Anatomy of a Floor

Your finish floor is essentially a “skin” or covering over the real floor of your house—the structural floor that holds up the walls, furnishings, and occupants and keeps out moisture and drafts. Many people never see this structural floor or know how it fits with the other parts of the house. But if you’re installing a new finish floor, you must know something about your existing subfloor and framing conditions to avoid covering up problems that may affect your new floor. We will now show you the anatomy of a concrete slab and a wood-frame subfloor, it is important that you find out which type is underneath your existing finish floor, because the two types are quite different, Each has specific characteristics of material and construction that will affect your new floor.

The illustrations below will help you visualize the floor structure as a series of layers. It is easy to see the importance of some layers; for instance, in wood-frame construction, the foundation and framing layers support the house, and the top layer provides the finished look. But the layer sandwiched in the middle also serves a critical function. This layer is the underlayment, and de pending on the materials used for the subfloor and finish floor, it serves one or more of the following purposes: (1) it adds stiffness and rigidity to the floor; (2) it provides a smooth surface for the flooring material; (3) it protects the floor from moisture, drafts, and dust; (4) it increases the floor’s resilience; and (5) it provides a suitable surface to which the finish floor can adhere. The upcoming discussions of underlayments will help you decide if your new flooring needs an underlayment, and which material is best.

Note: The term underlayment usually refers to the non-structural material added just before the finish floor is installed, but it is sometimes used to describe the layer immediately below the finish floor—the subfloor or an existing finish floor over which new flooring material will be laid. To avoid confusion, be specific about what you’re referring to when you discuss your underlayment or subfloor with a professional.

Concrete Slabs

Concrete is not a waterproof material. It contains a certain amount of moisture for months after it’s poured, and it can also wick moisture up from the ground. Therefore, special moisture barriers are often installed beneath the slab before it’s poured, to prevent moisture problems which can cause discomfort to you, and deterioration of the finish floor. Because concrete slabs are very strong and stable when properly constructed, they can provide excellent backing for many types of flooring. Rigid materials such as ceramic tiles and wood blocks, as well as flexible materials like resilient and carpet materials, can be glued down to a prepared concrete slab if there is no moisture problem. Laminated wood strip materials are specially manufactured for glued application to concrete slabs, though for traditional wood strip or plank materials, you’ll have to construct a wood subfloor over the slab in order to create a subsurface that you can nail into.

Anatomy of a Concrete Slab: Moisture barrier between sand bed and slab; Steel mesh embedded in slab when poured; Gravel and sand bed; Tackless strip; Baseboard

Wood-Frame Subfloors

A wood-frame subfloor has different characteristics. Its separation from the earth protects it from ground moisture, but it is vulnerable to atmospheric humidity and water from spills, plumbing leaks, or root leaks, which can cause the structure or subfloor to warp or distort from expansion. A wood subfloor is also easy to nail into, and it provides a resilient base for the finish floor.

Structurally, a wood subfloor is only as strong as the foundation and framing beneath it. If the foundation has settled over time, the subfloor might sag; this should be corrected before installing new flooring. If you use very heavy finish floor materials like brick or stone, you may need to reinforce the joist structure to support the additional load. As you can see, the subfloor characteristics, inherent in the structural materials themselves, have important implications for choosing and installing your finish floor. If you need to make structural additions or repairs, refer to our web site articles: Basic Carpentry Techniques and Basic Remodeling Techniques.

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