Hollow Spots and Bounce in a Glue-Down Engineered Floor
A 5,000-square-foot glue-down floor of 5/8-inch by 5-inch engineered hickory was installed over an 8-year-old concrete slab on grade. The occupant reports hollow sounds and a soft feel near a control joint. Moisture testing with an in-situ RH probe reads 72%, within the adhesive manufacturer's limit, and calcium chloride gives 3.5 lb per 1,000 square feet, also within limit. A 10-foot straightedge shows a 1/4-inch dip within the 10-foot span near the control joint and an 1/8-inch step at a patched area. Adhesive coverage is about 60% in the hollow zones and 90% elsewhere. Several end joints have opened 1/16 inch. The floor is otherwise bonded and flat away from the joint.
1. Evaluate the slab flatness against the applicable tolerance and explain the consequence for the installation.
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The slab does not meet the flatness tolerance.
- The commonly applied wood-flooring tolerance is 3/16 inch in 10 feet, or 1/8 inch in 6 feet. A 1/4-inch dip within a 10-foot span exceeds that tolerance, as does the 1/8-inch step at the patch.
- Flatness is an installation prerequisite, not an optional check. The installer is responsible for verifying it and for correcting low spots with a suitable patching compound before spreading adhesive.
- A dip near a control joint is a common location for both slab movement and poor finishing. The control joint itself must be honored according to the adhesive and flooring manufacturer's instructions.
- Because the dip was not corrected, the flooring could not be pressed into full contact with the adhesive across the low area, which explains the hollow sound and the soft feel.
The flatness deficiency is a direct, documented contributor to the complaint.
2. Distinguish among flatness, adhesive coverage, and structural deflection as causes of the hollow spots.
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Three mechanisms can produce a hollow sound, and the evidence separates them.
- Flatness: The 1/4-inch dip and the 1/8-inch step exceed tolerance and correlate with the hollow zones. This is the primary cause.
- Adhesive coverage: Coverage of 60% in the hollow areas is below the typical requirement of 80–90% for glue-down engineered flooring. Even on a flat slab, 60% transfer would leave voids. The installer may have compensated for the dip by using less adhesive or by failing to roll the floor.
- Structural deflection: A soft feel can also indicate slab or subbase movement. Test by loading the area and measuring vertical movement, and check for cracks, hollow-sounding concrete, or a suspended slab. If the slab itself deflects, that is a structural issue outside the flooring contractor's scope.
A practical sequence is: measure flatness, map adhesive transfer, then load-test for deflection. In this case flatness and coverage are both deficient and together explain the symptoms. Deflection should still be ruled out before issuing the report.
3. What corrective action and reporting conclusions are appropriate?
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Corrective action
- Remove and replace the affected flooring in the hollow areas.
- Grind or patch the slab to meet flatness tolerance before reinstalling.
- Honor the control joint with the specified separation or approved detail.
- Reinstall with full adhesive coverage, using the correct trowel notch and rolling procedure.
- Verify moisture again before reinstalling, even though the earlier tests passed.
Reporting conclusions
- The slab was out of flatness tolerance before installation, and the installer did not correct it.
- Adhesive coverage in the affected zones was insufficient.
- Moisture was within the adhesive manufacturer's limits, so moisture is not the cause.
- The flooring product itself is not implicated; the failure is installation-related.
Caution
- Do not recommend injecting adhesive as a universal fix. It may reduce the hollow sound temporarily but will not correct the slab dip or the underlying coverage problem.
- Document the straightedge measurements, coverage percentages, and moisture results with photographs and a sketch of the affected zones.