Flooring Problem
Hollow or Spongy Spots in Glue-Down Wood & LVP Floors
Why do parts of my hardwood or vinyl floor sound hollow or feel spongy when I walk on them?
The Short Answer
What Causes Hollow Spots?
A hollow or spongy spot is a void: the flooring has lost continuous contact with the substrate beneath it. In a glue-down wood or LVP floor it means the adhesive bond broke or never fully formed there, or the concrete or subfloor was not flat, so the plank bridges a low area with air underneath. Tapping sounds hollow because you are drumming on an unsupported span — the bond, not the board, is what failed.
The consequence is progressive. An unsupported plank flexes with every step, which fatigues the joint or adhesive further and can lead to cracked boards, loosened planks, or telegraphed movement on LVP. The honest framing is that the fix is dictated by why the void exists: a flatness or adhesive gap can be re-bonded or injected, but if the root cause is slab moisture or a wrong adhesive, spot fixes will keep failing until the substrate itself is addressed.
How to Recognize Hollow Spots
A drummy, hollow knock underfoot or when tapped, sometimes with a slight give or a click, in isolated spots or lanes while the rest of the floor feels solid.
Root Causes
Why Hollow Spots Happens
Substrate not flat
The leading cause — NWFA calls for the substrate within 3/16 inch over a 10-foot radius, and planks over a low area bridge a void from day one.
Insufficient trowel coverage
Too little adhesive, the wrong trowel notch, or a spread that skinned over before the plank was set leaves un-bonded zones (per flooring adhesive manufacturer specs).
Slab moisture breaking the bond
Excess concrete moisture, tested per ASTM F2170 RH or F1869 MVER, degrades adhesive and releases planks — a real risk on Sacramento Valley slab-on-grade.
Wrong or incompatible adhesive
A construction or non-flooring adhesive that cannot move with the wood cracks its bond and releases boards, creating hollow and loose spots.
Floating-floor or underlayment issues
On floating LVP or laminate, a crushed pad, debris under the floor, or a subfloor hump creates the same drumminess with no glue involved.
What You Can Check
- Tap test: tap across the floor with a knuckle, a golf ball, or a metal tool and map the hollow-sounding areas — a solid tap means good contact, a drummy tap means a void.
- Load test: press or step and feel for give or a click; movement plus hollow sound points to bond loss rather than a minor low spot.
- Read the pattern: isolated spots suggest local flatness or adhesive gaps, while large lanes or spreading areas suggest slab moisture or systemic adhesive failure.
- Test the substrate cause: an RH probe (ASTM F2170) on the slab and a straightedge for flatness tell you whether to expect recurrence — high RH or a wavy slab means spot fixes alone will not hold.
Can It Be Saved? Repair vs. Replace
Minor isolated hollow, floor otherwise sound: monitor — a small void that is not growing, cracking, or lifting often needs nothing beyond watching and a dated note.
Bonded floor with a local void: a pro can inject a low-viscosity flooring adhesive or void filler through a small hole to re-bond the plank without removing it, the least invasive repair when the substrate is otherwise sound and dry.
Loose, cracked, or clicking boards over a flatness gap: lift the affected planks, correct the substrate to the 3/16-inch-in-10-foot tolerance, then re-set with the correct adhesive and full coverage (NWFA).
Widespread hollowness from slab moisture or the wrong adhesive: this is a system failure — the field must come up, the slab tested and mitigated (ASTM F2170/F1869, moisture membrane), and the floor reinstalled per spec; patching spots will keep failing.
Prevention
How to Prevent Hollow Spots
- Flatten the substrate first — grind or patch to NWFA's 3/16 inch in 10 feet and verify with a straightedge before any glue-down goes down.
- Test slab moisture before installing: ASTM F2170 in-situ RH (commonly a 75% limit, or the adhesive maker's limit) and/or F1869 MVER, and use the specified moisture membrane if the slab is wet (ASTM F710).
- Use the flooring maker's specified adhesive and trowel, and set planks into wet, not skinned-over, adhesive at full coverage.
- Acclimate the flooring and follow the adhesive's open-time and working-time limits so the bond forms fully across the whole plank.
Good to Know
Frequently Asked Questions
Is a hollow-sounding spot always a problem I need to fix?
Not always. A small hollow area that is not cracking, lifting, or spreading is often just an acoustic quirk and can simply be monitored. It becomes a real problem when you also feel give or a click, when boards crack, or when the hollow zone grows, because those signal an active bond failure or a substrate issue that will worsen underfoot. Map it, date it, and watch whether it spreads.
Can hollow spots be fixed without tearing up the floor?
Sometimes, yes. If the substrate is flat and dry and only the bond is missing, a pro can inject adhesive or void filler through a tiny hole to re-anchor the plank invisibly. But if the void comes from a wavy slab, slab moisture, or the wrong adhesive, injection is a band-aid, and the boards have to come up so the real cause is corrected or it returns.
The installer says the adhesive failed — is that usually true?
Adhesive gets blamed far more often than it deserves. Industry guidance from NWFA finds most hollow and loose spots trace to a substrate that was not flat, not dry, or not prepped, or to too little adhesive, rather than a bad product. The honest diagnosis checks slab flatness, moisture (ASTM F2170), and trowel coverage before condemning the glue.
We have a slab-on-grade home in the Sacramento Valley — does that make hollow spots more likely?
Slab-on-grade is common here and it raises two specific risks: the slab must be ground or patched flat before glue-down, and it must be moisture-tested, because even dry-looking Valley slabs can push enough vapor to break an adhesive bond. Insist on ASTM F2170 RH testing and the specified moisture membrane on grade — that is the difference between a floor that stays bonded and one that goes drummy in a year.
Think You Have Hollow Spots?
We'll inspect your floor, find the moisture or structural source, and tell you honestly whether it's a repair or a replacement. Call (916) 749-0272 for an assessment.