Hollow-sounding or bouncy spots underfoot

A drum-like note when tapped, or a slight give underfoot, over a defined area. Tap along with a knuckle and the boundary is usually sharp.

What causes it

In rough order of likelihood. Each one has a tell — the observation that separates it from the others.

1. 1

   Adhesive failure on a glue-down floor

   How to tell: A defined hollow area on engineered flooring over concrete. The bond has let go and the area will grow.

2. 2

   The subfloor was not flat enough at installation

   How to tell: Hollow spots present from the beginning, at the high and low points of a wavy subfloor. An installation failure.

3. 3

   Normal behaviour of a floating floor

   How to tell: Mild hollowness across the whole floor on a click-together installation. Expected, not a defect.

4. 4

   Subfloor deterioration underneath

   How to tell: Genuine softness rather than just noise, particularly near a bathroom or an exterior wall. This one is structural.

What it actually means

Hollow means a gap between the floor and what should be supporting it. On a glue-down floor it spreads, because every footstep works the edges of the debonded area. Softness rather than sound is a different and more serious matter — that is the subfloor, not the finish floor.

Small debonded areas can be injected and weighted. Large ones mean lifting and re-laying that section. Where the subfloor itself has gone soft, the floor comes up regardless — and finding that out before you refinish rather than after is the difference between one job and two.

• Tap out the area with a knuckle and mark the boundary with tape. Whether it grows over a month is the key diagnostic.
• Note whether it sounds hollow or actually feels soft. Those are different problems with different costs.
• Keep heavy rolling loads off it — a debonded area spreads fastest under wheels.

• Gaps opening between boards in winter
• Squeaking or creaking hardwood floor