A hollow spot under glued-down vinyl planks — that telltale drumming or clicking sound underfoot — is usually caused by adhesive that has failed to bond, dried out before the plank was laid, or a small void in the subfloor beneath. The good news is that many hollow spots can be fixed without pulling up the floor, using a simple adhesive injection technique. Here's how to diagnose the problem and sort it out.
What Causes Hollow Spots?
Before you reach for the tools, it helps to understand why the hollow formed:
- Adhesive failure – The glue dried out (flashed off) before the plank was pressed down, or the wrong adhesive was used for the substrate.
- Subfloor voids – A dip, low spot, or delamination in the subfloor (common with older particleboard or plywood) means the adhesive never made full contact.
- Moisture movement – Seasonal swelling and shrinking of a timber subfloor can break the adhesive bond over time.
- Insufficient trowel coverage – If the original installer used too fine a trowel notch, there simply wasn't enough adhesive to grip the full plank.
Tap the floor gently with your knuckle or a small rubber mallet to map out the hollow area. Mark the edges lightly with masking tape so you know exactly what you're dealing with.
Tools and Materials You'll Need
- Large-gauge syringe (60 ml or larger, available from flooring suppliers or some hardware stores)
- Vinyl plank adhesive or flooring repair adhesive compatible with your existing glue
- Masking tape and a marker
- Clean cloths and mineral turpentine (for adhesive clean-up, if solvent-based)
- Roller or heavy book to apply pressure
- Weights (paint tins, heavy books) or floor roller
Check with your adhesive supplier or a store like Mitre 10 or Bunnings to confirm you're using an adhesive compatible with luxury vinyl plank (LVP). Using the wrong product can cause the plank to lift further or stain.
Step-by-Step: The Syringe Injection Method
This method works best when the hollow is small to medium (up to roughly 300 mm across) and the plank itself is undamaged.
1. Soften the Plank Edge (if needed)
For a hollow near the centre of a plank, you may be able to inject adhesive through the seam between planks without lifting anything. If the seam is tight, use a utility knife to very carefully open a small gap — just enough for the syringe tip.
2. Load the Syringe
Fill the syringe with a suitable vinyl flooring adhesive. Pressure-sensitive or contact adhesives designed for LVP are commonly used — ask your supplier for a recommendation suited to your specific plank type and subfloor.
3. Inject the Adhesive
Insert the syringe tip into the seam or a small drilled hole (2–3 mm diameter) and slowly inject adhesive into the void. Work from one side of the hollow to the other, moving the syringe tip as you go to distribute the glue evenly. Don't overfill — you want to fill the void, not force adhesive up through the seams.
4. Press and Weight the Plank
Immediately press the plank down firmly by hand, then place heavy weights (paint tins, stacked books) over the area. If you have access to a floor roller, roll over the area with firm pressure. Leave the weights in place for the time specified on the adhesive packaging — typically 24 hours.
5. Clean Up
Wipe away any adhesive that squeezes out of the seams straight away, using a damp cloth (for water-based adhesive) or a cloth lightly dampened with mineral turpentine (for solvent-based). Dried adhesive is much harder to remove and can damage the plank surface.
6. Test the Fix
Once cured, tap the area again. A solid sound means success. If it still sounds hollow, the void may be too large or the subfloor itself may need attention.
When the Subfloor Is the Problem
If multiple planks are hollow, or the hollow keeps coming back, the issue is likely with the subfloor rather than the adhesive. Common culprits in NZ homes include:
- Delaminating particleboard — very common in homes built from the 1970s to 1990s, especially where moisture has been present.
- Uneven concrete slabs — high spots can prevent full adhesive contact across the plank.
- Moisture from below — a crawl space without adequate ventilation can cause ongoing adhesive failure.
For subfloor issues, the planks may need to be lifted, the subfloor repaired or levelled with a suitable floor levelling compound, and the planks re-laid. BRANZ has useful guidance on subfloor preparation standards for NZ conditions.
Moisture in concrete slabs is a particular concern — the NZ Building Code (specifically clause E3) addresses internal moisture, and adhesive manufacturers specify maximum moisture vapour emission rates (MVER) for slabs before laying vinyl. If you suspect a moisture problem, have the slab tested before re-gluing.
Preventing Hollow Spots in Future
- Always follow the adhesive manufacturer's open time instructions — don't let the glue flash off before laying the plank.
- Ensure the subfloor is flat to within 3 mm over 1.8 m (a common industry tolerance for vinyl plank installation).
- Use the correct trowel notch size specified by the adhesive manufacturer.
- Allow the subfloor to acclimatise and check moisture levels before installation.
When to Call a Professional
If the hollow area is large, if multiple planks are affected, or if you suspect a subfloor or moisture problem, it's best to bring in a qualified flooring installer. Attempting to re-glue over a failing subfloor or a moisture-affected slab will only lead to the same problem recurring. A professional can assess the subfloor, identify the root cause, and carry out repairs correctly — including lifting and re-laying planks where needed.
You can find experienced local flooring installers on tradedirectory.co.nz.
