What’s a Subfloor?

Posted on Jan 19th 2012 by David — Comments ↓

In a Basement, or Not?

Because of the moisture issues that can affect solid hardwood flooring, you should only install a solid hardwood floor above ground level, or “above grade”. Definitely try not to go lower than 3″ below grade, below the level of the ground outside. If your room is below that, you’re better looking for another kind of material, engineered flooring, a moisture resistant laminate, or waterproof vinyl flooring, for instance. Though as long as you’re above grade, even water prone rooms like kitchens and bathrooms can be okay for solid hardwood flooring. There are protective finishes that can be used to protect the surfaces, so that you can have a fair amount of time to wipe up even major spills, and spilling (or flooding!) would be your only worry.


What Makes Up the Subfloor?

We should also consider what you already have in your room, what the new flooring will go over. Consider what subfloor material you have. First, what is a subfloor? Well, if you were to neatly slice down through a completely finished floor all the way to the Earth below, and take a look at the resultant cross section, going down from the top, here is what you would find:


Top layer: The part you walk on

This is what you see when you look down at your feet. This is the finished flooring, likely made of either hardwood, laminated wood, vinyl or stone tiles.


Second layer: Some kind of flooring “underlayment”

With wood type flooring, this will usually include some kind of a rosin paper, or possibly a felt paper, and what’s called a “moisture barrier” to keep natural moisture from affecting the wood products above. There may also be a type of board, made of a plywood or a cement type of material. These are put in place to create a level, almost planed surface on which to install your finished floor.

Third layer: Subfloor

This will either be wood or concrete, and its role is to be structurally sound, to support the weight of whatever will be put on it – people, ovens, things like that.


Bottom layer: “Joists”

“Joist” is an engineering term for the support crosspieces under a floor (or in a ceiling, depending on where you’re standing) that run between the walls of a room, or between the main support beams. If a subfloor supports the stuff in a room, joists could be said to support the room itself. They hold up the entire floor. These are going to be thick beams. Not all floors have these, as they are not needed with concrete subfloors.


So is your subfloor concrete, plywood or Oriented Strand Board (OSB)? OSB is an engineered type of panel, primarily wood-based. Strands of wood are pressed together in layers and bonded with a synthetic resin. The layers cross each other, some horizontal, some vertical, adding strength to the planks. Some may be suitable for installation of hardwood, while others may not. Older types of OSB are typically unsuitable for hardwood floor installs. check the manufacturer stamp on the actual product itself.


So, About That Subfloor?

OSB and Particle Board

Now OSB can be confused with particle board, but they are different things. Particle board is made of wood chips, planed off bits of softer wood and sawdust. It is not engineered so much as just jammed together really well. Particle board is very prone to moisture, expanding when it isn’t sealed properly. A particle board subfloor will not hold nails or staples, nor can it sustain a glued attachment, so it cannot be directly covered with hardwood flooring. You can put a floating floor over particle board. There are engineered hardwood floors that float, as well as laminates and vinyl floor types. Regardless of what kind you choose, you must replace any areas that are already moisture damaged. The expansion of any subfloor can damage the locking system of any type of floating floor.


With a plywood subfloor, all installation methods are deemed fair game. You can lay a floating floor, of course, but adhered floors are fine as well glue down floors, and staple or nail down floors wok on plywood subfloors.


For installation over concrete slab, the preference is for engineered hardwood flooring over solid. Solid hardwood flooring can expand and contract, even slightly warp due to temperature changes and differing moisture conditions (this is why it must also go above grade). Engineered wood flooring has a top layer of hardwood, which is the color, bonded to two or more layers of wood that is less costly, and often strong and resiliant because the layers are arranged with their grain patterns alternating. This is why engineered hardwood flooring is more stable, more resistant to those changes, and therefore may be installed above or below grade.


While there may be limitations, such as acceptable temperature ranges, most hardwood flooring can be installed over radiant heating.


Continue to Select your flooring

There is quite a selection of styles, species and sizes in hardwood flooring, and you also may have a choice of installation methods, which determines how much work you, or your paid installers, will do. There are also strips and planks, seven foot lengths and random lengths, solid and engineered, just to name a few. That’s our next post, Choosing Hardwood for Your Specific Needs.



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David is a Writer at Floors To Your Home (.com), as well as the PPC Manager, a Marketing Strategy Team member, a Researcher, Videographer, Social Strategist, Photographer and all around Resource Jitō. In my spare time I shoot and edit video, explore film history, mix music (as in ‘play with Beatles multi-tracks’) and write non-fiction for my friends. Connect with me on W. David Lichty’s Google+

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