumulate before the bond gives way.
Does this apply to LVT and tiles as well as sheet vinyl?
It applies to any impermeable covering and to the adhesive under it. Thresholds differ between products and systems, so the manufacturer’s stated limit for the specific adhesive and covering is the number to test against.
Tell us the slab age and what is down now.
We will set out the testing positions, where they sit in the programme, and what the preparation is likely to involve before any material is ordered.