Subfloor Moisture and Preparation Before Welded PVC Sheet Flooring

1. Why moisture decides the job

A welded PVC sheet is a vapour barrier. Once it is glued down and welded, any residual moisture in the slab has nowhere to go. It accumulates under the sheet until the vapour pressure exceeds the bond strength of the adhesive — and then it lifts, usually in the middle of a floor rather than at an edge where someone would notice early.

• Blistering — sheet lifts in domes 100–300 mm across, most often over a slab that passed a visual check but was never tested.
• Seam lifting — the weld stays intact but the sheet peels back from the weld line, because the adhesive beneath has been plasticised by moisture.
• Efflorescence — white salt deposits at seams and perimeter, carried up by water migrating through the slab.

None of these are visible at handover. That is why the moisture test is not a formality: it is the only point in the project where the risk is still cheap to fix.

2. Two test methods — pick one and keep it

There are two accepted ways to measure moisture in a concrete slab, and their results are not convertible. Choose one at specification stage and apply it to every reading on the project.

In-situ RH (F2170) has largely replaced the calcium chloride dish on commercial projects: it is faster, less sensitive to site air movement, and gives a number that is easier to write into a specification. The calcium chloride test remains common on refurbishment work and wherever the specification predates 2010.

3. What the numbers mean — and why “dry” is not a specification

Most resilient flooring adhesive data sheets describe the substrate as “dry, firm, load-bearing, dust-free”. That is a workmanship instruction, not a specification: it cannot be measured, and it cannot be argued about after a failure. A moisture requirement needs three things — a number, a method, and a test density.

The thresholds below are the values commonly written into commercial specifications and adhesive data sheets. They are industry practice rather than a claim unique to us; on any given project the governing figure is the one in the adhesive TDS supplied with the order.

Read the table as a gate, not a target. A slab at 78% RH is not “nearly there”; it is a slab that needs a moisture-suppression primer or a different construction. Planarity is a separate gate — covered in the installation sequence, where the limit is 2 mm under a 2 m straightedge.

4. When it fails: three remedies, in order of cost

1. Two-component epoxy moisture-suppression primer (DPM). The standard answer for readings in the 75–85% band. It seals the slab and lets the self-levelling compound go down on top. Two conditions apply: the slab must be sound enough to take it, and the DPM must be compatible with the levelling compound above it — mixing systems from different suppliers is where this goes wrong.

2. Self-levelling compound plus the correct primer. Fixes planarity and slows moisture in one operation, but only within limits. A cement-based levelling compound is not a moisture barrier; it slows vapour transmission rather than stopping it. Around 85% RH and above it is not enough on its own.

3. Change the construction. If the programme does not allow a seven-day DPM cure, a floating SPC floor takes the adhesive out of the equation — the boards are not bonded to the slab, so slab moisture has an escape path at the perimeter. The trade-offs are a different feel underfoot and a different acoustic profile, and SPC is not the right answer for a welded, coved, fully sealed hospital corridor.

The decision is a schedule decision as much as a technical one. A DPM adds roughly a week; changing to floating SPC changes the specification the client signed off.

5. Site checklist before the first sheet goes down

• Concrete at least 28 days old and visibly dry — test it anyway
• Moisture test done at the specified density, results recorded with location, method and date
• Surface laitance removed by mechanical preparation, not acid etching
• Oil, curing compound and old adhesive residue removed
• Planarity within 2 mm under a 2 m straightedge
• Primer applied at the supplier’s ratio, never diluted, and allowed to reach the specified tack time
• Substrate temperature and dew point recorded on the day of installation
• Adhesive open time respected — in hot climates it shortens dramatically

Frequently Asked Questions