Hardwood floors for centuries have been well regarded. They became a standard in pre-World-War-II homes, and remained very popular until the advent of plywood as a substrate. The use of hardwood floors was furthered diminished in the '60s and '70s when wall-to-wall shag carpet became a chic and affordable alternative.

Hardwood floors made a major comeback in the '80s, and their popularity continues to skyrocket. In fact, people with wall-to-wall carpets frequently are removing them to reveal buried treasure ö elegant hardwood floors. With refinishing, such floors can be restored to pristine condition.

Hardwood is not an inexpensive flooring choice. However, with proper care and maintenance, it can last a lifetime. This makes it a cost-effective home investment. And few finishes offer the warmth (literally), natural beauty and design alternatives of hardwood. What's more, a hardwood floor is versatile. A large oval braided area rug can lend quaintness to a family room, while an oriental rug can provide an elegant touch to a dining room or living room.

Hardwoods are natural insulators, having air spaces of microscopic hollow cells that provide insulation. It would take 15 inches of concrete to equal the insulating qualities of one inch of wood. This quality means energy savings.

Concrete floors, by design, allow moisture to travel through the surface. This moisture will cause solid hardwood planks or strips to twist, cup and crack. This is magnified because there is no method to connect the hardwood to the concrete other than with an adhesive–which doesn't solve the moisture problem. Therefore, hardwood should not be installed directly over a concrete floor.

There are a couple of exceptions to this rule, and a couple of installation alternatives that make for a professional, long-lasting installation over a concrete floor. The exceptions: Parquet and laminated flooring can be glued directly to concrete with the proper moisture retarder. Parquet flooring consists of small regimented strips of hardwood that are usually preassembled into uniform squares. The squares then are installed much like pieces to a puzzle. Because parquet is constructed of small strips, it more readily conforms to the contour of the slab.

Laminated flooring is constructed of several layers of wood veneer bonded together with glue and pressure. This construction technique makes the flooring material less resistant to the twisting, cupping and curling to which solid material is subject. Once installed, laminate flooring has essentially the same appearance as solid material. However, because its finish consists of a reasonably thin veneer, it does not offer the durability and refinishing qualities of solid wood flooring.

Solid wood flooring can be successfully installed over a concrete floor if one of the two following installation systems is used: plywood-on-slab or sleeper. In either case, the slab must be flat and level, with a trowel finish, free of grease, oil, stains and dust. New concrete is heavy with moisture, so be sure to test for dryness before beginning the sub-floor.

Either of the following systems is satisfactory for 3/4-inch flooring up to four inches wide. For plank flooring four inches and wider, use the plywood-on-slab system, or top the sleeper system with an additional nailing surface.

Plywood-on-slab consists of a 3/4-inch layer of plywood underlayment to which the hardwood is nailed. The slab first should be covered with a vapor barrier of either asphalt felt, building paper or polyethylene (sheet plastic). With the vapor barrier in place, loosely place a layer of 3/4-inch-thick sheet (4 feet by 8 feet) exterior plywood panels over the entire area, leaving a 3/4-inch space at the wall line and 1/4 inch to 1/2 inch between panels. Fasten the plywood to the slab with powder-actuated fasteners, securing the center of the panel first, then the edges, using nine or more fasteners per sheet.

Install a second vapor barrier on top of the plywood underlayment and below the hardwood flooring. The hardwood should be installed at a right angle to the long dimension of the plywood. Tongue-and-groove material can be blind-nailed, using 1 1/2-inch nails.

The alternative involves the installation of sleepers. They typically consist of flat, dry, pressure-treated two-by-fours laid flat on their sides. They not only act as a nailing surface, but eliminate the need for subfloor when installed in rows at 12-inch centers, as well. For the hardwood flooring installation, the sleepers should be 18 inches to 48 inches long. This will prevent curling and twisting.

Prior to installing the sleepers, apply an asphalt primer and allow it to dry. Next, embed the sleepers in rivers of hot (poured) or cold (cut-back) asphalt mastic. The sleepers should be installed at a right angle to the direction of the finished flooring. Stagger the end joints, overlapping the ends four inches.

Before installing the floor, loosely lay an additional vapor barrier of four-to-six-mil polyethylene sheet plastic over the sleepers, overlapping the edges on top of the two-by-fours. Nail the flooring to the sleepers through the film. For added insulation, install one-inch-thick rigid insulation panels between the rows of sleepers. The rigid insulation should be placed above the sheet plastic and below the hardwood flooring.

For more information on hardwood floor installation and care, contact the Oak Floor Institute, P.O. Box 3009, Memphis, Tenn. 38173-0009.