Hollow Spots Under Glued Floors

That dead-sounding patch when you walk across a glued-down board is the adhesive failing to grab the underside. Here's why it happens and how to stop it on your next install.

What hollow spots actually are

When you glue a timber board down with a notched trowel, the adhesive sits in ridges across the subfloor. The board lays into those ridges and -- in theory -- spreads them out as the board takes its weight. If the board lifts off the ridges before the adhesive cures, you end up with sealed air pockets under the board. Walk across it later and the pocket sounds dead, like tapping cardboard.

Why boards lift before cure

Three main reasons:

Adhesive Green Grab too low

Cheap or hard-elastic adhesives don't have enough initial tack to hold a board down against natural board cup, internal stress, or moisture movement. The board lifts a few millimetres in the first hour and you're left with hollows that no amount of weight will fix once cured.

Subfloor flatness

If the subfloor has dips and humps, the board only contacts the high points and bridges the dips. Adhesive pools in the dip without contact. Standard tolerance is around 3mm in 3m -- check with a long straight edge before you start spreading glue.

Wrong trowel notch

Notch too small and you don't get enough adhesive volume to fill out the dips. Notch too big and the ridges don't compress when the board goes down.

The fix on a finished floor

There isn't a clean one. You can't inject adhesive under a glued board without lifting it. If the hollows are a few isolated spots, mark them and accept it. If it's widespread, the answer is to lift the floor and re-do it. This is why prevention matters -- a hollow-spot callback eats the entire margin on a glued floor job.

How to stop it on the next install

Pick an adhesive built for high initial bond strength. Bona Quantum T has the highest Green Grab in the Bona range -- up to 150% more shear strength than standard elastic adhesives, and it holds boards flat to the subfloor through the cure window. Use the Bona OptiSpread UX gun to dispense from the 9kg sausage and a Bona notched trowel matched to your board thickness. Check subfloor flatness before you spread. Keep the room temperature stable through cure -- don't crank the heating or open windows.

Subfloor moisture matters too

If the subfloor is wetter than spec, the board will move as it equalises and even a great adhesive can lose contact. Quantum T is itself a moisture barrier but the substrate still has to be within tolerance. Test with a moisture meter before you start. If concrete is borderline, prime with R410 or R540 first.

Related: Bona Product FAQs →