SW-04 · Troubleshooting & RepairsSome hollow sound is normal in a floating floor; NWFA says no remedy is needed when there is no vertical movement.1 A hollow spot that flexes underfoot is a void, usually a dip in the slab beyond the guide's flatness tolerance, such as 3/16 inch in 10 feet.2 3 Movement over a void can break tongues and chip edges, so the fix is to lift the planks and fill the dip.1
Tap a heel across a click-lock vinyl floor on a concrete slab and the sound changes from place to place. Some areas are solid. Others sound hollow, or drummy. Owners notice it in quiet rooms, and facility managers notice it in corridors. The sound alone is not a defect. What matters is whether the floor moves where it sounds hollow.
Why a floating floor sounds hollow
A floating floor sits on the slab rather than being bonded to it. There is always a thin layer of air, pad or underlayment between them, and in places the plank bridges tiny variations in the surface. The NWFA's Wood Floor Facts sheet on hollow spots makes the point for floated wood floors. One of the most common causes of hollow sounds is what would be considered normal for a floating installation.1 It says hollow sounds are not a direct concern as long as no movement accompanies them, and that no remedy is necessary when vertical deflection is not present.1 The same logic applies to vinyl plank: listen, but judge by movement.
When it is a defect
The NWFA sheet draws the line clearly. When a void under the flooring is tied to vertical deflection, performance may become a factor, and movement could cause the flooring tongues to break or edges to chip.1 Among the common causes it lists slight undulations in the subfloor that make planks bridge a dip, causing hollow sounds or deflection.1 For a click-lock floor, a plank that flexes every time it is stepped on works its locking profile. Over months, that shows up as a joint that clicks, peaks, opens or breaks.
Timeless Designs warns that a subfloor not leveled to its specification puts extra stress on the tongue and groove, resulting in plank separation.4 It asks installers to check for dips or humps that can create a void or peak under the floor.4
The flatness tolerances
Every rigid-core guide sets a flatness limit for the substrate, and voids are usually dips beyond it.
| Source | Flatness tolerance |
|---|---|
| Marquis rigid vinyl plank | 3/16” in 10’ or 1/8” in 6’2 |
| EverStep click-lock LVP | Fill low spots greater than 3/16 inch in a 10 foot span3 |
| Timeless Designs | 1/8” in 10’4 |
| National Flooring Products | 3/16” in 10’ or 1/8” in a 6’ radius5 |
A concrete contractor's guide to floor preparation describes flatness as the requirement that avoids uneven wear, hollow spots or an irregular finished surface.6 It notes the difference from level: a floor can be flat without being level, and flatness is what the flooring needs.6 See the flatness guide for how to measure it.
Finding the void
- Sound and feel. Walk the area slowly. Mark spots that sound hollow, then step on each and watch the joints for movement.
- Look at the seams. A joint that rises or opens slightly under load sits over a void.
- Map the pattern. Hollow spots in a line often follow a slab joint, a trench or a patch. Scattered spots usually mean general unevenness.
- Check the slab. Where planks can be lifted, put a straightedge on the concrete and measure the gap under it against the guide's tolerance.
Record each spot with a photo and its location. If the floor goes to a warranty claim, the pattern matters more than the sound.
Common causes on concrete
Slabs are rarely as flat as their drawings suggest. Typical sources of voids are dished areas between high points and curled slab edges at joints. Trowel marks, patches left proud or low, and self-leveler that did not flow to a consistent plane add more. A self-leveling pour that ends short of the walls can leave a step. Old adhesive ridges and debris under the planks also lift them, creating small voids beside them. See soft spots in self-leveler for problems in the pour itself.
Too much cushion
Extra underlayment is a cause that sits in the floor rather than the slab. Many rigid planks come with a pad attached, and the joint is designed around it. Elite says its click end joints are designed to work with its attached pad, and it does not warrant the product with an additional pad installed under it.7 Marquis says additional underlayment may not be installed under products that already have attached cushions.2 A second soft layer lets the plank sink under each step, which sounds hollow and loads the lock the same way a dip does. If a floor sounds hollow everywhere and flexes evenly, check what is underneath before measuring the slab. See the underlayment guide.
The fix
For a floating floor, the reliable repair is to lift the planks back to the void, correct the slab and relay the floor. EverStep says low spots beyond its tolerance should be filled with a Portland cement leveling compound.3 Elite says high spots must be ground down and low spots filled with a Portland cement leveling compound.7 Unlock the rows carefully. Timeless warns that planks must stay flat and be slid apart, because lifting them damages the end joint.4
For glued floors, NWFA names adhesive injection into the void, or replacing individual boards, as the most common repairs for performance-related hollow sounds.1 That applies to adhered floors, not floating ones, because a floating plank is not meant to be bonded down.
Commercial floors
Hollow sounds draw more complaints in quiet offices, clinics and multifamily corridors, and rolling loads make any deflection worse. On commercial jobs, measure the slab against the guide before installation and record it, so a later complaint can be judged against that record. Specifications written for the concrete trade often use F-numbers instead; Daltile notes that overall values of FF 36 and FL 20 may be appropriate for resilient floor coverings.8 Make sure the slab spec and the flooring guide agree before the pour, not after. Where the floor will carry carts or beds, consider whether a glue-down product suits the space better than a floating one.
Sources
- National Wood Flooring Association (Wood Floor Facts, via Floor Supply Network), “Wood Floor Facts: Hollow Spots”. gehlflooring.com/wp-content/uploads/2019/02/WFF_-_Hollow_Spots.pdf
- Marquis Industries, “Rigid Vinyl Plank Installation Instructions (Angle)”. marquisind.com/static/resources/Angle_Angle%20Installation%20Instruction
- EverStep Flooring, “LVP Click Lock Installation Guidelines”. everstepflooring.com/lvp-click-installation-guidelines/
- Timeless Designs, “Click Floating Luxury Vinyl Installation Instructions”. timelessdesignsflooring.com/wp-content/uploads/2023/05/TD-Rigid-Core-Ins
- NFP, “Architect’s Choice LVP Installation Guide”. my-nfp.com/resource/architects-choice-lvp-installation-guide
- HP Subfloors, ““Flat is a Must – Level is a Plus”: A Guide to Perfect (Or Near-Perfect) Floors - Schonox | A Sika Brand”. hpsubfloors.com/flat-is-a-must-level-is-a-plus-a-guide-to-perfect-or-nea
- Elite Flooring, “Välinge Installation Guide (Woodlands)”. eliteflooringusa.com/links/install-instructions/Elite-Installation-Guide
- Daltile, “ProSeries Luxury Vinyl Flooring Installation & Care Guide (Rigid Core Click, Glue Down)”. s1.img-b.com/build.com/mediabase/specifications/daltile/1996309/daltile-
General information for the flooring trade. This article was drafted with AI assistance and checked against the sources listed above; the manufacturer's current instructions and the project specification govern any real job. If something here is wrong or out of date, tell us.
