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Answers

Laying 8 mm terrazzo over an existing floor: when it works

It works over a sound, clean, level floor, and it fails over a cracked one for the same reason every cement-bound terrazzo fails over a cracked slab: the floor is ground together with what is under it. There is no thick system available to hide behind — a sand cushion needs three inches and a finished floor has none to give — so the substrate is the whole risk.
Six terrazzo processes, drawn to one scaleEach column shows the layers of one terrazzo process. Precast and cement-bound are thick; epoxy is thin; porcelain is a fired body. Dimensions are published ranges, not our measured figures.substratePrecast20 mmsubstrateCement-based12 mmsubstrateEpoxy5 mmsubstratePoured10 mmsubstrateLiquid3 mmsubstratePorcelain9 mmSections drawn to one scale from published ranges. Our measured figures are listed separately.
The six processes drawn to one scale. Layer thicknesses are published ranges.

The figures, and where each one comes from

The floor under it Outcome
Sound, level, clean the normal case
Sound but cracked ⛔ the crack follows through, or the system has to change
Slab on a suspended frame ⛔ deflection, and the band has little thickness to span
Wet or damp seal or treat first, and a cement matrix absorbs
Level but not flat the panel follows the floor; flatness is inherited

Why the processes are not interchangeable

NTMA states the constraint that makes renovation terrazzo a narrow choice: a sand-cushion system cannot be installed in a renovation because the elevation cannot be found. What remains are epoxy, precast or a coating, and all three of them put the substrate directly into the finished floor — so whatever the slab is doing, the floor now does. A thin panel is the version of that with the least mass to distribute and therefore the least tolerance for a substrate that moves. The check that matters is not the flatness, which a panel will follow anyway; it is whether the slab has a crack in it.

What these figures are, and what they are not

The renovation limitation on sand-cushion systems and the crack-telegraphing behaviour are both from NTMA guidance. ⛔ We give NO deflection figure: the L/720 value in general circulation could not be traced to a standard and is not reproduced anywhere on this site. ⛔ We have not laid a floor over an existing slab and we do not install floors; a substrate assessment is the installer's work on site.

Measured data sheets and test reports

If you are specifying this

These are the forms we actually supply for this question, with production photographs where we have them:

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