Do I Need A Subfloor?

    Victuallers (CC BY-SA 3.0) crop/Wikimedia Commons

    Are you planning to replace your old floors? Or are you about to install flooring in a newly-built home? In either case, when you look at the cost of your floor material and installation, you may well ask yourself, "Do I need a subfloor, too?" To make an informed decision, take a closer look at what a subfloor does and why it's important.

    Do I Need a Subfloor? What Does It Actually Do?

    A subfloor is a structure attached to your floor joists which provides support for your finish (surface) flooring. Used alone, most finish flooring materials are not strong enough for the dead weight of furniture, cabinetry, appliances, and other household objects plus the live weight of people and pets.

    Your subfloor also provides stability and a level surface, which will make flooring installation easier, whether you hire a professional or do it yourself. If you install a ceramic tile floor, a level subfloor is essential to prevent tile cracking.

    Another function of subflooring is to protect your surface floor against moisture damage and mold in a damp environment.

    Electrical wiring and plumbing pipes may be run through the subfloor.

    What's the Difference between Subfloor and Underlayment?

    Don’t confuse the terms “subfloor” and “underlayment.” Underlayment is an additional thin layer that is placed on top of the subfloor when you’re having tile, carpet, or laminate flooring installed. The underlayment acts as a vapor barrier, helping to safeguard your floor against ambient moisture. It also adds an element of soundproofing and, in the case of carpeted floors, increases underfoot comfort.

    What Materials are Used for Subfloors?

    Plywood is the material most commonly used for subflooring. Manufactured from thin sheets of wood glued together with the grain of each layer perpendicular to the previous one, plywood provides a strong support, a smooth surface, and a reasonable cost.

    Oriented strand board (OSB) is another popular subfloor material. Because it's made up of large wood chips adhered with resin, OSB offers a dense, smooth surface compared to plywood. When exposed to moisture, though, OSB takes longer to dry, which could lead to water damage for your subflooring and even your finish floor. However,  insulated subfloor paneling, consisting of OSB sheathing backed by foam insulation, is available -- for a price.

    Particleboard is not used as frequently as the previous two options. Although it is also manufactured from wood and is inexpensive, particleboard is composed of small chips, resulting in a weaker product which absorbs more moisture.

    Do I Need a Subfloor Over a Concrete Slab?

    While a subfloor is not necessary to add structural strength when you'll be laying finish flooring on top of a concrete slab (as in a basement remodel), subflooring over concrete offers two other advantages:

    1. Dampness control. Install a vapor barrier of plastic sheeting before laying the subfloor. If necessary, place sleepers (mini-joists) to raise the subfloor above a very damp concrete slab. Waterproof sealer will also help keep out moisture.

    2. Temperature control. A subfloor will help insulate against a concrete floor that becomes overly cold in winter.

    The concrete slab must be clean, sound, and level, with any serious cracking repaired, before you install your subfloor.

    Subfloor Installation Tips

    • Always check your local building code before choosing a particular subflooring material or design.

    • As with many finish flooring materials, your subfloor may need 24 hours or more to acclimate to your home.

    • The thickness of subflooring depends on the spacing of your joists. Minimum thickness should be 5/8” for plywood and 23/32” for OSB. When joists are spaced 16-19.2 inches apart, subfloor should be ¾” for both materials. With joist spacing of over 19.2, thickness of plywood should be 7/8” and of OSB, 1”.

    • Always read through the subflooring manufacturer’s instructions prior to installation.

    • Your finish flooring is only as good as your subfloor. Ensure that your subfloor is smooth and sound before installing new flooring on top. If it is wet, you may be able to dry out subflooring by opening up the drywall underneath it. However, patching is your best option for a severely damaged subfloor.

    Laura Firszt writes for

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