How Terrazzo Works

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.

This type of terrazzo has several advantages over old-school terrazzo. The chips aren't just set into a thick cement -- they're laid in a matrix, or mixture, that's a combination of cement and latex. The latex strengthens the concrete, allowing the terrazzo topping to be spread as thinly as three-eighths of an inch. Instead of floating over the cement sub-floor on a cushion of sand, it's bonded directly to the concrete sub-layer below. Polyacrylate terrazzo systems cure quickly in comparison to traditional terrazzo -- often it can be installed in a single day -- especially because they're usually bonded to an existing concrete slab.

Because of the quick cure, they require less work and less product, so polyacrylate terrazzo floors are less expensive. If you wanted to put terrazzo in your home, it could be a good choice if you already have a level slab. Level is key -- since there's no sand cushion, a polyacrylate terrazzo floor can't be laid on an uneven floor. The existing concrete floor must have joints placed at specific distances (which vary depending on the size of the floor), and you have to put dividers over these joints. You can put more dividers if you want more color changes in the flooring, but unless they're over joints, they don't help prevent the floor from cracking. That's because the terrazzo topping layer is so incredibly thin. It weighs about four and a half pounds per square foot.

Since polyacrylate terrazzo is breathable, you can have it installed outside, too. And unlike cementitious terrazzo, some formulations of polyacrylate terrazzo allow you to include glass and mirrored chips -- including recycled glass. The aggregate can also be different sizes. Polyacrylate can be dyed to match just about any color, and in brighter colors than old-school cementitious terrazzo. People often use it on walls and other applications since it's so thin and cures quickly. If I could, I'd have polyacrylate terrazzo flooring in my house. It doesn't require any smelly solvents, either...unlike some types of epoxy terrazzo. But that has its own perks, too. Read on to find out why it's the most popular terrazzo installed today.

So if the traditional, cementitious terrazzo is great for basements and the outdoors, and polyacrylate terrazzo allows a wide variety of colors, why is there a third option? In the past, traditional was the only choice. Then epoxy, or resin-based, terrazzo came along in the 1970s and revolutionized the world of terrazzo flooring, making it much more affordable. According to the National Terrazzo & Mosaic Association, epoxy has more 70 percent of the terrazzo market. It's the most versatile, the strongest and the least susceptible to scratching, fading, cracking or staining.

You can have epoxy terrazzo flooring that's just a quarter of an inch thick, and it can go over a prepared concrete slab or even on top of plywood sub-flooring depending on the thickness. It's installed in multi-story buildings because it's so light -- about three or four pounds of weight per square foot. Epoxy terrazzo is available in any color and is the best for making intricate, multi-colored designs. You can install it with very few dividers, especially if it's being installed over plywood. It also can't be penetrated by moisture, so epoxy can't grow mold, mildew or bacteria.

That non-breathable quality is great for sanitary purposes, but it also means that moisture can get trapped between the concrete slab and the terrazzo topping. Some contractors will install a vapor barrier between them to help with this problem, which can lead to the topping becoming loose. It also can't be installed outdoors -- one, because it's non-breathable, and two, because the epoxy can fade with exposure to sunlight. Another downside is the solvent used in installation and cleanup with some types of epoxy terrazzo. There are some solvent-free versions, and only those kinds can be used in places with common ventilation systems like hospitals. Otherwise the fumes are too strong.

As you can see, each type of terrazzo has its pros and cons, depending on what you want. They're all going to be more expensive than many other types of flooring. We'll look at intensive process of installing terrazzo next.

No matter which type of terrazzo you choose, it's all going to be more intensive than putting in carpet or hardwood. Many types of flooring lend themselves to weekend projects or DIYers -- terrazzo isn't one of them. Unless you happen to be a mason. If you want terrazzo installed in an existing building, you need to call in contractors who specialize in it. Each one goes about it a little bit differently. While I've laid out the three main and most common types of terrazzo, there are all sorts of variations and contractors have their preferences and specialties. So let's pretend you're going get terrazzo.

First, your contractor will have to assess your current sub-flooring. If you want traditional terrazzo, you'll need a thick concrete base. That might mean excavating down under your house, because there's that sand layer on top of the cement before the actual terrazzo topping is laid and naturally you want that to jive with the existing floor level in the house. That's a lot of time and money spent, but if you're having it installed outside, it might not be as big of a deal. If you have a concrete slab already and you're getting polyacrylate or epoxy terrazzo, it'll just need to be checked to be sure it's sound and level. Your contractor will also locate existing joints in the concrete. And some types of epoxy terrazzo can be laid on top of plywood sub-flooring -- what many of us are most likely to have in our homes.

But let's go with the concrete slab, shall we? Workers will clean it, repair any defects, and prepare it for the type of terrazzo you're getting, a process known as creating the CSP (concrete surface profile). A thicker type, like traditional terrazzo, requires a rougher concrete surface, while thin-set systems like epoxy can go on a relatively smooth surface. You may also need a special membrane onto top of the cement to help prevent cracking, or in the case of epoxy terrazzo, to act as a moisture barrier.

If you're going with traditional, next you'll get the layer of sand, then a layer of cement, and then onto the design. But with polyacrylate and epoxy, it's straight onto the design. That'll all be planned out ahead of time, of course, differ depending on the type of terrazzo. Some contractors will print out the design on paper and lay it, full-sized, onto the floor. How detailed do you want it? Color changes in the design are separated by dividing strips, which might be made out of plastic, copper, zinc, brass or other materials -- the color can be part of the design, and they might also be different thicknesses. Really intricate designs might be laid out on panels of wire mesh ahead of time, or even use templates cut with water jets, while the rest is bent on-site. Workers solder joints together and then glue or otherwise attach the dividing strips to the concrete. The end is in sight...

Finally, the pretty part! The cement, cement matrix or epoxy is mixed on site, with colors added directly to the mix. Then the aggregate -- glass, marble, granite chips -- is mixed in. If you have lots of different colors, this means lots of different mixes. Epoxy cures quickly so it'll be done in smaller sections. The topping is spread evenly between the divider strips with trowels, or poured if it's a thinner aggregate. Next comes the curing. It could be less than a day, or several days in the case of traditional terrazzo.

Once the terrazzo is cured, you might be amazed at how, well, rustic it looks. Like lumpy concrete, actually. Go figure. You might like this look in some outdoor applications, and some people do leave their terrazzo this way. But that's probably not what you want to walk on in your house or office building. That's where grinding comes in. A huge grinding machine with disks covered in diamond or carborundum (silicon carbide) is run over the floor to expose the chips and create a smooth surface. Inevitably there will be some small holes left in the surface due to air pockets when the terrazzo was spread, so workers will go back and fill all of these in with grout. Then they apply a sealer to prevent any staining or moisture from getting in to the design.

Last comes polishing and waxing -- only then will you truly see the beauty of terrazzo. Now wasn't that worth the cost? But how much are we talking, you may ask? Using a cost estimator provided by the National Terrazzo and Mosaic Association, in my state it can cost between $10 and $16 per square foot. But terrazzo has what industry people call a "superior life cycle cost benefit." Although the initial install is pricey, you don't have to worry about replacing it or repairing it like you do with carpet, tile or just about any other type of flooring for at least 40 years. So enjoy. Or if you're like me, admire its beauty and durability anytime you walk across it.

I wasn't exactly enthused when I first found out that I'd be writing about flooring -- just being honest. I had a vague idea of what terrazzo was, but I didn't know anything about its history, complexity, or versatility. At best, I thought terrazzo was sort of a poor cousin to mosaic. Sure, that's sort of how it started, but it really has grown into an own art form and gained new life through its "green" possibilities. You might see utilitarian or ornate examples in all kinds of places. Terrazzo can be seen in a hospital, a university building, a sidewalk in New York, or in famous landmarks like St. Peter's Basilica or Mount Vernon, the estate of George Washington. I'll be paying closer attention to what's under my feet from now on!

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