The Purpose and Use of a Subfloor

The subfloor isn’t what most people worry about when they get ready to install new flooring. They’re usually thinking the differences of carpet vs. hardwood, not what goes beneath them. However, the subfloor is vital to the structure of your flooring, and what you see is just the topmost layer in a simple but crucial buildup of materials that make your floors work. But what is a subfloor? And what’s it made out of?

Subflooring is an important part of the architecture of a floor. Typically, when looked at as a cross-section, a floor would look kind of like a sandwich. You would see the visible “finish” floor at the top, and the joists at the bottom.

The joists are the support of the floor. Not all floors have joists, such as concrete floors, which do not require them. Between the subfloor and the finish, there’s an underlayment such as thin plywood, cement fiberboard, or cement board. Sometimes people say subfloor when they mean underlayment. Foam padding, cement board, waterproof layers, all of these materials are actually underlayment. When a contractor says subfloor, they mean the plywood or other materials that form the foundation of the floor.

When installing subflooring, keep plywood sheets as large as possible. This will help them stay strong. You don’t need to put down glue or nails between plywood and joists, but it can help prevent squeaky floors. Place the edges of the plywood sheets on top of the joists. Stagger plywood board to prevent four corners of plywood from meeting and causing spots of weakness in your subfloor.

No matter what flooring you put in, you must pay special attention to the subfloor, as without the proper choice of subflooring materials, you could end up ruining your new floor investment. If it needs sanding or repair, have it done as soon as possible.

Sometimes you don’t need to make alterations to the subflooring. For instance, if your existing subfloor is concrete, that’s fine for something like tile or floating install hardwood floors, though you may want to lay down a waterproof membrane. Concrete is porous, and moisture can seep up through it and cause problems like warping and mold, a danger in any home but especially one in which the occupants have allergies or any breathing issues such as asthma. Don’t skip this crucial step!

If you live in a newer house, your subfloors are most likely plywood with OSB over joists, particularly if your home has a crawlspace or basement. This type of subfloor is the most versatile. Any wood floors can be installed over it.

If your home is older, be careful of asbestos in your subflooring that could cause illness. Have a sample tested before you go ripping it up. If it tests positive, make the decision to build over the original floor or call in a special abatement company to come remove it for you.

A strong subfloor will help keep your flooring from degrading and will help protect your investment in the long term. Take care not to take shortcuts, or let your contractors do it, because weakness in your subfloor is a costly repair that can be avoided.