Guides
Can terrazzo be used outdoors, and what decides it
Yes, and the deciding variable is not the material but the binder. A resin system is light and takes water and chemicals well, and it is sensitive to ultraviolet light. A cement system is unaffected by the sun and is heavy enough to need a real base.
Terrazzo is used outdoors. The question is never whether the material survives the weather — it does — but which of the two systems is being specified and what is underneath it.
The two systems behave differently outdoors
A cement-bound system is unaffected by ultraviolet light and is heavy. The published assemblies run from 25 to 30 lbs per square foot for a sand-cushion build down to 5 to 7 for a monolithic one, and that weight is why an exterior job starts with the base rather than with the finish.
A resin-bound system is the opposite way round: 3 to 4 lbs per square foot, no sand cushion needed, and excellent chemical and water resistance. ⛔ It is also the one sensitive to ultraviolet light, which is why it is chosen indoors far more often than out.
So the choice outdoors falls out of the base. Where there is a real concrete base and the height is available, a cement-bound system is the durable answer. Where the depth is not available — a bridge deck, a retrofit, a terrace with a limited build-up — the resin system is chosen for its weight and fixedness, and the ultraviolet exposure has to be dealt with in the specification rather than afterwards.
The substrate question comes first
Whatever is chosen, an exterior floor is ground together with its substrate, so the substrate tolerance is not a detail. The finished flatness allowance for a cement-bound floor is 3 mm over a two-metre straightedge, and an outdoor base moves more than an indoor one — which is the reason movement joints are drawn at all.
What to ask before specifying
Ask what the base is and whether its height can change. Ask whether the surface is walked on when wet, and if so ask for the wet slip figure with the test method named beside it. Ask how the movement joints are spaced. Those three answers decide the system; the finish is decided afterwards and can be changed.
What we do not say here: no slip figure, no weight for our own material, no joint spacing and no ultraviolet test result for any product we supply. Every figure quoted above is a published system figure with the source named, and none of them describes a batch we have made.
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