Failed Concrete Moisture Test and Cupping in a Clubhouse
A 4,000 ft² engineered 7½ in. wide white oak plank floor is glue-down over an on-grade concrete slab in a newly constructed clubhouse. The adhesive manufacturer's limit is 80% RH per ASTM F2170. The flooring contractor performed three in-situ RH probes and reports 78%, 81%, and 84% at 40% depth; the average is 81%. The general contractor says the slab was poured 45 days earlier and “looks dry.” Six weeks after installation, planks cup along the exterior walls, especially on the south side, and adhesive ridges are visible at seams. Indoor RH is 45%. The flooring contractor blames the concrete; the GC blames the flooring contractor for installing too early. No vapor retarder was specified under the slab, and the building's HVAC ran only intermittently during construction.
1. What testing and documentation should have existed before installation, and did the readings meet the adhesive manufacturer's requirement?
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ASTM F2170 requires in-situ RH probes at the specified depth-typically 40% of slab thickness-with proper hole cleaning and equilibration before reading. The number and placement of probes should represent the slab, including exterior walls and areas of different pour dates. The contractor should have recorded probe locations, depths, equilibration time, calibration, slab age, and ambient conditions.
The readings did not meet the adhesive manufacturer's limit. Three probes are a start, but the result is not an average to be compared with 80%. Each location must meet the limit. The 81% and 84% probes exceed 80%, and the 84% probe alone is enough to fail the requirement. A 45-day-old slab that “looks dry” is not evidence; concrete can be dry at the surface and still emit vapor. Installation should have stopped until the slab was remediated or a vapor-retarder or moisture-mitigation system and compatible adhesive were specified.
2. What is the most probable cause of the cupping, and how should responsibility be allocated under NWFA guidelines?
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The cupping pattern-edges high, centers low, concentrated along exterior walls-is classic moisture vapor drive from the slab. The south side warms more and may have higher vapor pressure; no under-slab vapor retarder and intermittent HVAC allow moisture to move into the adhesive and flooring. Adhesive ridges at seams suggest the adhesive was still tacky or the trowel notch and coverage were wrong, but that alone does not explain broad cupping.
Responsibility is shared. The GC or building owner controls the slab, vapor retarder, HVAC, and schedule; the flooring contractor controls acceptance testing and installation. Under NWFA guidelines, the installer must verify that the substrate meets the adhesive manufacturer's moisture limit before proceeding. The flooring contractor proceeded despite a failed probe; the GC provided a slab without a vapor retarder and without stable HVAC. The inspector should report both, not assign 100% to one party.
3. What re-testing and remedial options would you recommend?
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Remediation:
- Stabilize the building: run HVAC continuously, control RH, and dry the slab. Do not rely on surface drying.
- Re-test with ASTM F2170 after stabilization, using more probes, especially along exterior walls and near the 84% location. Also verify the adhesive manufacturer's current limit; some products allow 85% or require a specific mitigation system.
- If readings remain above the limit, install a qualified moisture-mitigation system or remove the flooring and adhesive, then reinstall with a compatible system.
- For affected planks, remove and replace cupped boards after the slab is dry; sanding a cupped floor before it is dry will cause crowning.
- Document probe locations, readings, and photos. Consider a joint site meeting with the GC, flooring contractor, and adhesive manufacturer.