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The substrate decides more than the surface does

The finish is the part of a terrazzo floor that is chosen. The substrate is the part that is discovered, usually late, and it decides more.

October 18, 2026 · 2 min read

Reference photograph of a terrazzo surface, s06. Not a photograph of our goods.

A floor specification starts with a finish, and the finish is what gets discussed. But the finish is applied to something, and on terrazzo that something is ground together with the surface.

What happens because of this

A cement-bound terrazzo floor is ground together with its substrate. The finished flatness is therefore the substrate tolerance plus whatever the topping adds, plus the effect of grinding — and the code allows 3 mm on the finished floor with a two-metre straightedge.

That means the slab cannot be given the slack it would get under a tile installation. A dip invisible under a 9 mm tile telegraphs straight through a ground surface, because the grinding follows the dip rather than correcting it.

What the North American codes ask for

The American practice is mechanical preparation to a stated profile: ASTM F710 with ICRI CSP 3 to 4, concrete of at least 3,626 psi which is 25 MPa, and for a resin floor a relative humidity reading under ASTM F2170, above which a barrier is required.

Note that these are two different moisture measurements in two systems, and they are not interchangeable. A water content on the slab and a relative humidity reading through it answer different questions.

Where the choice actually gets made

Where a slab cannot be brought to the condition a ground surface needs, the answer is not to try harder on the terrazzo. It is to change system. An epoxy floor is thin and bonded, and it tolerates a substrate that is sound but not perfect.

That decision has to be made before the finish is chosen, which is why a conversation that starts with the finish has already started at the wrong end.

The question to ask first

Not “what terrazzo”. Ask what the substrate currently is, what condition it is in, and what it would take to bring it to a profile a ground surface can sit on. The answer to the second question usually determines the first.

What we do not say here: we do not prepare substrates, inspect slabs or test floors we supplied. Everything above is a published requirement with the standard named, and substrate condition is established by testing on site by the party installing.