Anatomy of a Floor

If you're planning to change the flooring in your home, don't choose a new surface without looking at what goes underneath. Here's a quick guide to a typical floor anatomy.

Squeaks, sags and other annoying floor problems can be avoided when you understand the anatomy of a floor and how your new floor covering will interact with what's below it. Here's a quick guide to a typical floor anatomy:


Joists are the cross-beams that make up the frame of the floor and sit atop the foundation of the house. On higher floors of a house, the joists hold up the higher floors, acting as the skeleton underneath.

"The biggest complaint homeowners have about floors is that they're noisy, squeak or bounce," says John Peavey, director of applied technology at the National Association of Home Builders Research Center in Upper Marlboro, Md. "If you don't install the joists correctly, it can feel like the floor is bouncing, or it'll make those noises."


Often made of plywood or composite material, the subfloor is placed atop the joists, preventing damage to the joists and providing a stiffer base for the floor covering.

The spacing in the subfloor creates the right amount of deflection (the amount of vertical space the floor dips between two joists) for your floor covering. The spacing specifications differ with different floor coverings.

"If you're installing a floor yourself, start by finding out your local building codes, which will tell you what type of wood grading or lumber grading you need to build the subfloor, as well as the span size," Peavey advises. "The span size defines the size of the lumber and the spacing between different parts of the subfloor."

Most local building departments reference a version of the International Residential Code. Click here for general code information.

Organizations like the Southern Forest Products Association and the APA Engineered Wood Association are good resources for information on subfloor design and requirements.


Underlayment, which is placed over the subfloor, helps to deaden sound and slightly softens the feel when walking on the floor. Most are made of foam or cork. Underlayment is required for the installation of "floating floors" like laminate or engineered wood flooring, which are not attached to the subfloor.

"If you're putting in ceramic tile or natural stone that's grouted, you need an underlayment that's moisture resistant," explains Tom Jennings, technical adviser and former chairman of the World Floor Covering Association, "as there could be a pinhole in the grouting and moisture could seep through. For carpet, the padding underneath isn't seen, but the quality of the finished product is affected by it."

Floor Covering

This is the design element that you've chosen to look at and walk on. Floor coverings include vinyl, laminate, hardwoods, tile, stone and carpet.

The cost of materials will run an average of $3 to $4 a square foot, regardless of the floor covering you choose. Installation costs will vary, depending on the condition of the joists, subfloor and type of underlayment required.

"We like to say that most flooring does not wear out," Jennings says. "It uglies out by going out of style. Existing conditions can dictate using one product over another. Someone needs to look at the existing condition of the floor first. Most people are not served well by installing their own floors. If you don't follow the manufacturer's instructions correctly, you have no or limited warranties."

Several manufacturing trade associations can answer questions about installation and product selection, including:

Several manufacturing trade associations can answer questions about installation and product selection, including:

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