Crowning After Sanding a Cupped Kitchen Floor
A 3/4 in. solid white oak floor in a 12 ft by 18 ft kitchen was installed nail-down over plywood and finished on site 14 months ago. In June the homeowner noticed cupping: board edges about 1/16 in. high across the dishwasher and sink area. A plumber found a slow supply-line leak under the sink and repaired it. Two weeks later, the flooring contractor sanded the entire floor flat and applied two coats of water-based finish. Three weeks after sanding, the center of each board is now high and the edges are low, with visible ridges along seams. Moisture readings: flooring 9.5%, subfloor 8.2%, indoor RH 52%. The finish is intact but the floor feels uneven.
1. Explain the mechanism that produced the crowning after sanding.
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Crowning after sanding a cupped floor is a geometry and moisture problem. A cupped board is concave: the edges are high and the center is low. When the contractor sands the floor “flat,” the sander removes the high edges and leaves the lower center closer to its original thickness. If the boards then dry and flatten-or if the center was still swollen-the center sits higher than the sanded edges. The result is a crowned appearance and ridges at seams.
Sanding did not fix the moisture; it removed the evidence of cupping and changed the board profile. The elevated flooring MC (9.5%) versus subfloor (8.2%) and 52% RH show the floor was not at equilibrium. The correct approach is to correct the moisture source, dry the floor to equilibrium, and only then evaluate whether sanding is needed.
2. What should the inspector measure and document, and what is the correct corrective sequence?
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Measure:
- MC of flooring across the affected boards and unaffected areas, using several readings per board and a calibrated meter.
- Subfloor and joist MC; check for elevated moisture under the sink and along the leak path.
- Indoor RH and temperature over 24 hours.
- Flatness with a straightedge and feeler gauge; record crown height and seam ridges.
- Finish thickness and whether the finish is bridging.
- Verify the leak repair and any remaining moisture.
Document with photos, a floor plan, and dates.
Correct sequence:
- Confirm the leak is repaired and the area is dry.
- Dry the building and floor to equilibrium; do not sand while MC is elevated.
- Re-measure until flooring and subfloor are stable and within an acceptable differential.
- If the boards are still crowned from the previous sanding, replace affected boards or resand only after equilibrium; then apply the full finish system.
- Do not simply recoat; the unevenness is in the wood, not the finish.
3. Was sanding the cupped floor appropriate? Explain.
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No. Sanding a cupped floor before the moisture source is corrected and the floor is at equilibrium is a known cause of crowning and unevenness. The contractor should have documented the cupping, identified the leak, dried the floor, and waited for stable MC before sanding. Sanding removed the high edges, so when the boards flattened the centers appeared high. This is a workmanship issue as well as a moisture issue. The inspector should note the sequence and the MC readings; the appropriate remedy is replacement of damaged boards or a controlled resand after drying, not another coat of finish.