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How Terrazzo Works

Traditional Terrazzo
Terrazzo, Venetian style.
Terrazzo, Venetian style.

This type of terrazzo is basically what the Venetians used back in the day -- with a few changes. It was cheap for them because it used leftovers, clay, and goat's milk as a sealant. Of course, it also required lots of hours of labor as the workers did everything by hand, including polishing what was initially a very rugged surface into something smooth and comfortable for walking. That's part of what makes the traditional terrazzo expensive today -- the cost of labor. It's a time consuming, intensive process and requires skilled workers.

Traditional (also known as cementitious) terrazzo is heavy and requires laying a thick cement foundation slab to start. There are a couple of different variations, but according to the National Terrazzo & Mosaic Association, sand-cushion is the best of these. On top of the cement slab, there's a membrane to block moisture, followed by a layer of sand. Dividers -- strips of metal or plastic -- are placed to both make color changes in the terrazzo and provide room for the cement to expand and contract, preventing cracking. Then the terrazzo "topping", a mixture of cement and aggregate (chips of marble, granite, et cetera) is added. In total, you're looking at around three inches of flooring and a weight of about 25 pounds per square foot. The cement also takes a long time to cure, or dry.

So why choose this big heavy stuff? You can install it outdoors as well as indoors, and you generally don't have any problems with it staying level and even once installed. It breathes, so you don't have to worry as much about moisture problems. This is important if it's being installed outdoors or in a basement. It can accommodate big chips but the aggregate choices are limited -- things like glass can't be used because it's not porous and can't hold onto the stiff concrete. In addition, color choices are limited because the cement itself is dyed, and this type of terrazzo does best with simple shapes like squares because of the way the cement cures.

If old-school terrazzo turns you off because it's heavy and complicated, next up we'll look at what's sort of the "middle ground" of main terrazzo types.

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