Subfloor moisture is the silent failure mode of every engineered floor. A new slab can read four per cent relative humidity at the surface and nine per cent three centimetres down; the floor lays cleanly, looks correct for a year, and then cups. The fix is patience — and a calcium chloride test before delivery.
This article is for builders and fitouts who specify timber over concrete. It is not a comprehensive installation manual; consult Topdeck's installation guide and AS 1684 for the full specification. What follows are the questions we are asked most often by people running the program on site.
Why slab moisture matters
Engineered timber is a multi-layer construction: a hardwood wear layer (commonly 3–4mm) bonded to a multi-ply plywood core. The plywood is dimensionally stable in normal humidity, but it is not waterproof. Moisture migrating from a slab into the underside of the floor will be absorbed by the plywood; the plywood will expand; and the expansion will manifest at the surface as cupping (board edges lift), crowning (centres lift), or — at worst — adhesive failure.
The challenge is that surface moisture is not the same as subsurface moisture. A slab can be touch-dry on top and still be saturated below. The standard residential reading instrument — a pin or pinless surface meter — only measures the top millimetre or two. For the full picture, calcium chloride is required.
The calcium chloride test
A calcium chloride test (per ASTM F1869) measures the moisture vapour emission rate from a slab. The procedure: a sealed dish containing pre-weighed calcium chloride salt is taped to the cleaned slab for 60 to 72 hours. The salt absorbs moisture vapour rising from the slab. After the test period, the dish is reweighed and the gain converted to pounds per 1,000 square feet per 24 hours.
For engineered timber, the threshold is typically 3 lbs / 1,000 sq ft / 24 hours or less. Above this, installation is deferred until the slab dries further. There are products — moisture-tolerant adhesives and vapour barrier sheets — that allow installation above 3 lbs, but they sit outside the standard warranty and should be a contingency, not a default.
Newer slabs typically test high. A 100mm slab in average Sydney conditions takes roughly 30 days per inch to reach 80% RH internal — so an industrial 200mm slab is often six months from pour to readiness. Specifiers are advised to write this into the program early.
A slab can be touch-dry on top and still be saturated below.
Three failure modes to recognise
One. Cupping at twelve months. Visible board-edge lift across most of the floor. Root cause: moisture rising into the plywood core. Fix: lift, dry the slab to spec, relay (often with full board replacement, as cupped boards rarely flatten reliably).
Two. Crowning at six to eighteen months. Visible board-centre lift, often patchy. Root cause: moisture from a localised source (plumbing leak, dishwasher leak, slab penetration). Fix: source first, then lift and dry. Crowning is harder to diagnose because it can be local rather than global.
Three. Adhesive failure with hollow-sounding boards. Walk the floor; tap suspect areas. Root cause: usually a combination of moisture and inadequate adhesive coverage at install. Fix: lift, reglue.
Specification language for the program
When writing the construction program, the following clauses are useful:
- Slab to be cured a minimum of 28 days from pour before flooring trade attends site.
- Calcium chloride test (ASTM F1869) to be conducted no more than 72 hours before flooring delivery.
- Test result to be supplied in writing; threshold for installation is 3 lbs / 1,000 sq ft / 24 hours.
- If the slab exceeds threshold, installation is rescheduled and the slab re-tested.
- Builder responsible for site-protection of the dried slab between test and installation.
For most residential projects, this is sufficient. For commercial work — a hotel lobby, a retail fitout — a more conservative threshold (2 lbs) is sensible, and a vapour barrier may be specified regardless.
The patient version
The shortest version of this article: do not install timber over green concrete. The longest version: every slab is a chemistry experiment with its own timeline, and the only reliable instrument is a calibrated test. Patience saves the floor.
