A floor is nothing less than a room's fifth wall. Today more and more architects and designers are focusing their considerable attention on this most visible component of a room: Floors are wearing unpredictable new clothes. Increasingly, you can glance down and gaze at something unexpected: cork or recycled glass, linoleum or concrete aswirl with color, or, yes, even leather and bamboo. If a material is substantial, easy to clean, and can tolerate being trampled upon by people and pets, it may now be a candidate for the floor -- all of which is opening up new, undreamed of possibilities for renovating and decorating.

There are many good reasons to consider something other than the customary coverings when choosing a floor. After designing bamboo cabinets for a house in the Caribbean, Donato Savoie and Antonio Morello of the New York City design firm Studio MORSA decided to use bamboo for the floor of the Rodney Telford boutique in New York City. With its pale color and tight grain, punctuated by bamboo's characteristic "knuckles," the floor is elegant and distinctive. The designers were also attracted to bamboo's environmentally friendly attributes: It's a grass that does not require replanting after harvesting. In fact, many of today's unconventional floors are alternatives to nonrenewable natural materials like stone, wood from dwindling forests, or synthetic petroleum-based solid and laminating substances. And some of the prettiest comprise recycled matter rescued from landfills, like metal shavings and glass bottles.

Some of the best places for floor gazing are the newest commercial and industrial buildings, which often serve as incubators for adventurous materials and designs. Slowly but surely, these experiments are finding their way into houses and apartments.

There are just a few ground rules to observe. Besides being durable, a floor must suit the purpose of the room it graces. Leather, lovely in a library, is impractical in a foyer, where it will get dirty and scuffed. Because floor coverings can be expensive to install, consider colors and materials that you won't tire of seeing each time you look down.

And don't forget a floor's acoustic properties. Clodagh, a designer in New York City, likes the bright sound of stainless steel -- in the right setting. She has used it in a lot of boutiques, where, she says, "you don't want to feel as though you're the only person in the store." But you probably wouldn't want it in your bedroom.

Floor Materials
Cork
This is a classic covering enjoying new respect. In 1936, Frank Lloyd Wright used cork lavishly throughout Fallingwater, in western Pennsylvania, perhaps his masterpiece and surely his design most in harmony with nature. Cork remains attractive, durable, and environmentally appealing; what's more, it's warm in the winter and cool in the summer.

Cork is the soft brown meat beneath the bark of oak trees grown in Spain, Portugal, and Sardinia. Harvested when the trees' outer bark naturally loosens every decade or so, most of it is used in wine bottles. The rest is ground up for use in soundproofing, shoe soles, and flooring. Available in tiles and tongue-and-groove planks, cork starts at about $3 per square foot and comes in a range of patterns. It can be laid out in checkerboard or herringbone patterns, dressed up with borders, or stenciled. And you care for it as you do wood: with varnish, wax, or polyurethane.

Old Wine Bottles
No one thinks of old wine bottles as jewels, but precious stones will instantly come to mind when you see a field of recycled-glass tiles in turquoise or amber. They can be as pristine as new glass. The tiniest mosaic pieces measure one inch square. Large tiles start at $24 per square foot. They are deliberately flawed, and delightfully so, with cracks and bubbles reminiscent of a Roman mosaic. You can devise an entire floor of glass or use it as accents on terra-cotta or concrete. Glass is easy to maintain -- wash it with vinegar and water or spritz it with glass cleaner. But don't drop a heavy object, like a hammer, on it.

Bamboo
At first glance, a bamboo floor is similar in appearance to wood. But look closely: Bamboo's grain is always tight and straight, interrupted by those distinctive nodes.

Like wood flooring, bamboo is sold in tongue-and-groove planks. The finest start at $6 per square foot and are made in China, where reeds grow to the six-inch diameter required for tongue-and-groove planks. Caring for a bamboo floor is similar to caring for a wood one. Because moisture does not cause it to expand or contract excessively, consider bamboo for kitchens or bathrooms, where wood may be impractical.

Linoleum
This is often confused with vinyl, but they're two different materials. Vinyl is a relatively modern synthetic. Real linoleum is made from natural ingredients: wood flour, rosins, ground limestone, powdered cork, pigments, jute, and linseed oil. No longer manufactured in the United States, it is available through companies like Forbo and Gerbert, which import it from the Netherlands, Scotland, and Germany.

The recipe for linoleum has changed little since its invention in 1860. The High Victorian name blends the Latin words for flax (linum) and oil (oleum). Linseed oil is the primary component; it also lends linoleum its distinctive scent. Today's linoleum is sumptuous, available in vivid colors or in marbleized, sweetly retro concoctions unimaginable during the Depression or the fifties. Starting at $32 per square yard, linoleum is no longer a budget floor. But it can last a half century if it is properly installed (the floor underneath must be completely smooth), damp-mopped regularly with a neutral detergent, and never scrubbed with abrasive cleansers. Kitchens are still its primary habitat, but linoleum can be laser-cut in intricate patterns worthy of an entryway or dining room.

Concrete
The idea of a concrete floor may take some convincing. But the newest concretes are stylish, colorful, and versatile. "You can make a concrete floor as rough or as sensuous as you like," says Clodagh. "And it will last forever." The Ardex company makes a quick-drying, self-leveling concrete topping that starts at $4.50 per square foot. It can be poured over an existing concrete floor, and you can tint it. Syndecrete, made by Syndesis, is another kind of concrete flooring; it's lightweight, colored, and incorporates recycled materials, like metal shavings and glass chips. Starting at $15 per square foot, the final product, produced in tiles, looks like a twenty-first-century terrazzo floor.

Leather
For something a bit more buttery, there's always leather. Needless to say, it's expensive, from about $18 for an eight-by-eight-inch tile. It should be waxed twice a year and buffed every few weeks. But if you can live with leather's demands, it is beautiful. Tiles from the Edelman company come in six shades, from rust to hunter green. And, of course, there's that subtle scent.