Terrazzo that sounds hollow when you tap it

A hollow, drummy sound when you tap a terrazzo floor means the topping has lost its bond with the slab in that spot. Small areas may be stable and can be watched; loose, cracked or growing areas are cut out and replaced before any grinding. A small repair falls in the usual $75 to $250 per patch range; a larger section replaced from strip to strip is quoted after the area is mapped.

Check this first

Do the golf-ball test. Drop a golf ball from knee height, or roll a steel ball or tap a coin, across the floor in a grid. Solid floor gives a sharp click. Hollow floor gives a dull, lower thud.
Mark the edges. Use painter’s tape to outline each hollow area. You will often find they are smaller than they sounded at first, or that they stop at a divider strip.
Press on it. Stand on the area and shift your weight. Any movement, rocking or crunching sound means it is loose, not just hollow.
Look for cracks. A hollow area with a crack through it, or a crack around its edge, is more likely to break up.
Note where it is. Next to an exterior door, a bathroom, a kitchen sink, or along an old plumbing route is a clue about the cause.

What is behind it

What you see or hear	Likely cause	Fix and typical cost
Small dull spot, no crack, no movement	Minor bond loss	Often watched, not repaired
Hollow area bounded by divider strips	Panel lost bond with slab	Replace the panel, quoted after mapping
Hollow area with a crack through it	Bond loss plus movement	Cut out and replace strip to strip
Hollow near a bathroom or kitchen	Old leak under the floor	Find the source first, then replace
Hollow after tile removal	Bond weakened by demolition	Cut out and replace, quoted after mapping
Rocking or crunching underfoot	Topping broken loose	Replace before any grinding

Why this matters before grinding

A planetary grinder is heavy and spins hard against the floor. Over a well-bonded topping it cuts cleanly. Over a section that has let go of the slab, it can crack or break up the loose piece and leave a bigger repair than you started with. That is why hollow areas are mapped as soon as the covering is off, and why the NTMA’s restoration specification includes “repairs to delaminated areas” as a distinct part of the job.

How replacement is done

The NTMA approach is to replace terrazzo to an architectural break, usually the nearest divider strip, or to a jagged, saw-tooth edge where there is none. The loose topping comes out, the concrete underneath is cleaned and repaired, cracks in the slab are filled, and a new topping of chips and matrix matched to the floor goes in. It cures, then is ground and polished along with the rest of the floor. A replaced panel reads as one panel in a slightly different shade, since matching existing aggregates may not be exact. See the crack and hole repair page for how chips are matched.

Local causes worth checking

Hillsborough’s terrazzo floors are sixty to seventy years old. In our pull of county property records, every house that listed terrazzo was built between 1954 and 1970, so these slabs have been through decades of wet seasons and dry ones. Two local causes are worth ruling out before a hollow area is replaced:

An old plumbing leak under the slab. If the hollow area sits along a bathroom or kitchen line, have a plumber confirm nothing is still leaking. Replacing terrazzo over an active leak only repeats the job.
Storm water. Floors that sat under water in Helene or Milton in 2024 are worth tapping out once they dry, since a long soak can loosen a weak bond. The flood-damaged terrazzo page has the order of work.

When to call

As soon as the covering is off, and before anyone grinds. Send a photo of the floor with the hollow areas outlined in tape, a coin for scale, and a note on any movement or cracks. We will tell you which areas look stable and which need replacing, priced from the pricing page. Then the floor can go through a normal restoration.

Frequently Asked Questions

Why does my terrazzo sound hollow in some spots?

The topping has lost its bond with the concrete slab underneath in that area, leaving a thin gap. It can come from slab movement, an old leak, a weak original bond, or impact. A hollow area that is not cracked or loose may be stable for years; one that is cracked, rocking or getting bigger needs repair.

Can you grind and polish a floor with hollow spots?

Not over a loose section. A planetary grinder can break up material that is not bonded to the slab. Hollow areas are mapped first, and anything loose or cracked is cut out and replaced before grinding. Sound but slightly hollow areas are watched and sometimes left alone.

What does it cost to fix a hollow spot in terrazzo?

It depends on the size. A small area may be a patch at $75 to $250. A larger section is replaced, ideally from divider strip to divider strip, and quoted after it has been mapped, because the size is unknown until you tap it out.