Hollow spots and lifting terrazzo: how to test it and what it takes to fix in St. Petersburg

Terrazzo that sounds hollow when you tap it has lost its bond to what is underneath, and the fix depends entirely on how big the area is and whether it has started to move. Small, flat hollow areas are stabilized and ground with the rest of the floor at $4 to $8 per square foot. Sections that have lifted or cracked get cut out to a divider strip and replaced, and the honest answer is that the number comes from a sounding survey and not from a price table.

Check this first

1. Sound the whole floor. Drag a golf ball slowly in overlapping lines. Tight terrazzo clicks. Hollow terrazzo goes dull and drummy, and the sound changes at a sharp line rather than fading.
2. Tape the boundaries. Mark the edge of every dull area with painter’s tape. Take a photo from a doorway. That map is the single most useful thing you can send.
3. Stand on it and rock. If the area flexes, clicks, or grits under your weight, it is loose, not merely hollow, and it is going to be replaced rather than filled.
4. Look for a crack around the edge. A hollow area ringed by a crack has been working for a while. A hollow area with no crack at all is often stable and old.
5. Check whether it is near water. Hollow spots along an exterior wall, under a slider, at a bathroom wall, or in a room that flooded are worth a moisture check before anything is repaired.

The full restoration page explains what happens after the survey.

What is usually wrong

Patch and grind numbers are the same as on the pricing page.

Why this happens in Pinellas houses

Cement terrazzo in a mid-century block house is a topping placed on a slab, and its whole performance depends on the bond between the two. The NTMA describes the monolithic system, the thin one used over house slabs, as a half-inch cement matrix veneer placed upon a provided concrete slab that is dependent on the concrete quality for flatness and crack prevention. Where the slab was dusty, damp, or badly finished on the day the terrazzo crew arrived in 1958, the bond was weaker from the start, and it can stay quietly hollow for sixty years without a problem.

What turns a quiet hollow spot into a repair is water. This county is sand over limestone with a shallow water table, and a slab-on-grade floor here is a moisture path. Vapor moving up through the slab carries dissolved salts, they crystallize at the bond line, and crystal growth pries the topping off the concrete from below. The same mechanism that leaves white powder on the surface of damp concrete works at the interface, and Wikipedia’s summary of efflorescence puts it directly: efflorescence may clog the pores of porous materials, resulting in their destruction by internal water pressure. That is why hollow spots cluster along exterior walls, near plumbing, and in rooms that took surge in Gulfport or on the barrier islands.

The NTMA restoration specification lists repairs to delaminated areas as a distinct scope item, separate from crack repair, and requires the contractor to reevaluate conditions once substrates are uncovered. That is the honest shape of this job.

What will not fully come out

A replaced section is new terrazzo. It is matched by chip and matrix, ground flush, and polished with the rest of the floor, and it is still new terrazzo next to sixty-year-old terrazzo. The NTMA specification handles the seam by requiring a saw tooth, a jagged edge, wherever the replacement does not run to an architectural break or an existing divider strip, precisely because a straight seam is what the eye finds. Expect a visible but irregular boundary, not an invisible one.

And the underlying limit: if vapor is still driving through the slab, a replaced section can delaminate again. Moisture gets tested before repair and before sealing, not after. The white haze page covers what that testing looks like.

What to expect on the call

Send the taped map photo, say roughly what percentage of the floor sounds hollow, and mention any flooding history or plumbing work. You will get a range rather than a firm number, because the extent of a delamination is not knowable until the loose material comes up. What you should also get, before any money changes hands, is a plain statement of which areas will be replaced and which will be left alone. The cracks page covers the related symptom, and the restoration cost calculator gives the grind-and-polish side of the number.