Cupping Planks on a 1920s Slab
A 1920s brick-and-timber boutique hotel is renovating 42 guestrooms on floors two through four. The designer specified a 7-inch engineered oak plank with a 2 mm wear layer, full-spread glue-down over a 10-mil vapor retarder on the existing concrete floors. The ground floor is slab-on-grade; upper floors are slab on metal deck. Before installation the flooring contractor ran two calcium chloride tests on the ground-floor slab, which read 5.8 and 6.2 pounds per 1,000 square feet per 24 hours, and recorded 86 percent relative humidity with in-situ probes. The specification was issued 14 months before installation, names a specific pressure-sensitive adhesive and a 3/16-inch trowel, and says nothing about moisture limits, a mockup, or a warranty. Six weeks after the ground-floor rooms opened, planks are cupping at the seams, adhesive is transferring to the concrete, and two rooms smell musty. The contractor has submitted a $61,000 change order to replace the ground floor.
1. Diagnose the failure. What is the most likely mechanism, and what testing would you require before accepting that diagnosis?
Read the worked answer
The numbers already tell most of the story. Calcium chloride results of 5.8 and 6.2 pounds are roughly double the 3-pound threshold most adhesive and flooring manufacturers publish, and 86 percent relative humidity exceeds the typical 75 to 80 percent limit for in-situ probes. The slab is emitting vapor faster than the adhesive and the plank back can release it, so vapor pressure builds under the flooring, plasticizes the pressure-sensitive adhesive, and drives differential swelling at the plank edges. That produces exactly the observed trio: cupping at seams, adhesive transfer to the slab, and a musty odor from biological growth in the damp adhesive layer.
Before accepting it, require: a properly distributed in-situ probe survey per ASTM F2170 at 40 percent of slab depth, not two probes; pH and alkalinity testing, since high pH attacks adhesives; confirmation of whether a vapor retarder exists under the slab; adhesive and plank retained samples sent to the manufacturer for analysis with the lot numbers; and a moisture content check on the planks themselves to rule out a wet install or a building that was not dried in. Also verify the HVAC was operating and stable at service conditions before, during, and after installation. Two probes in 42 rooms is not a survey, and the contractor's own data was never escalated.
2. Where did the specification and the construction-phase process fail? What should have been required at each stage?
Read the worked answer
The specification failed in four places, and each is a documented lesson rather than a hindsight complaint.
- No moisture criteria. A wood flooring section over concrete must state the maximum slab emission and relative humidity, the test methods, and the number and location of tests, and must require compliance with the manufacturer's published limits, whichever is stricter.
- Stale product data. A 14-month-old spec naming one adhesive is a risk; adhesives are reformulated and the named product may no longer be the manufacturer's recommendation for a high-moisture slab. The section should require the contractor to submit current manufacturer instructions for the specific assembly and to confirm the adhesive is approved for the measured conditions.
- No mockup and no pre-installation conference. A single-room mockup held for the duration of the project, with the same slab, adhesive, and conditioning, would have failed visibly before 42 rooms were installed.
- No warranty and no field quality control. Require a manufacturer's system warranty, documented slab readings logged by room, and a hold point where installation cannot proceed until the readings are accepted in writing.
The construction-phase failure was procedural: the contractor recorded failing numbers and installed anyway, and nobody issued a nonconformance. That is the contractor's error, but the spec gave no hold point to trigger it.
3. How do you document the failure and advise the owner on cost allocation and next steps?
Read the worked answer
Do not approve the change order as written. Document first.
- Issue a written notice to the owner and contractor describing the observed condition and the recorded test data, and state that the cause is under investigation.
- Photograph and log every affected room; cut a test area to expose the adhesive and slab; retain sealed samples of plank, adhesive, and slab core.
- Request the contractor's daily reports, adhesive lot data, and the manufacturer's technical opinion.
- Notify your professional liability carrier if you issued the specification, because the missing moisture criteria is a design document gap.
On allocation: the contractor installed over readings that failed any reasonable manufacturer limit without written manufacturer approval, which is a strong contributor. The designer's omission of moisture criteria and a hold point is also a real contributor. Expect a shared resolution rather than a clean win.
On next steps: the fix is not new plank over the same slab. Specify a moisture mitigation system with a documented perm rating, a new mockup, a pre-installation conference, a revised adhesive selected for the mitigated slab, logged readings per room, and a manufacturer's system warranty. Award the repair as a negotiated change with the mitigation scope priced separately from the replacement.